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ABSTRACT: The compounds Ca(Fe1-xNix)2As2 with the tetragonal ThCr2Si2-type structure
(space group I4/mmm) show a continuous transition of the interlayer As-As
distances from a non-bonding state in CaFe2As2 (dAs-As = 313 pm) to
single-bonded As2-dimers in CaNi2As2 (dAs-As = 260 pm). Magnetic measurements
reveal weak ferromagnetism which develops near the composition
Ca(Fe0.5Ni0.5)2As2, while the compounds with lower and higher nickel
concentrations both are Pauli-paramagnetic. DFT band structure calculations
reveal that the As2-dimer formation is a consequence of weaker metal-metal in
MAs4-layers (M = Fe1-xNix) of Ni-richer compounds, and depends not on
depopulation or shift of As-As antibonding states as suggested earlier. Our
results also indicate that the ferromagnetism of Ca(Fe0.5Ni0.5)2As2 and related
compounds like SrCo2(Ge0.5P0.5)2 is probably not induced by dimer breaking as
recently suggested, but arises from the high density of states generated by the
transition metal 3d bands near the Fermi level without contribution of the
dimers.
02/2013;
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ABSTRACT: Polycrystalline samples of the phosphide oxides RE(2)AuP(2)O (RE = La, Ce, Pr, Nd) were obtained from mixtures of the rare earth elements, binary rare earth oxides, gold powder, and red phosphorus in sealed silica tubes. Small single crystals were grown in NaCl/KCl fluxes. The samples were studied by powder X-ray diffraction, and the structures were refined from single crystal diffractometer data: La(2)AuP(2)O type, space group C2/m, a = 1515.2(4), b = 424.63(8), c = 999.2(2) pm, β = 130.90(2)°, wR2 = 0.0410, 1050 F(2) values for Ce(2)AuP(2)O, and a = 1503.6(4), b = 422.77(8), c = 993.0(2) pm, β = 130.88(2)°, wR2 = 0.0401, 1037 F(2) values for Pr(2)AuP(2)O, and a = 1501.87(5), b = 420.85(5), c = 990.3(3) pm, β = 131.12(1)°, wR2 = 0.0944, 1143 F(2) values for Nd(2)AuP(2)O with 38 variables per refinement. The structures are composed of [RE(2)O](4+) polycationic chains of cis-edge-sharing ORE(4/2) tetrahedra and polyanionic strands [AuP(2)](4-), which contain gold in almost trigonal-planar phosphorus coordination by P(3-) and P(2)(4-) entities. The isolated phosphorus atoms and the P(2) pairs in La(2)AuP(2)O could clearly be distinguished by (31)P solid state NMR spectroscopy and assigned on the basis of a double quantum NMR technique. Also, the two crystallographically inequivalent La sites could be distinguished by static (139)La NMR in conjunction with theoretical electric field gradient calculations. Temperature-dependent magnetic susceptibility measurements show diamagnetic behavior for La(2)AuP(2)O. Ce(2)AuP(2)O and Pr(2)AuP(2)O are Curie-Weiss paramagnets with experimental magnetic moments of 2.35 and 3.48 μ(B) per rare earth atom, respectively. Their solid state (31)P MAS NMR spectra are strongly influenced by paramagnetic interactions. Ce(2)AuP(2)O orders antiferromagnetically at 13.1(5) K and shows a metamagnetic transition at 11.5 kOe. Pr(2)AuP(2)O orders ferromagnetically at 7.0 K.
Inorganic Chemistry 02/2013; · 4.60 Impact Factor
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ABSTRACT: New nitridosilicates Ca(3)Sm(3)[Si(9)N(17)] and Ca(3)Yb(3)[Si(9)N(17)] were synthesized from the reactions of the pure metals (calcium and samarium/ytterbium) with silicon diimide "Si(NH)(2) " in a radio-frequency (rf) furnace at temperatures of up to 1650 °C. These isotypic compounds crystallize in the cubic space group P4(-)3m (no. 215) with lattice parameters a=739.50(3) pm; V=0.4044(1) nm(3); Z=1; wR(2) =0.029 (240 diffraction data, 26 parameters) for Ca(3)Sm(3)[Si(9)N(17)] and a=730.20(2) pm; V=0.3893(1) nm(3); wR(2) =0.039 (387 diffraction data, 27 parameters) for Ca(3)Yb(3)[Si(9)N(17)]. The new structure type of Ca(3)RE(3)[Si(9)N(17)] (RE=Sm, Yb) consists of two independent infinite networks, each of which have an expanded sphalerite (ZnS) topology in which the positions of the Zn and S atoms are replaced by voluminous [N([4])(SiN(3))(4)] units and [Si(5)N(16)] supertetrahedra, respectively, thereby displaying twofold interpenetration. As well, a structural description of Ca(3)Yb(3)[Si(9)N(17)], its thermal stability, and magnetic properties, as well as UV/Vis, IR, and Raman spectra, are presented.
