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Superconductivity in Se-doped new materials EuSr2Bi2S4F4 and Eu2SrBi2S4F4
Authors:
Zeba Haque,
Gohil S. Thakur,
Rainer Pöttgen,
Ganesan Kalai Selvan,
Rangasamy Parthasarathy,
Sonachalam Arumugam,
Laxmi Chand Gupta,
Ashok Kumar Ganguli
Abstract:
From our powder x ray diffraction pattern, electrical transport and magnetic studies we report the effect of isovalent Se substitution at S sites in the newly discovered systems EuSr2Bi2S4F4 and Eu2SrBi2S4F4. We have synthesized two new variants of 3244 type superconductor with Eu replaced by Sr which is reported elsewhere [Z. Haque et. al.]. We observe superconductivity at Tc 2.9 K (resistivity)…
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From our powder x ray diffraction pattern, electrical transport and magnetic studies we report the effect of isovalent Se substitution at S sites in the newly discovered systems EuSr2Bi2S4F4 and Eu2SrBi2S4F4. We have synthesized two new variants of 3244 type superconductor with Eu replaced by Sr which is reported elsewhere [Z. Haque et. al.]. We observe superconductivity at Tc 2.9 K (resistivity) and 2.3 K (susceptibility) in EuSr2Bi2S4-xSexF4 series for x = 2. In the other series Eu2SrBi2S4-xSexF4, two materials (x= 1.5; Tc = 2.6 K and x = 2; Tc = 2.75 K) exhibit superconductivity.
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Submitted 11 December, 2016;
originally announced December 2016.
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Unusual mixed valence of Eu in two new materials EuSr2Bi2S4F4 and Eu2SrBi2S4F4: Mössbauer and X-ray photoemission Spectroscopy investigations
Authors:
Zeba Haque,
Gohil Singh Thakur,
Rangasamy Parthasarathy,
Birgit Gerke,
Theresa Block,
Lukas Heletta,
Rainer Pöttgen,
Amish G. Joshi,
Ganesan Kalai Selvan,
Sonachalam Arumugam,
Laxmi Chand Gupta,
Ashok Kumar Ganguli
Abstract:
We have synthesized two new Eu-based compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4 which are derivatives of Eu3Bi2S4F4, an intrinsic superconductor with Tc = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu3Bi2S4F4. Our structural and 151Eu Mössbauer spectroscopy studies show that in EuSr2Bi2S4F4, Eu-atoms exclusively occupy the crystallographic 2a-sit…
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We have synthesized two new Eu-based compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4 which are derivatives of Eu3Bi2S4F4, an intrinsic superconductor with Tc = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu3Bi2S4F4. Our structural and 151Eu Mössbauer spectroscopy studies show that in EuSr2Bi2S4F4, Eu-atoms exclusively occupy the crystallographic 2a-sites. In Eu2SrBi2S4F4, 2a-sites are fully occupied by Eu-atoms and the other half of Eu-atoms and Sr-atoms together fully occupy 4e-sites in a statistical distribution. In both compounds Eu atoms occupying the crystallographic 2a-sites are in a homogeneous mixed valent state ~ 2.6 - 2.7. From our magnetization studies in an applied H = 9 Tesla, we infer that the valence of Eu-atoms in Eu2SrBi2S4F4 at the 2a-sites exhibits a shift towards 2+. Our XPS studies corroborate the occurrence of valence fluctuations of Eu and after Ar-ion sputtering show evidence of enhanced population of Eu2+-states. Resistivity measurements, down to 2 K suggest a semi-metallic nature for both compounds.
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Submitted 2 December, 2016;
originally announced December 2016.
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Synthesis, Crystal Structure and Magnetism of Eu3Sc2O5Fe2As2
Authors:
Franziska Hummel,
Marcus Tegel,
Birgit Gerke,
Rainer Pöttgen,
Dirk Johrendt
Abstract:
The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173-1373 K in a resistance furnace and characterized by X-ray powder diffraction with Rietveld analysis: Sr3Fe2O5Cu2S2-type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm. Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like oxide blocks. No structural transition occurs in the temperature range from 10 to 300 K. Magnetic measu…
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The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173-1373 K in a resistance furnace and characterized by X-ray powder diffraction with Rietveld analysis: Sr3Fe2O5Cu2S2-type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm. Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like oxide blocks. No structural transition occurs in the temperature range from 10 to 300 K. Magnetic measurements have revealed Curie-Weiss behavior with an effective magnetic moment of 7.79 muB per europium atom in agreement with the theoretical value of 7.94 muB for Eu2+. A drop in the magnetic susceptibility at 5 K indicates possible antiferromagnetic ordering. 151Eu and 57Fe Mössbauer spectroscopic measurements have confirmed a beginning cooperative magnetic phenomenon by showing significantly broadened spectra at 4.8 K compared to those at 78 K.
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Submitted 9 June, 2015;
originally announced June 2015.
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Valence modulations in CeRuSn
Authors:
R. Feyerherm,
E. Dudzik,
K. Prokes,
J. A. Mydosh,
Y. -K. Huang,
R. Pöttgen
Abstract:
CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-δ)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structur…
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CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-δ)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L$_{3}$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to a previous report, the new data exhibit small but significant variations as function of temperature that are consistent with a transition of a fraction of Ce$^{3+}$ ions to the intermediate valence state, analogous to the $γ\rightarrow α$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.
