Les antimonites antiferromagnetiques MnSb2O4 et NiSb2O4

Neutron diffraction has been used to study the variation of antiferromagnetic order in the antimony isomorphous MnSb₂O₄ (T_N ～ 60 K) and NiSb₂O₄ (T_N ～ 46 K). The magnetic moments have been related to the Mn²⁺ and Ni²⁺ spins and magnetostrictive effects have been interpreted. The influence of the method of synthesis is mentioned: polycrystalline MnSb₂O₄ has been obtained from hydrothermal synthesis. Orthorhombic distortions are not connected with magnetic interactions but with structural defects.     

Selective substitution of vanadium for molybdenum in Sr2(Fe1−xVx)MoO6 double perovskites

The crystal and magnetic structures of Sr₂(Fe_1−xV_x)MoO₆ (0.03?x?0.1) compounds are refined by alternately using X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) data collected at room temperature. The refinement results reveal that the V atoms selectively occupy the Mo sites instead of the Fe sites for x?0.1. The 3d/4d cation ordering decreases with the increase of the V content. Slight distortions in the lattice and metal octahedra are shown at 300 K, and the distortions increase at 4 K. The magnetic structure at 4 K can be modeled equally well with the moments aligning along [001], [110] or [111] directions. The total moments derived from the NPD data for the [110] and [111] direction models agree well with the magnetic measurements, whereas the [001] model leads to a smaller total moment. Bond valence analysis indicates that Sr ions are properly located in the structure and Mo ions are compatible with both the Fe sites and the Mo sites. The electronic effects are suggested to be responsible for the selective occupation of the V on the Mo sites due to the different distortions of the FeO₆ and MoO₆ octahedra.     

Pressure-Induced Structural Deformations in SeO2

The high-pressure behavior of low-dimensional selenium dioxide SeO₂ (P4₂/mbc, Z=8) is studied with Raman scattering and synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell up to 23 GPa at room temperature. Pressure-induced transformations in this material involve a sequence of structural distortions of the chain structure. The transformation occuring above 7.0 GPa is due to symmetry lowering to space group Pbam (Z=8) without major changes of the crystal lattice dimensions and coordination around the Se atoms. Like in the ambient pressure polymorph, the structural unit is a SeO₃E polyhedron, where E is a Se non-bonded electron lone pair, or an irregular tetrahedron with the O atoms and Se lone pair at the vertices. Further structural transitions above 17 GPa are likely to be the result of additional distortions leading to monoclinic symmetry of the crystal structure. All transformations are reversible with little hysteresis.     

CdCu3(OH)6Cl2: A new layered hydroxide chloride

A new transition metal hydroxide chloride containing kagomé layers of magnetic ions, CdCu₃(OH)₆Cl₂, has been synthesized and structurally characterized. The actual low symmetry P2₁/n structure can be derived from the ideal trigonal one with a change in cation distribution and coherent distortions of the anion framework. The result is a fundamentally different Cu²⁺ kagomé framework than found in the related Herbertsmithite and Kapellasite minerals. Magnetization measurements show no transition to long range magnetic order above T=2 K, despite strong antiferromagnetic interactions with a Weiss temperature of θ_w=−150 K. Furthermore, we show that the structure of CdCu₃(OH)₆Cl₂ and related hydroxide chlorides can be rationalized on the basis of [(OH)₃Cl]⁴⁻ pseudopolyatomic anions that pack and rotate, in much the same way as do traditional polyatomic anions. This opens the door to rational design of new and useful hydroxide chloride materials.     

Evolution of structural distortions in solid solutions between BiMnO3 and BiScO3

Crystal structures of solid solutions of BiMn_1−xSc_xO₃ with x=0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7 were studied with synchrotron X-ray powder diffraction. The strong Jahn-Teller distortion, observed in BiMnO₃ at 300 K and associated with orbital order, disappeared already in BiMn_0.95Sc_0.05O₃. The orbital-ordered phase did not appear in BiMn_0.95Sc_0.05O₃ down to 90 K. Almost the same octahedral distortions were observed in BiMn_1−xSc_xO₃ with 0.05?x?0.7 at room temperature and in BiMnO₃ at 550 K above the orbital ordering temperature T_OO=473 K. These results allowed us to conclude that the remaining octahedral distortions observed in BiMnO₃ above T_OO are the structural feature originated from the highly distorted monoclinic structure.     

