Crystal structures and chemistry of double perovskites Ba2M(II)M′(VI)O6 (M=Ca, Sr, M′=Te, W, U)

Structures of the double perovskites Ba₂M(II)M ′(VI)O₆ (M=Ca, Sr, M′=Te, W, U) at room temperature have been investigated by the Rietveld method using X-ray and neutron powder diffraction data. For double perovskites with M=Sr, the observed space groups are I2/m (M′ =W) and (M′=Te), respectively. In the case of M=Ca, the space groups are either monoclinic P2₁/n (M′=U) or cubic (M′=W and Te). The tetragonal and orthorhombic symmetry reported earlier for Ba₂SrTeO₆ and Ba₂CaUO₆, respectively, were not observed. In addition, non-ambient X-ray diffraction data were collected and analyzed for Ba₂SrWO₆ and Ba₂CaWO₆ in the temperature range between 80 and 723 K. It was found that the rhombohedral structure exists in Ba₂SrWO₆ above room temperature between the monoclinic and the cubic structure, whereas the cubic Ba₂CaWO₆ undergoes a structural phase transition at low temperature to the tetragonal I4/m structure.     

Magnetic properties of quadruple perovskites Ba4LnRu3O12 (Ln=La, Nd, Sm-Gd, Dy-Lu)

Quadruple perovskites Ba₄LnRu₃O₁₂ (Ln=La, Nd, Sm-Gd, Dy-Lu) were prepared and their magnetic properties were investigated. They adopt the 12L-perovskite-type structure consisting of Ru₃O₁₂ trimers and LnO₆ octahedra. All of these compounds show an antiferromagnetic transition at 2.5-30 K. For Ba₄NdRu₃O₁₂, ferrimagnetic ordering has been observed at 11.5 K. The observed magnetic transition is due to the magnetic behavior of the Ru^4.33+₃O₁₂ trimer with S=. Magnetic properties of Ba₄LnRu₃O₁₂ were compared with those of triple perovskites Ba₃LnRu₂O₉ and double perovskites Ba₂LnRuO₆.     

Synthesis, structure, and magnetic properties of Ba2Cu2US5

The alkaline-earth uranium chalcogenide Ba₂Cu₂US₅ was obtained in a two-step reaction from BaS, Cu₂S, and US₂. Ba₂Cu₂US₅ crystallizes in a new structure type in space group C2/m of the monoclinic system with two formula units in a cell of dimensions a=13.606(3) Å, b=4.0825(8) Å, c=9.3217(19) Å, and β=116.32(3)° (153 K). The structure consists of layers separated by Ba atoms in bicapped trigonal-prismatic coordination. The two-dimensional layer is built from US₆ octahedra and CuS₄ tetrahedra. The connectivity of the MS_n polyhedra within the layer in the [001] direction is oct tet tet oct tet tet. A μ_eff value of 2.69(2) μ_B/U was obtained from the magnetic susceptibility data. No magnetic transition was observed for Ba₂Cu₂US₅ down to 2 K.     

On the symmetry and crystal structures of Ba2LaIrO6

Accurate profile analysis of X-ray diffraction data was carried out to settle recent dispute on the symmetry and crystal structures of the double perovskite Ba₂LaIrO₆. Even through careful comparison of the full-width at half-maximum values, we found no evidence for Ba₂LaIrO₆ adopting either monoclinic (I2/m) or mixed rhombohedral and monoclinic (I2/m) structures at room temperature, becoming triclinic at below about 200 K. The correct space group is just at temperatures between 82 and 653 K. Furthermore, the phase transition does occur in Ba₂LaIrO₆, but the transition temperature is found to be much higher than the reported value.     

Structures and crystal chemistry of the double perovskites Ba2LnB′O6 (Ln=lanthanide B′=Nb and Ta): Part I. Investigation of Ba2LnTaO6 using synchrotron X-ray and neutron powder diffraction

The structure of 14 compounds in the series Ba₂LnTaO₆ have been examined using synchrotron X-ray diffraction and found to undergo a sequence of phase transitions from I2/m monoclinic to I4/m tetragonal to cubic symmetry with decreasing ionic radii of the lanthanides. Ba₂LaTaO₆ is an exception to this with variable temperature neutron diffraction being used to establish that the full series of phases adopted over the range of 15-500 K is P2₁/n monoclinic to I2/m monoclinic to rhombohedral. The chemical environments of these compounds have also been investigated and the overbonding to the lanthanide cations is due to the unusually large size for the B-site in these perovskites.     