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Submitted 14 May, 2014;
originally announced May 2014.
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Superconductivity and Crystal Structure of the Palladium-Iron-Arsenides Ca10(Fe1-xPdxAs)10Pd3As8
Authors:
C. Hieke,
J. Lippmann,
T. Stürzer,
G. M. Friederichs,
F. Nitsche,
F. Winter,
R. Pöttgen,
D. Johrendt
Abstract:
The palladium-iron-arsenides Ca10(Fe1-xPdxAs)10(Pd3As8) were synthesized by solid state methods and characterized by X-ray powder and single crystal diffraction. The triclinic crystal structure (space group P-1) is isotypic to the homologue platinum 1038 type superconductors with alternating FeAs4/4- and Pd3As8-layers, each separated by layers of calcium atoms. Iron is tetrahedral and palladium is…
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The palladium-iron-arsenides Ca10(Fe1-xPdxAs)10(Pd3As8) were synthesized by solid state methods and characterized by X-ray powder and single crystal diffraction. The triclinic crystal structure (space group P-1) is isotypic to the homologue platinum 1038 type superconductors with alternating FeAs4/4- and Pd3As8-layers, each separated by layers of calcium atoms. Iron is tetrahedral and palladium is planar coordinated by four arsenic atoms. As2-dimers (dAs-As = 250 pm) are present in the Pd3As8-layer. Even though each layer itself has a fourfold rotational symmetry, the shifted layer stacking causes the triclinic space group. Resistivity measurements of La-doped samples show the onset of superconductivity at 17 K and zero resistivity below 10 K. The magnetic shielding fraction is about 20 % at 3.5 K. 57Fe-Mössbauer spectra exhibit one absorption line and show no hint to magnetic ordering. The electronic structure is very similar to the known iron-arsenides with cylinder-like Fermi surfaces and partial nesting between hole- and electron-like sheets. Our results show that superconductivity in the palladium-iron-compounds is present but complicated by too high substitution of iron by palladium in the active FeAs-layers. Since the electronic preconditions are satisfied, we expect higher critical temperatures in Pd1038-compounds with lower or even without Pd-doping in the FeAs-layer.
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Submitted 26 March, 2013;
originally announced March 2013.
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Crystal Structure of BaFe2Se3 as a Function of Temperature and Pressure: Phase Transition Phenomena and High-Order Expansion of Landau Potential
Authors:
V. Svitlyk,
D. Chernyshov,
E. Pomjakushina,
A. Krzton-Maziopa,
K. Conder,
V. Pomjakushin,
R. Pottgen,
V. Dmitriev
Abstract:
BaFe2Se3 (Pnma, CsAg2I3-type structure), recently assumed to show superconductivity at ~ 11 K, exhibits a pressure-dependent structural transition to the CsCu2Cl3-type structure (Cmcm space group) around 60 kbar, as evidenced from pressure-dependent synchrotron powder diffraction data. Temperature-dependent synchrotron powder diffraction data indicate an evolution of the room-temperature BaFe2Se3…
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BaFe2Se3 (Pnma, CsAg2I3-type structure), recently assumed to show superconductivity at ~ 11 K, exhibits a pressure-dependent structural transition to the CsCu2Cl3-type structure (Cmcm space group) around 60 kbar, as evidenced from pressure-dependent synchrotron powder diffraction data. Temperature-dependent synchrotron powder diffraction data indicate an evolution of the room-temperature BaFe2Se3 structure towards a high symmetry CsCu2Cl3 form upon heating. Around 425 K BaFe2Se3 undergoes a reversible, first order isostructural transition, that is supported by the differential scanning calorimetry data. The temperature-dependent structural changes occur in two stages, as determined by the alignment of the FeSe4 tetrahedra and corresponding adjustments of the positions of Ba atoms. On further heating, a second order phase transformation into the Cmcm structure is observed at 660 K. A rather unusual combination of isostructural and second-order phase transformations is parameterized within phenomenological theory assuming high-order expansion of Landau potential. A generic phase diagram mapping observed structures is proposed on the basis of the parameterization.
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Submitted 3 January, 2013;
originally announced January 2013.
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Structural and magnetic phase transitions in triclinic Ca10(FeAs)10(Pt3As8)
Authors:
T. Stürzer,
G. M. Friederichs,
H. Luetkens,
A. Amato,
H. -H. Klauss,
F. Winter,
R. Pöttgen,
D. Johrendt
Abstract:
We report the structural and magnetic phase transitions of triclinic Ca10(FeAs)10(Pt3As8). High-resolution X-ray powder diffraction reveals splitting of the in-plane (a,b) lattice parameters at Ts ~ 120 K. Platinum-doping weakens the distortion and shifts the transition temperature to 80 K in Ca10(Fe1-xPt_xAs)10(Pt3As8) with x = 0.03. muSR experiments show the onset of magnetic order near Ts and a…
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We report the structural and magnetic phase transitions of triclinic Ca10(FeAs)10(Pt3As8). High-resolution X-ray powder diffraction reveals splitting of the in-plane (a,b) lattice parameters at Ts ~ 120 K. Platinum-doping weakens the distortion and shifts the transition temperature to 80 K in Ca10(Fe1-xPt_xAs)10(Pt3As8) with x = 0.03. muSR experiments show the onset of magnetic order near Ts and a broad magnetic phase transition. No symmetry breaking is associated to the structural transition in Ca10(FeAs)10(Pt3As8) in contrast to the other parent compounds of iron arsenide superconductors.