Chemistry 07/2012; 18(35):10857-64. · 5.93 Impact Factor
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ABSTRACT: The topochemical deintercalation of Na(+) ions from solid NaFeAs at room temperature in THF with iodine yields the superconducting phase Na(1-y)Fe(2-x)As(2) (T(c) ≈ 11 K). This metastable iron arsenide decomposes at 120 °C and is not accessible by high-temperature solid-state synthesis. X-ray powder diffraction confirms the ThCr(2)Si(2)-type structure, but reveals very small coherently scattering domains with a mean composition Na(0.9(2))Fe(1.7(1))As(2). HRTEM investigations show crystalline as well as strongly distorted areas with planar defects. The latter are probably due to sodium loss and disorder which is also detected by (23)Na solid state NMR. The (57)Fe-Mössbauer spectrum of Na(1-y)Fe(2-x)As(2) shows one type of iron atoms in tetrahedral coordination. All results point to one crystallographic phase with very small domains due to fluctuations of the chemical composition. From electronic reasons we suggest the superconducting phase is presumably NaFe(2)As(2) in the ordered fractions of the sample.
Inorganic Chemistry 07/2012; 51(15):8161-7. · 4.60 Impact Factor
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ABSTRACT: The recently discovered compounds Ca10(Fe1-xPtxAs)10(Pt3+yAs8) exhibit
superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum
arsenide (Pt3+yAs8) layers separated by layers of Ca atoms. We show that high
Tc's above 15 K only emerge if the iron-arsenide layers are at most free of
platinum-substitution (x \rightarrow 0) in contrast to recent reports. In fact
Pt-substitution is detrimental to higher Tc, which increases up to 38 K only by
charge doping of pure FeAs layers. We point out, that two different negatively
charged layers [(FeAs)10]n- and (Pt3+yAs8)m- compete for the electrons provided
by the Ca2+ ions, which is unique in the field of iron-based superconductors.
In the parent compound Ca10(FeAs)10(Pt3As8), no excess charge dopes the
FeAs-layer, and superconductivity has to be induced by Pt-substitution, albeit
below 15 K. In contrast, the additional Pt-atom in the Pt4As8layer shifts the
charge balance between the layers equivalent to charge doping by 0.2 electrons
per FeAs. Only in this case Tc raises to 38 K, but decreases again if
additionally platinum is substituted for iron. This charge doping scenario is
supported by our discovery of superconductivity at 30 K in the electron-doped
La-1038 compound (Ca0.8La0.2)10(FeAs)10(Pt3As8) without significant
Pt-substitution.
03/2012;
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ABSTRACT: The effects of internal chemical pressure on electron doped iron arsenide
superconductors are studied in the series Ba(Fe(1-y)Coy)2(As(1-x)Px)2.
Combinations of both dopants induce superconductivity also in such areas where
only one would not suffice, and can likewise move the system into an overdoped
state, while no higher critical temperature than 31 K in BaFe2(As(1-x)Px)2 was
found. The phase diagram gives no evidence of holes in BaFe2(As(1-x)Px)2 as
suggested by recent photoemission experiments. Chemical and physical pressure
act similarly in Ba(Fe(1-y)Coy)2(As(1-x)Px)2, but our data reveal that the most
important control parameter is the length of the Fe-As bond and not the unit
cell volume. This emphasizes that differences between chemical and physical
pressure which manifest oneself as the non-linear reduction of the Fe-As
distance in BaFe2(As(1-x)Px)2 are strongly linked to the superconducting
properties also in the Co doped compounds.
03/2012;
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ABSTRACT: It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba(1-x)K(x)Fe(2)As(2) exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba(1-x)K(x)Fe(2)As(2) showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.