X-ray crystal analysis of tricyclohexyltin trifluoroacetate and Mössbauer data for (cyclo-C6H11)3SnO2CR′ (R′ = CH3, CH2CL,CHCL2, CCl3 and CF3)

The halogenocarboxylates (cyclo-C₆H₁₁)₃SnO₂CR′, (R′ = CH₃, CH₂Cl, CHCl₂, CCl₃ and CF₃) have been prepared, and characterized by Mössbauer and IR spectroscopy. The crystal structure of (cyclo-C₆H₁₁)₃SnO₂CCF₃ has been determined by X-ray analysis. The crystals are orthorhombic, space group Pcmn, with unit cell parameters a 14.390 ± 0.004, b, 13.427 ± 0.004, c 11.516 ± 0.003 Å. The structure was resolved by Patterson methods and refined to an R value of 0.147. The coordination about the tin atom can be considered distorted trigonal-pyramidal or distorted tetrahedral. Mössbauer data are explained in terms of distortions of bond angles about the tin atom.     

The crystal structure of β-Ga2Se3

From new X-ray powder diffraction data reported in this paper, the structure of the ordered, superstructural phase of Ga₂Se₃ is found to be different from the structures stated in the literature up to now. The difference relates essentially to the structure distortion involved in the formation of the superstructure. This distortion is clarified in a way which, in general, is suitable for investigations of small distortions of cubic structures. The superstructure cell turns out to be monoclinic, with a = 6.6608(3), b = 11.6516(4), c = 6.6491(3) Å, β = 108.840(5)°, and Z = 4. Furthermore, the coordinates of the Ga and Se positions in this cell are deduced. The space group is shown to be C⁴_s-Cc (No. 9).     

Structure and properties of rhombohedral CePd3Ga8: A variant of the cubic parent compound with BaHg11 structure type

Single crystals of a new intermetallic gallide, R-CePd₃Ga₈, have been synthesized from excess molten gallium. Single-crystal X-ray diffraction reveals that R-CePd₃Ga₈ crystallizes in the R-3m space group with a=b=c=8.4903(10) Å and α=β=γ=89.993(17). R-CePd₃Ga₈ is a variant of the cubic BaHg₁₁ structure type with three structural units: a Ce-centered polyhedron, a distorted cube of Pd₂Ga₆ and a Pd-centered cuboctahedron. The distortions of these units are compared to undistorted analogous units in intermetallic compounds with BaHg₁₁ structure type. Field and temperature-dependent magnetization measurements on R-CePd₃Ga₈ reveal a paramagnetic material with strong antiferromagnetic correlations and a magnetization consistent with Ce³⁺. Electrical resistance measurements indicate Kondo behavior between localized Ce³⁺ magnetic moments.     

Low-temperature single crystal X-ray diffraction and high-pressure Raman studies on [(CH3)2NH2]2[SbCl5]

The structure of bis(dimethylammonium) pentachloroantimonate(III), [(CH₃)₂NH₂]₂[SbCl₅], BDP, was studied at 15 K and ambient pressure by single-crystal X-ray diffraction as well as at ambient temperature and high pressures up to 4.87(5) GPa by Raman spectroscopy. BDP crystallizes in the orthorhombic Pnma space group with a=8.4069(4), b=11.7973(7), c=14.8496(7) Å, and Z=4; R₁=0.0381, wR₂=0.0764. The structure consists of distorted [SbCl₆]³⁻ octahedra forming zig-zag [{SbCl₅}_n]²ⁿ⁻ chains that are cross-linked by dimethylammonium [(CH₃)₂NH₂]⁺ cations. The organic and inorganic substructures are bound together by the N-H…Cl hydrogen bonds. The distortions of [SbCl₆]³⁻ units increase, partly due to the influence of the hydrogen bonds which became stronger, with decreasing temperature. The preliminary room temperature, high-pressure X-ray diffraction experiments suggest that BDP undergoes a first-order phase transition below ca. 0.44(5) GPa that destroys single-crystal samples. The transition is accompanied by changes in the intensities and positions of the Raman lines below 400 cm⁻¹.     