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Submitted 21 December, 2012;
originally announced December 2012.
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119Sn solid state NMR and Mössbauer spectroscopic studies of the intermediate-valent stannide CeRuSn
Authors:
F. M. Schappacher,
P. Khuntia,
A. K. Rajarajan,
M. Baenitz,
J. A. Mydosh,
B. Chevalier,
S. F. Matar,
R. Pöttgen
Abstract:
The ternary stannide CeRuSn is a static mixed-valent cerium compound with an or-dering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4.2 K) two tin sites can clearly be distinguished. 119Sn solid state NMR experiments are performed to probe t…
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The ternary stannide CeRuSn is a static mixed-valent cerium compound with an or-dering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4.2 K) two tin sites can clearly be distinguished. 119Sn solid state NMR experiments are performed to probe the local hyperfine fields at the two different Sn sites. 119Sn NMR powder spectra are nicely fitted with two Sn sites with nearly the same magnetic anisotropy, but with different absolute shift values. Both Sn sites are strongly affected by crossover-like transitions between 100 and 280 K. This local-site study confirms the superstructure modulations found in previous investiga-tions. Towards lower temperatures the powder spectra are broadened giving strong evidence for the antiferromagnetically ordered ground state.
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Submitted 15 June, 2012;
originally announced June 2012.
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Complex charge ordering in CeRuSn
Authors:
R. Feyerherm,
E. Dudzik,
S. Valencia,
J. A. Mydosh,
Y. -K. Huang,
W. Hermes,
R. Pöttgen
Abstract:
At ambient temperatures, CeRuSn exhibits an extraordinary structure with a coexistence of two types of Ce ions in a metallic environment, namely trivalent Ce3+ and intermediate valent Ce(4-x)+. Charge ordering produces a doubling of the unit cell along the c-axis with respect to the basic monoclinic CeCoAl type structure. Below room temperature, a phase transition with very broad hysteresis has be…
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At ambient temperatures, CeRuSn exhibits an extraordinary structure with a coexistence of two types of Ce ions in a metallic environment, namely trivalent Ce3+ and intermediate valent Ce(4-x)+. Charge ordering produces a doubling of the unit cell along the c-axis with respect to the basic monoclinic CeCoAl type structure. Below room temperature, a phase transition with very broad hysteresis has been observed in various bulk properties like electrical resistivity, magnetic susceptibility, and specific heat. The present x-ray diffraction results show that at low temperatures the doubling of the CeCoAl type structure is replaced by an ill-defined modulated ground state. In this state, at least three different modulation periods compete, with the dominant mode close to a tripling of the basic cell. The transition is accompanied by a significant contraction of the c axis. XANES data suggest that the average Ce valence remains constant, thus the observed c axis contraction is not due to any valence transition. We propose a qualitative structure model with modified stacking sequences of Ce3+ and Ce(4-x)+ layers in the various modulated phases. Surprisingly, far below 100 K the modulated state is sensitive to x-ray irradiation at photon fluxes available at a synchrotron. With photon fluxes of order 10E12/s, the modulated ground state can be destroyed on a timescale of minutes and the doubling of the CeCoAl cell observed at room temperature is recovered. The final state is metastable at 10 K. Heating the sample above 60 K again leads to a recovery of the modulated state. Thus, CeRuSn exhibits both thermally and x-ray induced reversible transformations of the Ce3+/Ce(4-x)+ charge ordering pattern. Such a behavior is unique among any know intermetallic compound.
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Submitted 24 November, 2011;
originally announced November 2011.