Synthesis and magnetic properties of 12L-perovskites Ba4LnIr3O12 (Ln=lanthanides)

New quadruple perovskite oxides Ba₄LnIr₃O₁₂ (Ln=lanthanides) were prepared and their magnetic properties were investigated. They crystallize in the monoclinic 12L-perovskite-type structure with space group C2/m. The Ir₃O₁₂ trimers and LnO₆ octahedra are alternately linked by corner-sharing and form the perovskite-type structure with 12 layers. The Ln and Ir ions are both in the tetravalent state for Ln=Ce, Pr, and Tb compounds , and for other compounds (Ln=La, Nd, Sm-Gd, Dy-Lu), Ln ions are in the trivalent state and the mean oxidation state of Ir ions is . An antiferromagnetic transition has been observed for Ln=Ce, Pr, and Tb compounds at 10.5, 35, and 16 K, respectively, while the other compounds are paramagnetic down to 1.8 K.     

Crystal structures and magnetic properties of 6H-perovskite-type oxides Ba3MIr2O9 (M=Mg, Ca, Sc, Ti, Zn, Sr, Zr, Cd and In)

Crystal structures and magnetic properties of quaternary oxides Ba₃MIr₂O₉ (M=Mg, Ca, Sc, Ti, Zn, Sr, Zr, Cd and In) were investigated. Rietveld analyses of their X-ray diffraction data indicate that they adopt the 6H-perovskite-type structure with space group P6₃/mmc or, in the case of M=Ca, Sr and Cd, a monoclinically distorted structure with space group C2/c. The Ir valence configurations are (M=Mg, Ca, Zn, Sr and Cd), (M=Sc and In) and (M=Ti and Zr). Magnetic susceptibility and specific heat measurements were carried out. In the , the Ir⁵⁺ ions have a non-magnetic ground state and the magnetic behavior for these compounds is explained by the Kotani's theory. For , the effective magnetic moment of these compounds is significantly small, although the Ir⁴⁺ ions have magnetic moment, which indicates the existence of the strong antiferromagnetic interaction between Ir⁴⁺ ions in the Ir⁴⁺₂O₉ face-shared bioctahedra. In the case of , a specific heat anomaly was found at about 10 K (M=Sc) and 1.6 K (M=In), which suggests the magnetic ordering of the magnetic moments of Ir⁴⁺ in the (Ir⁴⁺Ir⁵⁺)O₉ bioctahedra.     

From ordered antiferromagnet to spin glass: A new phase Mn4Ir7−xMnxGe6

A new ternary phase, Mn₄Ir_7−xMn_xGe₆ (0?x?1.3), was studied by X-ray and neutron powder diffraction and SQUID magnetometry. The crystal structure is cubic, of the U₄Re₇Si₆ type, space group , Z=2, with the lattice parameter at 295 K. Within the limited range of homogeneity small variations of the composition yield dramatic changes of the magnetic structure. For x=0 long-range antiferromagnetic order is formed below the transition temperature 228 K, with large magnetic moments on Mn, 4.11(9) μ_B at 10 K, in a magnetic unit cell , c_M=2a_C. In contrast, for x=1.3 spin glass behavior is observed below 90 K. The Mn atoms form an ideal cubic framework, on which geometric frustration of competing nearest and next nearest neighbor antiferromagnetic interactions is suggested to explain the composition sensitive magnetic properties. A TiNiSi-type phase, IrMnGe, is found in samples of 1:1:1 composition quenched from the melt.     