Phase transitions in the perovskite methylammonium lead bromide, CH3ND3PbBr3

The structure of phase IV of methylammonium lead bromide, CH₃ND₃PbBr₃, is shown from Rietveld refinement of neutron powder diffraction data to be centrosymmetric, with space group Pnma: Z=4; a=7.9434(4) Å, b=11.8499(5) Å, c=8.5918(4) Å at 11 K; R_wp=2.34% R_p=1.81%. This corresponds to one of the pure tilt transitions, a^-b⁺a⁻, commonly observed in perovskites. Additional distortions not required by pure tilting are found in the PbBr₆ octahedra, and it appears that the structure optimizes the hydrogen bonding between the methylammonium cation and the framework. It is likely that the lowest temperature phase of the corresponding iodide also has this structure. The structure is compared to the available data for that of other Pnma perovskites. A brief comparison to the higher temperature phases in which the methylammonium ion is disordered is given.     

Kristall- und molekülstruktur von hexaphenyldistannylselenid (C6H5)3SnSeSn(C6H5)3

The crystal and molecular structure of hexaphenylditin selenide (C₆H₅)₃SnSeSn(G₆H₅)₃ was determined by X-ray diffraction data and was refined to R  0.055. The compound is monoclinic, space group P2₁, with a  9.950(4), b  18.650(7), c  18.066(6) Å, β  106.81(4)°, Z  4. The two molecules in the asymmetric unit differ slightly in their conformations, both having approximate C₂ symmetry. Bond lengths and angles are: SnSe 2.526 (2.521(3) ? 2.538(3)) Å; SnC 2.138 (2.107(16)?2.168(19)) Å; SnSeSn 103.4(1)°, 105.2(1)°. There are only slight angular distortions at the SnSeC₃ tetrahedra (SeSnC angles: 104.3(5)?114.8(4)°). The bond data indicate essentially single bonds around the Sn atoms.     

Crystal Structures and Magnetic Properties of the Two Misfit Layer Compounds: [SrGd0.5S1.5]1.16 NbS2 and [Sr(Fe,Nb)0.5S1.5]1.13 NbS2

Crystal structures of two new misfit compounds, [SrGd_0.5S_1.5]_1.16NbS₂ and [Sr(Fe,Nb)_0.5S_1.5]_1.13NbS₂, were determined through the composite approach, i.e., by refining each subpart (Q, H-parts, and the common part) of these composite materials, separately. The Q-part is a three-atom-thick layer, with the NaCl-type structure, where external SrS planes enclose the inner GdS or (Fe,Nb)S plane; the structural difference between these two compounds lies in the central layer within the Q-part: Gd and S atoms are in special positions (octahedral coordination), while Fe and S atoms are statistically distributed on split (×4) positions (tetrahedral coordination) around a central unique site (=special position occupied by Nb). The H-part is a sandwich of sulfur planes enclosing the inner Nb plane as observed for the structure of the binary compound NbS₂ itself. The Sr-Gd derivative shows a paramagnetic behavior in the whole studied temperature range (2-300 K). On the other hand, antiferromagnetic interactions occur in the Sr-Fe derivative; the complex magnetic behavior of this compound is related to the statistical distribution of Fe atoms which leads to frustration of the magnetic interactions. At room temperature, experimental values obtained from Mössbauer spectrum correspond to Fe³⁺ in tetrahedral sulfur environment: isomer shift δ=0.32 mm s⁻¹, and quadrupole splitting ΔE=0.48 mm s⁻¹.     

Structure cristalline de K2SO4(SbF3)2

The crystal structure of K₂SO₄(SbF₃)₂ was determined by X-ray diffraction on a single crystal (R = 0.035 for 2264 reflections). There are two families of antimony atoms showing two different environments: AX₅E octahedron (6 coordination) and AX₆E 3.3.1 monocapped octahedron (7 coordination). The SO^2?₄ unit weakly bonded to four antimony atoms is not very distorted. This arrangement permits the minimization of π-E interactions. Infrared and Raman spectra are discussed in terms of diffraction results.     