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Ternary Antmonides RE2Ir3Sb4 (RE = La, Ce, Pr, Nd)
Authors:
Konrad Schäfer,
Wilfried Hermes,
Ute Ch. Rodewald,
Rolf-Dieter Hoffmann,
Rainer Pöttgen
Abstract:
The antimonides RE2Ir3Sb4 (RE = La, Ce, Pr, Nd) were synthesized by arc-melting of the elements and subsequent annealing or via high-frequence melting. The samples were characterized by X-ray powder diffraction and the four structures were refined from sin-gle-crystal X-ray diffraction data: Pr2Ir3Sb4-type, Pnma, Z = 4, a = 1621.9(2), b = 458.60(8), c = 1099.8(1) pm, wR2 = 0.036, 1558 F2 values fo…
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The antimonides RE2Ir3Sb4 (RE = La, Ce, Pr, Nd) were synthesized by arc-melting of the elements and subsequent annealing or via high-frequence melting. The samples were characterized by X-ray powder diffraction and the four structures were refined from sin-gle-crystal X-ray diffraction data: Pr2Ir3Sb4-type, Pnma, Z = 4, a = 1621.9(2), b = 458.60(8), c = 1099.8(1) pm, wR2 = 0.036, 1558 F2 values for La2Ir3Sb4, a = 1616.6(8), b = 456.5(2), c = 1094.8(5) pm, wR2 = 0.092, 1080 F2 values for Ce2Ir3Sb4, a = 1613.0(5), b = 454.9(2), c = 1094.1(5) pm, wR2 = 0.057, 1428 F2 values for Pr2Ir3Sb4, and a = 1609.8(6), b = 452.9(2), c = 1092.3(5) pm, wR2 = 0.052, 1472 F2 values for Nd2Ir3Sb4, with 56 parameters per refinement. The Sb1 atoms show enhanced displace-ment off the mirror planes at y = 1/4 and y = 3/4. A series of temperature dependent structure refinements of Pr2Ir3Sb4 down to 90 K are indicative of static disorder. The iridium and antimony atoms build up complex covalently bonded three-dimensional [Ir3Sb4] networks with Ir-Sb distances ranging from 256-269 pm (Nd2Ir3Sb4). The two crystallographically independent rare earth sites fill cavities of coordination numbers 17 (8 Ir + 9 Sb) and 15 (6 Ir + 9 Sb) within the [Ir3Sb4] polyanions. Temperature dependent magnetic susceptibility measurements indicate a stable trivalent ground state for the ce-rium compound. No magnetic ordering was evident down to 3 K.
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Submitted 17 June, 2011;
originally announced June 2011.
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Local moments and symmetry breaking in metallic PrMnSbO
Authors:
Simon A. J. Kimber,
Adrian H. Hill,
Yu-Zhong Zhang,
Harald O. Jeschke,
Roser Valentí,
Clemens Ritter,
Inga Schellenberg,
Wilfred Hermes,
Rainer Pöttgen,
Dimitri N. Argyriou
Abstract:
We report a combined experimental and theoretical investigation of the layered antimonide PrMnSbO which is isostructural to the parent phase of the iron pnictide superconductors. We find linear resistivity near room temperature and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K, followed by a spin…
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We report a combined experimental and theoretical investigation of the layered antimonide PrMnSbO which is isostructural to the parent phase of the iron pnictide superconductors. We find linear resistivity near room temperature and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K, followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a tetragonal to orthorhombic (T-O) transition. First principles calculations show that the large magnetic moments observed in this metallic compound are of local origin. Our results are thus inconsistent with either the itinerant or frustrated models proposed for symmetry breaking in the iron pnictides. We show that PrMnSbO is instead a rare example of a metal where structural distortions are driven by f-electron degrees of freedom.
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Submitted 2 June, 2010;
originally announced June 2010.
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Low Temperature Crystal Structure and 57Fe Moessbauer Spectroscopy of Sr3Sc2O5Fe2As2
Authors:
Marcus Tegel,
Inga Schellenberg,
Franziska Hummel,
Rainer Poettgen,
Dirk Johrendt
Abstract:
The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to delta(c)/c : delta(a)/a = 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Moessbauer spectroscopic d…
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The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to delta(c)/c : delta(a)/a = 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Moessbauer spectroscopic data show a single signal at 4.2, 77, and 298 K, respectively, subjected to quadrupole splitting. The isomer shift increases from 0.36(1) mm/s at 298 K to 0.49(1) mm/s at 4.2 K. No indication for magnetic ordering was found.
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Submitted 4 May, 2009;
originally announced May 2009.
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The layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs
Authors:
Marcus Tegel,
Franziska Hummel,
Sebastian Lackner,
Inga Schellenberg,
Rainer Poettgen,
Dirk Johrendt
Abstract:
Polycrystalline samples of the layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs were synthesized by high temperature solid state reactions and their crystal structures determined by the X-ray powder diffraction. Their structures are tetragonal (P4/nmm; Sr2CrO3FeAs: a = 391.12(1) pm, c = 1579.05(3) pm; Ba2ScO3FeAs: a = 412.66(5) pm, c = 1680.0(2) pm, Z = 2) and isotypic to Sr2ScO3CuS. Iron arse…
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Polycrystalline samples of the layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs were synthesized by high temperature solid state reactions and their crystal structures determined by the X-ray powder diffraction. Their structures are tetragonal (P4/nmm; Sr2CrO3FeAs: a = 391.12(1) pm, c = 1579.05(3) pm; Ba2ScO3FeAs: a = 412.66(5) pm, c = 1680.0(2) pm, Z = 2) and isotypic to Sr2ScO3CuS. Iron arsenide layers are sandwiched between perowskite-like oxide blocks and separated by ~1600 pm, which is much larger compared to the 1111 iron arsenide superconductors. The bond length and angles within the FeAs layers are adapted to the space requirements of the oxide blocks. Measurements of the magnetic susceptibility and electrical resistivity show no hint for a SDW-like anomaly in both compounds. Sr2CrO3FeAs shows Curie-Weiss paramagnetism above 160 K with an effective magnetic moment of 3.83(3) muB in good agreement with the theoretical value of 3.87 muB for Cr3+. Antiferromagnetic ordering was detected below TN ~ 31 K. 57Fe Moessbauer spectra of Sr2CrO3FeAs show a single signal that broadens below the magnetic ordering temperature due to a small transferred hyperfine field induced by the magnetic ordering of the chromium atoms. 57Fe-Moessbauer spectra of Ba2ScO3FeAs show single signals at 298, 77, and 4.2 K which are only subject to weak quadrupole splitting.