Crystal structure, phase transition and anisotropic thermal expansion of barium zirconium diorthophosphate, BaZr(PO4)2

The crystal structure of BaZr(PO₄)₂ at 298 K was determined from conventional X-ray powder diffraction data using direct methods, and it was further refined by the Rietveld method. The structure was monoclinic (space group C2/m, Z=2) with , , , β=93.086(1)° and . Final reliability indices were R_wp=8.21%, R_p=5.64% and R_B=2.92%. The atom arrangement is similar to that of yavapaiite (KFe(SO₄)₂), however, these crystal structures differ distinctly in the coordination numbers of barium and potassium atoms; the former is tenfold coordinated, whereas the latter is sixfold coordinated. The powder specimens were also examined by high-temperature XRD and DTA to reveal the occurrence of a phase transition from monoclinic to orthorhombic at 732 K during heating. Upon cooling the reverse transition occurred at 710 K. The monoclinic crystal expanded almost one-dimensionally along [503] during the heating process. The orthorhombic phase also showed a tendency to expand one-dimensionally along the c-axis above 732 K.     

Large magnetoresistance in a ferromagnetic cobaltite: The 12H Ba0.9CoO2.6

The magnetic and transport properties of a hexagonal cobaltite related to the perovskite structure have been studied. By combining transmission electron microscopy, X-ray powder diffraction and iodometric titration, it is found that Ba_0.9CoO_2.6 crystallizes in the 12H structure [P6₃/mmc, a=5.6612 (1) Å and c=28.4627(8) Å]. Interestingly, this compound is a ferromagnet with a Curie temperature T_C=50 K and a saturation magnetization . This value is smaller than expected from the effective paramagnetic moment, μ_eff=3.7μ_B/Co, corresponding to an average spin per Co, from which one would expect . This suggests either a canted structure or a strong local magnetic anisotropy related to the crystal field of the CoO_n polyhedra. A clear transition in the electrical resistivity is found at T_C consistent with a spin scattering reduction as the sample becomes ferromagnetic. The spin-charge coupling is evidenced by the large negative magnetoresistance effect optimum near T_C=50 K, with .     

The structural investigation of Ba4Bi3F17

The anion-excess ordered fluorite-related phase Ba₄Bi₃F₁₇ has been synthesized by a solid state reaction of BaF₂ and BiF₃ at 873 K. The crystal structure of Ba₄Bi₃F₁₇ has been studied using electron diffraction and X-ray powder diffraction (a=11.2300(2) Å, c=20.7766(5) Å, S.G. , R_I=0.020, R_P=0.036). Interstitial fluorine atoms in the Ba₄Bi₃F₁₇ structure are considered to form isolated cuboctahedral 8 : 12 : 1 clusters. The structural relationship between Ba₄Bi₃F₁₇ and similar rare-earth-based phases is discussed.     

Long-range magnetic ordering of S=1/2 linear trimers in A3Cu3(PO4)4 (A=Ca, Sr, and Pb)

Magnetic properties of S=1/2 linear trimer cluster compounds A₃Cu₃(PO₄)₄ (A=Ca, Sr, and Pb) were investigated. Magnetic susceptibility data for the three compounds showed that paramagnetic copper spins form trimers with the total spin of 1/2 below about 45 K. Specific heat and magnetization measurements indicated that the trimer clusters undergo ferromagnetic long-range ordering at for A=Ca and antiferromagnetic long-range ordering at for A=Sr and for A=Pb. A₃Cu₃(PO₄)₄ exhibited 1/3-magnetization plateau at least up to magnetic field of 55 T at 1.3 and 4.2 K. A₃Cu₃(PO₄)₄ with A=Sr and Pb showed a spin-flop transition near 0.03 T in the antiferromagnetic state at 0.08 K. Specific heat data at magnetic fields clearly showed broad maxima at low temperatures due to the finite intra-chain interaction in one-dimensional arrays of the trimers.     

Structural investigation of Sr2LiReO6. Evidence for a continuous tetragonal-cubic phase transition

The presence of a continuous and reversible -I4/m phase transition in a polycrystalline sample of the ordered double perovskite Sr₂LiReO₆ is described. The transition that occurs near 300 °C is a consequence of in-phase tilting of the BO₆ octahedra. The temperature dependence of both the lattice parameters and the spontaneous strains are consistent with a second order phase transition as would be expected for a soft mode transition.     