Physical Review Letters 12/2011; 107(23):237001. · 7.37 Impact Factor
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Angewandte Chemie International Edition 09/2011; 50(39):9195-9. · 13.45 Impact Factor
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ABSTRACT: It is widely believed that, in contrast to its electron doped counterparts,
the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of
magnetism and superconductivity in the underdoped region of the phase diagram.
Here, we report a combined high-resolution x-ray powder diffraction and volume
sensitive muon spin rotation study of underdoped Ba1-xKxFe2As2 (0 \leq x \leq
0.25) showing that this paradigm is wrong. Instead we find a microscopic
coexistence of the two forms of order. A competition of magnetism and
superconductivity is evident from a significant reduction of the magnetic
moment and a concomitant decrease of the magneto-elastically coupled
orthorhombic lattice distortion below the superconducting phase transition.
08/2011;
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ABSTRACT: We report the synthesis and crystal structures of three new superconducting
iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 (x = 0-0.15, y = 0-0.4). The
structures are stacking variants of FeAs- and slightly puckered Pt4-yAs8-layers
with square coordinated platinum separated by calcium-layers, respectively.
Arsenic atoms in the Pt4-yAs8-layers form (As2)4- dumbbells according to
Zintl's concept, providing charge balance in (Ca2+Fe2+As3-)10(Pt2+)3[(As2)4-]4.
Superconductivity was observed at 13-35 K. We suggest that the highest Tc above
30 K occurs in the 1048 phase with clean FeAs-layers that are indirectly
electron-doped according to (Ca2+Fe2+As3-)10(Pt2+)4[(As2)4-]4*2e-. We also
suggest that the lower critical temperatures occur in the 1038- and a-1048
phases due to Pt-doping at the Fe-site. DFT band structure calculations
indicate that the contribution of the Pt4-yAs8-layers to the Fermi surface is
small and that the Fermi energy is slightly either below or above a quasi-gap
in the Pt-states. The new platinum-iron compounds represent the first
iron-based superconductors with so far unknown structure types and can serve as
a new platform for further studies that go beyond the known systems.
07/2011;
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Angewandte Chemie International Edition 06/2011; 50(34):7919-23. · 13.45 Impact Factor
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ABSTRACT: The phosphide La4Rh8P9 was synthesized from the elements in a bismuth flux. The structure was refined from single crystal diffractometer data: space group Cmcm, a = 1303.1(2), b = 1893.2(2), c = 576.70(6) pm, wR2 = 0.0277, 1380 F2 values, 65 variables. The rhodium and phosphorus atoms build up a three-dimensional [Rh8P9] polyanion which leaves larger cages for the three crystallographically independent lanthanum sites. The rhodium atoms have between four and six phosphorus neighbors at Rh−P distance ranging from 229 to 254 pm. Three of the four crystallographically independent phosphorus atoms are isolated (P3− units), while the P4 atoms form dimers with double bond character (208 pm P−P). The P22− diphosphenide units bond side-on to a Rh3 and end-on to four Rh5 atoms. 31P magic angle spinning (MAS) NMR spectroscopy is able to resolve three of the four crystallographically distinct phosphorus sites. The doubly bonded phosphorus site P4 is characterized by an axially symmetric shielding tensor of moderate anisotropy Δσ = σ33 − σiso = 257 ppm. Electronic band structure calculations prove the metallic character and reveal the significant difference between the isolated P3− and the phosphorus atoms of the P22− units. Magnetic susceptibility measurement reveals Pauli paramagnetism.
02/2011;
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ABSTRACT: The phosphide La(4)Rh(8)P(9) was synthesized from the elements in a bismuth flux. The structure was refined from single crystal diffractometer data: space group Cmcm, a = 1303.1(2), b = 1893.2(2), c = 576.70(6) pm, wR2 = 0.0277, 1380 F(2) values, 65 variables. The rhodium and phosphorus atoms build up a three-dimensional [Rh(8)P(9)] polyanion which leaves larger cages for the three crystallographically independent lanthanum sites. The rhodium atoms have between four and six phosphorus neighbors at Rh-P distance ranging from 229 to 254 pm. Three of the four crystallographically independent phosphorus atoms are isolated (P(3-) units), while the P4 atoms form dimers with double bond character (208 pm P-P). The P(2)(2-) diphosphenide units bond side-on to a Rh3 and end-on to four Rh5 atoms. (31)P magic angle spinning (MAS) NMR spectroscopy is able to resolve three of the four crystallographically distinct phosphorus sites. The doubly bonded phosphorus site P4 is characterized by an axially symmetric shielding tensor of moderate anisotropy Δσ = σ(33) - σ(iso) = 257 ppm. Electronic band structure calculations prove the metallic character and reveal the significant difference between the isolated P(3-) and the phosphorus atoms of the P(2)(2-) units. Magnetic susceptibility measurement reveals Pauli paramagnetism.