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Submitted 6 April, 2009; v1 submitted 2 April, 2009;
originally announced April 2009.
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Superconductivity, magnetism and crystal chemistry of Ba1-xKxFe2As2
Authors:
Dirk Johrendt,
Rainer Poettgen
Abstract:
BaFe2As2 is the parent compound of the '122' iron arsenide superconductors and crystallizes with the tetragonal ThCr2Si2 type structure, space group I4/mmm. A spin density wave transition at 140 K is accompanied by a symmetry reduction to space group Fmmm and simultaneously by antiferromagnetic ordering. Hole-doping induces superconductivity in Ba1-xKxFe2As2 with a maximum Tc of 38 K at x = 0.4.…
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BaFe2As2 is the parent compound of the '122' iron arsenide superconductors and crystallizes with the tetragonal ThCr2Si2 type structure, space group I4/mmm. A spin density wave transition at 140 K is accompanied by a symmetry reduction to space group Fmmm and simultaneously by antiferromagnetic ordering. Hole-doping induces superconductivity in Ba1-xKxFe2As2 with a maximum Tc of 38 K at x = 0.4. The upper critical fields approach 75 T with rather small anisotropy of Hc2. At low potassium concentrations (x <= 0.2), superconductivity apparently co-exists with the orthorhombic distorted and magnetically ordered phase. At doping levels x >= 0.3, the structural distortion and antiferromagnetic ordering is completely suppressed and the Tc is maximized. No magnetically ordered domains could be detected in optimally doped Ba1-xKxFe2As2 (x >= 0.3) by 57Fe-Moessbauer spectroscopy in contrast muSR results obtained with single crystals. The magnetic hyperfine interactions investigated by 57Fe Moessbauer spectroscopy are discussed and compared to the ZrCuSiAs-type materials.
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Submitted 6 February, 2009;
originally announced February 2009.
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Competition of magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2
Authors:
Marianne Rotter,
Marcus Tegel,
Inga Schellenberg,
Falko M. Schappacher,
Rainer Poettgen,
Joachim Deisenhofer,
Axel Guenther,
Florian Schrettle,
Alois Loidl,
Dirk Johrendt
Abstract:
Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superco…
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Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad SDW phase transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2, where C changes by 0.1 J/K at Tc = 37.3 K. 57Fe-Moessbauer-spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic ordering at 4.2 K in samples with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF magnetic ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about single crystal (Ba1-xKx)Fe2As2.
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Submitted 15 December, 2008;
originally announced December 2008.
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Synthesis, crystal structure and spin-density-wave anomaly of the iron arsenide-fluoride SrFeAsF
Authors:
M. Tegel,
S. Johansson,
V. Weiss,
I. Schellenberg,
W. Hermes,
R. Poettgen,
Dirk Johrendt
Abstract:
The new quaternary iron arsenide-fluoride SrFeAsF with the tetragonal ZrCuSiAs-type structure was synthesized and the crystal structure was determined by X-ray powder diffraction (P4/nmm, a = 399.30(1), c = 895.46(1) pm). SrFeAsF undergoes a structural and magnetic phase transition at 175 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility.…
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The new quaternary iron arsenide-fluoride SrFeAsF with the tetragonal ZrCuSiAs-type structure was synthesized and the crystal structure was determined by X-ray powder diffraction (P4/nmm, a = 399.30(1), c = 895.46(1) pm). SrFeAsF undergoes a structural and magnetic phase transition at 175 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (P4/nmm) to orthorhombic (Cmme). 57Fe Moessbauer spectroscopy experiments show a single signal at room temperature at an isomer shift of 0.30(1) mm/s and magnetic hyperfine-field splitting below the phase transition temperature. Our results clearly show that SrFeAsF exhibits a spin density wave (SDW) anomaly at 175 K very similar to LaFeAsO, the parent compound of the iron arsenide-oxide superconductors and thus SrFeAsF may serve as a further parent compound for oxygen-free iron arsenide superconductors.
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Submitted 13 October, 2008;
originally announced October 2008.
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Magnetic ordering in the static intermediate-valent cerium compound Ce2RuZn4
Authors:
Volker Eyert,
Ernst-Wilhelm Scheidt,
Wolfgang Scherer,
Wilfried Hermes,
Rainer Poettgen
Abstract:
The low-temperature behavior of Ce2RuZn4 has been investigated. Specific heat and magnetic susceptibility data reveal an antiferromagnetic transition at a Neel temperature of 2 K. Ce2RuZn4 is a static intermediate-valent compound with two crystallographically independent cerium atoms. The magnetic data clearly show that only one cerium site is magnetic (Ce^3+), while the second one carries no ma…
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The low-temperature behavior of Ce2RuZn4 has been investigated. Specific heat and magnetic susceptibility data reveal an antiferromagnetic transition at a Neel temperature of 2 K. Ce2RuZn4 is a static intermediate-valent compound with two crystallographically independent cerium atoms. The magnetic data clearly show that only one cerium site is magnetic (Ce^3+), while the second one carries no magnetic moment. The experimental data are interpreted with the help of first principles electronic structure calculations using density functional theory and the augmented spherical wave method. The calculations reveal the occurrence of two different cerium sites, which are characterized by strongly localized magneticmoments and strong Ce-Ru bonding, respectively.