Hydrothermal Synthesis and Spectroscopic and Magnetic Behavior of the Mn7(HOXO3)4(XO4)2(X=As, P) Compounds. Crystal Structure of Mn7(HOAsO3)4(AsO4)2

The Mn₇(HOXO₃)₄(XO₄)₂ (X=As, P) compounds have been synthesized by using hydrothermal conditions. The arsenate phase was obtained under autogeneous pressure at 170°C. However, more drastic conditions at both pressure and temperature were necessary in the attainment of the phosphate compound. The crystal structure of Mn₇(HOAsO₃)₄(AsO₄)₂ was solved using single-crystal data. The unit-cell parameters are a=6.810(3) Å, b=8.239(2) Å, c=10.011(4) Å, α=104.31(2)°, β=108.94(3)°, γ=101.25(2)°. Triclinic, P-1 with Z=1. The isostructural Mn₇(HOPO₃)₄(PO₄)₂ phase was characterized from X-ray powder diffraction techniques. The crystal structure of both compounds consists of zig-zag chains constructed by dimeric edge-sharing Mn₂O₁₀ octahedra linked through the MnO₅ trigonal bipyramids. The three-dimensional framework is completed by the connection between isolated MnO₆ entities to the dimers octahedra and trigonal bipyramids. The existence of hydrogenarsenate and hydrogenphosphate anions has been confirmed by IR and Raman spectroscopies. Magnetic measurements indicate the existence of antiferromagnetic interactions in both compounds, which are slightly stronger in the arsenate phase.     

The relevancy of the [B2N4] substructure in the electronic structure of the metal-rich lanthanum nitridoborate La3(B2N4)

Lanthanoide nitridoborates of the general formula Ln₃(B₂N₄) with Ln=La, Ce, Pr, and Nd occur as black crystalline materials. Their structures contain oxalate-like [B₂N₄]⁸⁻ ions being stacked in an eclipsed formation along one crystallographic direction. Electronic structures were calculated for a molecular [B₂N₄]⁸⁻, for the [B₂N₄] partial structure, and for the complete La₃(B₂N₄) structure with the extended Hückel algorithm to analyze the bonding characteristics and to trace the necessity and properties of one surplus electron of (La³⁺)₃(B₂N₄⁸⁻)(e⁻). The HOMO of a [B₂N₄]⁸⁻ is B-B σ bonding, and the LUMO is B-B π bonding but B-N antibonding. The energy band of the solid state [B₂N₄] partial structure corresponding to the LUMO is broadened as a result of intermolecular B?B interactions between adjacent [B₂N₄] units along the stacking direction. Due to bonding interactions with La d orbitals, this band is significantly lowered in energy and occupied with one electron in the band structure of La₃(B₂N₄). This singly occupied band exhibits no band crossings but creates a semimetal-like band structure situation.     

Neutron diffraction, specific heat and magnetic susceptibility of Ni3(PO4)2

The Ni₃(PO₄)₂ phosphate was synthesized by the ceramic method in air atmosphere. The crystal structure consists of a three-dimensional skeleton constructed from Ni₃O₁₄ edge-sharing octahedra, which are interconnected by (PO₄)³⁻ oxoanions with tetrahedral geometry. The magnetic behavior was studied on powdered sample by using susceptibility, specific heat and neutron diffraction data. The nickel(II) orthophosphate exhibits a three-dimensional magnetic ordering at approximately 17.1 K. However, its complex crystal structure hampers any parametrization of the J-exchange parameter. The specific heat measurements of Ni₃(PO₄)₂ exhibit a three-dimensional magnetic ordering (λ-type) peak at 17.1 K. Measurements above T_N suggest the presence of a small short-range order in this phase. The total magnetic entropy was found to be 28.1 KJ/mol at 50 K. The magnetic structure of the nickel(II) phosphate exhibits ferromagnetic interactions inside the Ni₃O₁₄ trimers which are antiferromagnetically coupled between them, giving rise to a purely antiferromagnetic structure.     