Inorganic Chemistry 02/2011; 50(7):3044-51. · 4.60 Impact Factor
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ABSTRACT: Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries are determined by combined neutron- and x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V mixing, which interferes with or even suppresses superconductivity. This inhomogeneity may be intrinsic and explains scattered reports regarding Tc and reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data collected at 4 K indicates incommensurate mag- netic ordering of the V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly correlated vanadium, which does not contribute significantly to the Fermi surface of Sr2VO3FeAs. Comment: text revised, magnetic q-vector added, one reference added 4 pages, 4 figures
Physical Review B 08/2010; · 3.69 Impact Factor
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ABSTRACT: Superconductivity up to 30 K in charge neutrally doped BaFe2[As(1-x)P(x)]2 has been ascribed to chemical pressure, caused by the shrinking unit cell. But the latter induces no superconductivity in [Ba(1-x)Sr(x)]Fe2As2 in spite of the same volume range. We show that the spin-density-wave (SDW) state of BaFe2As2 becomes suppressed in BaFe2[As(1-x)P(x)]2 by a subtle reorganization of the crystal structure, where arsenic and phosphorus are located at different coordinates z(As), z(P). High-resolution X-ray diffraction experiments with BaFe2[As(1-x)P(x)]2 single crystals reveal almost unchanged Fe-P bonds, but a contraction of the Fe-As bonds, which remain nearly unchanged in [Ba(1-x)Sr(x)]Fe2As2. Since the Fe-As bond length is a gauge for the magnetic moment, our results show why the SDW is suppressed by P-, but not by Sr-doping. Only the Fe-P interaction increases the width of the iron 3d bands, which destabilizes the magnetic SDW ground state. The simultaneous contraction of the Fe{As bonds is rather a consequence of the vanishing magnetism. Ordered structure models of BaFe2[As(1-x)P(x)]2 obtained by DFT calculations agree perfectly with the single-crystal X-ray structure determinations. The contraction of the Fe-As bonds saturates at doping levels above x = 0.3, which corrects the unreasonable linear decrease of the so-called pnictide height. Comment: 7 Pages, 8 Figures, citations added, text revised
05/2010;
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ABSTRACT: The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type, P4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022, wR2=0.041, 173 F^2). Significantly different z-coordinates of tellurium and selenium at the 2c site are clearly discernible and were refined to z_Te=0.2868(3) and z_Se=0.2468(7). Thus the chalcogen heights differ by 0.24 A and the Fe-Se bonds are by 0.154 A shorter than the Fe-Te bonds, while three independent (Te,Se)-Fe-(Te,Se) bond angles occur. An elevated U33 displacement parameter of the iron atom is suggestive of a slightly puckered Fe layer resulting from different combinations of Se or Te neighbors. Such strong disorder underlines the robustness of superconductivity against structural randomness and has not yet been considered in theoretical studies of this system. Comment: 9 pages, 2 figures, 3 tables
12/2009;
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ABSTRACT: The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range antiferromagnetic ordering below T_N = 36 K with a magnetic propagation vector of q = (1/2, 1/2, 0). The Cr3+ ions form a collinear magnetic structure of the C-type in the magnetic space group C_Pmma' (a' = a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g (0, 1/4, z) Wyckoff position. The magnetic moments are aligned along the orthorhombic a'-axis. At 3.5 K, an ordered magnetic moment of 2.75+/-0.05 mu_B for the Cr3+-sublattice was refined. Comment: 6 pages, 6 figures
EPL (Europhysics Letters) 11/2009; · 2.17 Impact Factor