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Submitted 21 August, 2008;
originally announced August 2008.
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Materials with ZrCuSiAs Type Structure
Authors:
Rainer Pottgen,
Dirk Johrendt
Abstract:
The crystal chemistry and physical properties of materials with ZrCuSiAs type structure is reviewed.
The crystal chemistry and physical properties of materials with ZrCuSiAs type structure is reviewed.
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Submitted 14 July, 2008;
originally announced July 2008.
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Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2 and EuFe2As2
Authors:
Marcus Tegel,
Marianne Rotter,
Veronika Weiss,
Falko M. Schappacher,
Rainer Poettgen,
Dirk Johrendt
Abstract:
The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe2As2) or 190 K (EuFe2As2) to orthorhombic lattice sym…
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The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe2As2) or 190 K (EuFe2As2) to orthorhombic lattice symmetry in agreement with the group-subgroup relationship between I4/mmm and Fmmm. 57-Fe Moessbauer spectroscopy experiments with SrFe2 As2 show full hyperfine field splitting below the phase transition temperature (8.91(1) T at 4.2 K). Order parameters were extracted from detailed measurements of the lattice parameters and fitted to a simple power law. We find a relation between the critical exponents and the transition temperatures for AFe2As2 compounds, which shows that the transition of BaFe2As2 is indeed more continuous than the transition of SrFe2As2 but it remains second order even in the latter case.
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Submitted 1 September, 2008; v1 submitted 29 June, 2008;
originally announced June 2008.
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Spin density wave anomaly at 140 K in the ternary iron arsenide BaFe2As2
Authors:
Marianne Rotter,
Marcus Tegel,
Inga Schellenberg,
Wilfried Hermes,
Rainer Pöttgen,
Dirk Johrendt
Abstract:
The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resista…
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The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (I4/mmm) to orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single signal at room temperature and full hyperfine field splitting below the phase transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can serve as a new parent compound for oxygen-free iron arsenide superconductors.
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Submitted 12 June, 2008; v1 submitted 26 May, 2008;
originally announced May 2008.
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A 57Fe Moessbauer Spectroscopy Study of the 7 K Superconductor LaFePO
Authors:
Marcus Tegel,
Inga Schellenberg,
Rainer Pöttgen,
Dirk Johrendt
Abstract:
A polycrystalline sample of superconducting LaFePO was prepared in a tin flux at 1123 K. The structure was determined from single crystal data (ZrCuSiAs-type, P4/nmm, a = 3.9610(1), c = 8.5158(2) A, Z = 2) and the phase analysis was performed by the Rietveld method. LaFePO is Pauli-paramagnetic and becomes superconducting at 7 K after removing the ferromagnetic impurity phase Fe2P from the sampl…
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A polycrystalline sample of superconducting LaFePO was prepared in a tin flux at 1123 K. The structure was determined from single crystal data (ZrCuSiAs-type, P4/nmm, a = 3.9610(1), c = 8.5158(2) A, Z = 2) and the phase analysis was performed by the Rietveld method. LaFePO is Pauli-paramagnetic and becomes superconducting at 7 K after removing the ferromagnetic impurity phase Fe2P from the sample. 57Fe Moessbauer spectroscopy measurements at 298, 77, 4.2 and 4 K show single signals at isomer shifts around 0.35 mm/s, subject to weak quadrupole splitting. At 4 K, a symmetric line broadening appears, resulting from a small transferred magnetic hyperfine field of 1.15(1) T and accompanied by an angle of 54.7(5) between Bhf and Vzz, the main component of the electric field gradient tensor.
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Submitted 8 May, 2008;
originally announced May 2008.
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Structural and 121Sb Mössbauer Spectroscopic Investigations of the Antimonide Oxides REMnSbO (RE = La, Ce, Pr, Nd, Sm, Gd, Tb) and REZnSbO (RE = La, Ce, Pr)
Authors:
Inga Schellenberg,
Tom Nilges,
Rainer Pöttgen
Abstract:
The quaternary antimonide oxides REMnSbO (RE = La, Ce, Pr, Nd, Sm, Gd, Tb) and REZnSbO (RE = La, Ce, Pr) were synthesized from the RESb monoantimonides and MnO, respectively ZnO. We report on single crystal X-ray data and a detailed 121Sb Mössbauer spectroscopic investigation.
The quaternary antimonide oxides REMnSbO (RE = La, Ce, Pr, Nd, Sm, Gd, Tb) and REZnSbO (RE = La, Ce, Pr) were synthesized from the RESb monoantimonides and MnO, respectively ZnO. We report on single crystal X-ray data and a detailed 121Sb Mössbauer spectroscopic investigation.