Preparation and properties of the systems Fe2−xCrxWO6, Fe2−xRhxWO6, and Cr2−xRhxWO6

Solid solutions of the end members Fe₂WO₆, Cr₂WO₆, and Rh₂WO₆ have been prepared and their crystallographic and magnetic properties studied. All solid solutions crystallize with the trirutile structure, and their magnetic behavior is characterized by the existence of antiferromagnetic interactions and effective molar Curie constants corresponding to those expected from contributions of the spinonly moments of high-spin Fe³⁺, Cr³⁺, and diamagnetic low-spin Rh³⁺ ions. Fe₂WO₆ crystallizes with the tri-α-PbO₂ structure and is antiferromagnetic and conducting. The random rutile Rh₂WO₆ is conducting, and the difference between its magnetic and electric properties and those of the inverse trirutile Cr₂WO₆ are discussed in terms of possible interactions between Cr³⁺(3d) or Rh³⁺(4d) orbitals and W⁶⁺(5d) orbitals.     

Crystal structure and antiferromagnetism of EuSb2

EuSb₂ crystallizes in the monoclinic CaSb₂ type of structure, space group $\text{P2}{}_1\text{m}$, with a = 4.768(2), b = 4.299(2), c = 8.9703(3) Å, β = 103.01(3)°. The structural distortions of the ZrSi₂ type provide the Sb chains required for a Mooser-Pearson phase. EuSb₂ is antiferromagnetic below T_N = 26.2°K. From high-field magnetization measurements a weak anisotropy (H_anis. ≈ 6 kOe at 1.5°K) is deduced. The spins are aligned perpendicular to the (001) planes. Susceptibility measurements between 30 and 1100°K gave no indication of a Eu²⁺ → Eu³⁺ transition.     

Conductivity and Structural Investigations in Lacunary Pb6Ca2Li2(PO4)6 Apatite

Prismatic crystals of Pb₆Li₂Ca₂(PO₄)₆ were obtained by solid-state reaction. They were characterized by IR spectroscopy and chemical analyses. The structure as determined by X-ray diffraction study on single crystal revealed that the compound is isostructural to the hexagonal phase Pb₈Na₂(PO₄)₆. Crystal data for Pb₆Li₂Ca₂(PO₄)₆: space group P63/m (No. 176), a=b=9.6790(15) Å, c=7.1130(7), Z=1, R=0.039. In the compound, lithium was found to preferentially occupy the site (I) and the structure is stabilized by interactions between electron lone pairs of lead (II) ions. Electrical conductivity measured in a wide range of temperature is governed by a hopping mechanism of Li ions in tunnels.     

The Ternary Compounds Pd13ln5.25Sb3.75 and Pdln1.26Sb0.74: Crystal Structure and Electronic Structure Calculations

The new ternary compound Pd₁₃In_5.25Sb_3.75 was found. Its crystal structure was determined using a CCD diffractometer at room temperature. Evaluations and refinements finally yielded a C-centered monoclinic structure (space group, C2/c; Pearson symbol, mC88, Z=4) with a=15.189(2) Å, b=8.799(1) Å, c=13.602(2) Å, and β=123.83(1)°. For the entire data set of 3706 independent reflections residual values are R=0.0461 and R_w=0.0789. The structure was found to be isotypic to Pd₁₃Pb₉ with In and Sb on the Pb sites. The existence of a further ternary compound, which was already described as Pd₃In₄Sb₂, could be confirmed. Its composition range was determined by EPMA to be PdIn_1.2-1.3Sb_0.8-0.7. It does not melt congruently and we were not able to find suitable single crystals. However, we were able to prepare the pure ternary compound in order to perform X-ray powder diffraction using a Guinier image plate technique. The entire diffraction spectrum was refined by full profile Rietveld method using the program Fullprof. The α-PdSn₂ structure type (space group, I4₁/acd; Pearson symbol, t148, Z=16), proposed for this compound, was confirmed and the lattice parameters are a=6.4350(1) Å and c=24.3638(3) Å. The residual values were R_p=5.34 and R_wp=6.70. The tetragonal PdSn₂ structure type is a mixed variant of the CaF₂ type and the CuAl₂ type structure. Also in this ternary compound we assumed a random contribution of In and Sb over the 16e and 16f positions. The electronic structures of both compounds were investigated by extended Hückel calculations. Crystal orbital overlap populations show extended bonding interactions between the main group elements. The bonding interactions of the main group elements are almost optimized at the experimentally observed In/Sb ratio of the ternary compound. The In/Sb ratio in Pd₁₃In_5.25Sb_3.75 can thus be rationalized on the basis of the electronic structure.