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ABSTRACT: A new structure type of nitridosilicates with an interrupted framework has been identified for M(7)Si(6)N(15) with M = La, Ce, and Pr. The materials have been synthesized in a radio-frequency furnace at temperatures between 1550-1625 degrees C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce(7)Si(6)N(15) has been determined by using single-crystal X-ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P1, no. 2; a = 13.009(3), b = 25.483(5), c = 25.508(10) A; alpha = 117.35(3), beta = 99.59(3), gamma = 99.63(3) degrees; V = 7114(2) A(3); Z = 18; R1 = 0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R3, no. 148) have also been observed (a = 43.420(6), c = 6.506(2) A; V = 10 623(3) A(3); Z = 27; R1 = 0.0309). Pr(7)Si(6)N(15) (3) and La(7)Si(6)N(15) (4) are isostructural with 1 as evidenced by twinned single-crystal data for 3 (P1, no. 2; a = 12.966(3), b = 25.449(10), c = 25.459(10) A; alpha = 117.28(3), beta = 99.70(4), gamma = 99.60(4) degrees; V = 7068(4) A(3); Z = 18; R1 = 0.0526) and powder diffraction data for 4 (P1, no. 2; a = 13.109(9), b = 25.606(18), c = 25.609(18) A; V = 7223(12) A(3); Z = 18; R(P) = 0.0194; R(F)(2) = 0.0936). The crystal structure of M(7)Si(6)N(15) (M = La, Ce, Pr) is built up exclusively of corner-sharing tetrahedrons that appear as Q(2)-, Q(3)-, and Q(4)-type tetrahedrons forming different ring sizes within a less condensed three-dimensional network. Among the characteristic structural motifs are saw-blade-shaped 12-rings and finite chains consisting of four corner-sharing SiN(4) tetrahedrons. High-resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce(7)Si(6)N(15) show paramagnetic behavior with an experimental magnetic moment of 2.29 mu(B) per Ce, thereby corroborating the existence of Ce(3+).
Chemistry 08/2009; 15(36):9215 - 9222. · 5.93 Impact Factor
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ABSTRACT: The mixed valence europium nitridosilicate Eu(2)SiN(3) has been synthesized at 900 degrees C in welded tantalum ampules starting from europium and silicon diimide Si(NH)(2) in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a = 542.3(11) pm, b = 1061.0(2) pm, c = 1162.9(2) pm, Z = 8, 767 independent reflections, 37 parameters, R1 = 0.017, wR2 = 0.032). Eu(2)SiN(3) is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN(4) tetrahedra running parallel [100] with a maximum stretching factor f(s) = 1.0. The compound is isostructural with Ca(2)PN(3) and Rb(2)TiO(3), and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu(2+) and Eu(3+), respectively. (151)Eu Mössbauer spectroscopy of Eu(2)SiN(3) differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu(2+) and Eu(3+) signals over the whole temperature range. The Eu(2+) site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 mu(B)/f.u. and a small contribution of Eu(3+), in accordance with Eu(2+) and Eu(3+) in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T(C) = 24 K. DFT calculations of Eu(2)SiN(3) reveal a band gap of approximately 0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu(2+) and Eu(3+).
Journal of the American Chemical Society 08/2009; 131(31):11242-8. · 9.91 Impact Factor
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ABSTRACT: The solid solution Ga1-xGexV4S8 (x = 0 - 1) was synthesized by solid state reactions and characterized by temperature-dependent x-ray powder diffraction and static magnetic susceptibility measurements. The compounds crystallize in the cubic GaMo4S8-type structure (space group F-43m), built up by heterocubane-like [V4S4](5-x)+ cubes and [Ga1-xGexS4](5-x)- tetrahedra arranged in a NaCl-like manner. The successive substitution of Ga3+ by Ge4+ increases the electron count in the molecular orbital (MO) of the V4-cluster gradually from seven to eight. We observe an almost linear increase of the magnetic moments, connected with a transition from ferromagnetic to antiferromagnetic ordering around x <= 0.5. Remarkably, the low temperature structural phase transitions as known from the ternary compounds were also detected in the solid solution. The gallium-rich compounds (0 < x < 0.5) undergo rhombohedral distortions like GaV4S8 (space group R3m), whereas distortions to orthorhombic symmetry (space group Imm2) as known from GeV4S8 occur in the germanium-rich part of the solid solution (0.5 <= x < 1). Comment: 14 pages, 9 figures
06/2009;