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Submitted 11 April, 2008;
originally announced April 2008.
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Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds
Authors:
Frederick Casper,
Claudia Felser,
Ram Seshadri,
C. Peter Sebastian,
Rainer Poettgen
Abstract:
The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understand the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li$^+$[AlSi]$^-$) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti$^{4+}$[CoSb]…
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The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understand the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li$^+$[AlSi]$^-$) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti$^{4+}$[CoSb]$^{4-}$). The basis for this is that [AlSi]$^-$ (with the same electron count as Si$_2$) and [CoSb]$^{4-}$ (the same electron count as GaSb), are both structurally and electronically, zinc-blende semiconductors. The electronic structure of half-metallic ferromagnets in this structure type can then be described as semiconductors with stuffing magnetic ions which have a local moment: For example, 22 electron MnNiSb can be written Mn$^{3+}$[NiSb]$^{3-}$. The tendency in the 18 electron compound for a semiconducting gap -- believed to arise from strong covalency -- is carried over in MnNiSb to a tendency for a gap in one spin direction. Here we similarly propose the systematic examination of 18-electron hexagonal compounds for semiconducting gaps; these would be the "stuffed wurtzite" analogues of the "stuffed zinc blende" half-Heusler compounds. These semiconductors could then serve as the basis for possibly new families of half-metallic compounds, attained through appropriate replacement of non-magnetic ions by magnetic ones. These semiconductors and semimetals with tunable charge carrier concentrations could also be interesting in the context of magnetoresistive and thermoelectric materials.
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Submitted 30 October, 2007;
originally announced October 2007.
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Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)
Authors:
J. Rohrkamp,
O. Heyer,
T. Fickenscher,
R. Poettgen,
S. Jodlauk,
H. Hartmann,
T. Lorenz,
J. A. Mydosh
Abstract:
We report measurements of the thermal expansion for two Eu$^{+2}$- and two Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal expansion coefficient $α$ which become smeared and/or displaced in an external magnetic fie…
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We report measurements of the thermal expansion for two Eu$^{+2}$- and two Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal expansion coefficient $α$ which become smeared and/or displaced in an external magnetic field. Together with specific heat data we determine the initial pressure dependences of the transition temperatures at ambient pressure using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure dependences indicating strong spin-phonon coupling, in particular for GdAgMg and EuAuMg where a quantum phase transition might be reached at moderate pressures of a few GPa.
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Submitted 21 August, 2007;
originally announced August 2007.
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First principles investigations of the electronic, magnetic and chemical bonding properties of CeTSn (T=Rh,Ru)
Authors:
S. F. Matar,
J. F. Riecken,
B. Chevalier,
R. Poettgen,
V. Eyert
Abstract:
The electronic structures of CeRhSn and CeRuSn are self-consistently calculated within density functional theory using the local spin density approximation for exchange and correlation. In agreement with experimental findings, the analyses of the electronic structures and of the chemical bonding properties point to the absence of magnetization within the mixed valent Rh based system while a fini…
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The electronic structures of CeRhSn and CeRuSn are self-consistently calculated within density functional theory using the local spin density approximation for exchange and correlation. In agreement with experimental findings, the analyses of the electronic structures and of the chemical bonding properties point to the absence of magnetization within the mixed valent Rh based system while a finite magnetic moment is observed for trivalent cerium within the Ru-based stannide, which contains both trivalent and intermediate valent Ce.
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Submitted 1 August, 2007;
originally announced August 2007.
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Structure and properties of alpha- and beta- CeCuSn: A single-crystal and Mossbauer spectroscopic investigation
Authors:
C. Peter Sebastian,
Sudhindra Rayaprol,
Rolf-Dieter Hoffmann,
Ute Ch. Rodewald,
Tania Pape,
Rainer Pottgen
Abstract:
Two modifications of CeCuSn were prepared from the elements: the high-temperature (beta) modification crystallizes directly from the quenched sample, while the low-temperature (alpha) modification forms after annealing at 700 deg C for one month. Both modifications were investigated by X-ray powder and single crystal diffraction. We find for alpha-CeCuSn a structure of ZrBeSi type, space group P…
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Two modifications of CeCuSn were prepared from the elements: the high-temperature (beta) modification crystallizes directly from the quenched sample, while the low-temperature (alpha) modification forms after annealing at 700 deg C for one month. Both modifications were investigated by X-ray powder and single crystal diffraction. We find for alpha-CeCuSn a structure of ZrBeSi type, space group P63/mmc, a = 458.2(1), c = 793.7(2) pm, wR2 = 0.0727, 148 F2 values, 8 variable parameters. In the case of beta-CeCuSn we find the NdPtSb type structure, space group P63mc, a = 458.4(1), c = 785.8(2) pm, wR2 = 0.0764, 233 F2 values, 11 variable parameters. The copper and tin atoms build up layers of ordered [Cu3Sn3] hexagons. The layers are planar in beta-CeCuSn, however, with highly anisotropic displacements of the copper and tin atoms. In alpha-CeCuSn a puckering effect is observed resulting in a decrease of the c lattice parameter. Both modifications of CeCuSn exhibit antiferromagnetic ordering, however, there is a considerable difference in their magnetic behaviour. We show the anomalies in the physical properties of the alpha- and beta- modifications of CeCuSn by Mossbauer spectroscopy,magnetic and specific heat measurements and explain their structure-property relations.
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Submitted 9 December, 2006;
originally announced December 2006.
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Electronic structure of $RE$AuMg and $RE$AgMg ($RE$ = Eu, Gd, Yb)
Authors:
Jan Gegner,
T. C. Koethe,
Hua Wu,
H. Hartmann,
T. Lorenz,
T. Fickenscher,
R. Pöttgen,
L. H. Tjeng
Abstract:
We have investigated the electronic structure of the equiatomic EuAuMg, GdAuMg, YbAuMg and GdAgMg intermetallics using x-ray photoelectron spectroscopy. The spectra revealed that the Yb and Eu are divalent while the Gd is trivalent. The spectral weight in the vicinity of the Fermi level is dominated by the mix of Mg $s$, Au/Ag $sp$ and $RE$ $spd$ bands, and not by the $RE$ $4f$. We also found th…
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We have investigated the electronic structure of the equiatomic EuAuMg, GdAuMg, YbAuMg and GdAgMg intermetallics using x-ray photoelectron spectroscopy. The spectra revealed that the Yb and Eu are divalent while the Gd is trivalent. The spectral weight in the vicinity of the Fermi level is dominated by the mix of Mg $s$, Au/Ag $sp$ and $RE$ $spd$ bands, and not by the $RE$ $4f$. We also found that the Au and Ag $d$ bands are extraordinarily narrow, as if the noble metal atoms were impurities submerged in a low density $sp$ metal host. The experimental results were compared with band structure calculations, and we found good agreement provided that the spin-orbit interaction in the Au an Ag $d$ bands is included and correlation effects in an open $4f$ shell are accounted for using the local density approximation + Hubbard $U$ scheme. Nevertheless, limitations of such a mean-field scheme to explain excitation spectra are also evident.
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Submitted 21 November, 2005;
originally announced November 2005.
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Antiferromagnetic ordering in heavy fermion system Ce2Au2Cd
Authors:
S. Rayaprol,
R. Poettgen
Abstract:
La2Au2Cd and Ce2Au2Cd were prepared from the elements by reactions in sealed tantalum tubes in a water-cooled sample chamber of an induction furnace. These intermetallics crystallize with the tetragonal Mo2FeB2 type, space group P4/mbm. While La2Au2Cd is Pauli paramagnetic, Ce2Au2Cd shows Curie-Weiss behaviour above 100 K with an experimental magnetic moment of 2.41(2) muB/Ce atom, indicating tr…
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La2Au2Cd and Ce2Au2Cd were prepared from the elements by reactions in sealed tantalum tubes in a water-cooled sample chamber of an induction furnace. These intermetallics crystallize with the tetragonal Mo2FeB2 type, space group P4/mbm. While La2Au2Cd is Pauli paramagnetic, Ce2Au2Cd shows Curie-Weiss behaviour above 100 K with an experimental magnetic moment of 2.41(2) muB/Ce atom, indicating trivalent cerium. Antiferromagnetic ordering is detected for Ce2Au2Cd at 5.01(2) K and magnetization measurements reveal a metamagnetic transition at 3 K at a critical field of around 20 kOe with a saturation moment of 1.50(2)muB/Ce atom at 80 kOe. The low-temperature heat capacity properties characterize Ce2Au2Cd as a heavy fermion material with an electronic specific heat coefficient (gamma) = 807(5) mJ/mol K2 as compared to La2Au2Cd with gamma = 6(5) mJ/mol K2.
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Submitted 16 November, 2005; v1 submitted 27 September, 2005;
originally announced September 2005.
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Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds
Authors:
H. Hartmann,
K. Berggold,
S. Jodlauk,
I. Klassen,
K. Kordonis,
T. Fickenscher,
R. Poettgen,
A. Freimuth,
T. Lorenz
Abstract:
We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic $RET$Mg intermetallics with $RE = {\rm La}$, Eu, Gd, Yb and $T = {\rm Ag}$, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic ($RE = {\rm La}$, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K.…
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We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic $RET$Mg intermetallics with $RE = {\rm La}$, Eu, Gd, Yb and $T = {\rm Ag}$, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic ($RE = {\rm La}$, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K. All of them are metallic, but the resistivity varies over 3 orders of magnitude. The magnetic order causes a strong decrease of the resistivity and around the ordering temperature we find pronounced magnetoresistance effects. The magnetic ordering also leads to well-defined anomalies in the specific heat. An analysis of the entropy change leads to the conclusions that generally the magnetic transition can be described by an ordering of localized $S=7/2$ moments arising from the half-filled $4f^7$ shells of Eu$^{2+}$ or Gd$^{3+}$. However, for GdAgMg we find clear evidence for two phase transitions indicating that the magnetic ordering sets in partially below about 125 K and is completed via an almost first-order transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.
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Submitted 22 August, 2005;
originally announced August 2005.