Heat capacity and standard molar enthalpy of formation of crystalline 2,6-dicarboxypyridine (C7H5NO4)

Low-temperature heat capacity C_p,m of 2,6-dicarboxypyridine (C₇H₅NO₄; CAS 499-83-2) was precisely measured in the temperature range from (80 to 378) K with a high precision automated adiabatic calorimeter. No phase transition or thermal anomaly was observed in this range. The thermodynamic functions [H_T − H_298.15] and [S_T − S_298.15] were calculated in the range from (80 to 378) K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined, and , by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 K. The thermodynamic properties of the compound were further investigated through differential scanning calorimeter (DSC) and the thermogravimetric (TG) analysis.     

Synthesis, structure, and magnetism of Tb4PdGa12 and Tb4PtGa12

Single crystals of Tb₄MGa₁₂ (M=Pd, Pt) have been synthesized. The isostructural compounds crystallize in the cubic space group , with Z=2 and lattice parameters: a=8.5940(5) and 8.5850(3) Å for Tb₄PdGa₁₂ and Tb₄PtGa₁₂, respectively. The crystal structure consists of corner-sharing MGa₆ octahedra and TbGa₃ cuboctahedra. Magnetic measurements suggest that Tb₄PdGa₁₂ is an antiferromagnetic metamagnet with a Néel temperature of 16 K, while the Pt analog orders at T_N=12 K.     

Characterization by X-ray diffraction, magnetic susceptibility and Mössbauer spectroscopy of a new alluaudite-like phosphate:: Na4CaFe4(PO4)6

A single crystal of a new sodium calcium iron (III) phosphate, Na₄CaFe₄(PO₄)₆, has been synthesized by a flux method and characterized by X-ray diffraction, Mössbauer spectroscopy and magnetic susceptibility measurements. The compound crystallizes in the monoclinic space group C2/c(a=12.099(5) Å, b=12.480(5) Å, c=6.404(2) Å, β=113.77(3)°, Z=2, R₁=0.022, R_w2=0.066). The crystal structure belongs to the alluaudite type, characterized by the X(2)X(1)M(1)M(2)₂(PO₄)₃ general formula. The open framework results from Fe₂O₁₀ units of edge-sharing FeO₆ octahedra, which alternate with M(1)O₆ octahedra (M(1)=Na+Ca) that form infinite chains. These chains are linked together through the common corners of PO₄ tetrahedra yielding two distinct tunnels of sodium cation occupation. This compound is antiferromagnetic with a Néel temperature of 35 K. Mössbauer parameters are consistent with the structural results.     

Synthesis, crystal structure and characterization of new 12H hexagonal perovskite-related oxides Ba6M2Na2X2O17 (M=Ru, Nb, Ta, Sb; X=V, Cr, Mn, P, As)

The new Ba₆Ru₂Na₂X₂O₁₇ (X=V, Mn) compounds have been prepared by electrosynthesis in molten NaOH and their crystal structures have been refined from single crystals X-ray diffraction, space group P6₃/mmc, Z=2, for X=V: , , R₁=4.76%, for X=Mn : , , R₁=3.48%. The crystal structure is a 12H-type perovskite with a (c′cchcc)₂ stacking sequence of [BaO₃]_c, [BaO₃]_h and [BaO₂]_c′ layers. The tridimensional edifice is formed by blocks of Ru₂O₉ dimers that share corners with NaO₆ octahedra. These blocks sandwich double sheets of X⁵⁺O₄ tetrahedra. Several isotypic Ba₆M⁵⁺₂Na₂X⁵⁺₂O₁₇ materials (X=V, Cr, Mn, P, As) and (M=Ru, Nb, Ta, Sb) have been prepared by solid state reaction and characterized by Rietveld analysis. The magnetic and electric properties have been investigated and show besides the Ru⁵⁺₂O₉ typical intradimer antiferromagnetic couplings, discrepancies of both χ and ρ versus T at 50 and 100 K for Ba₆Ru₂Na₂X₂O₁₇ (X=V, As). In this work, a review of the identified Ru-hexagonal perovskite materials is also reported in order to overview the wide variety of possibilities in the field of new compounds synthesis.     

M1−x[W2O2X6] with M=K, Tl, Ag, Hg, Pb; X=Cl, Br—A class of mixed valence tungsten (IV,V) compounds with layered structures, W-W bonds and high conductivity

The crystal structure of WOCl₃, determined on the basis of powder diffraction data (tetragonal, P4₂/mnm, a=10.6856(6), c=3.8537(2)), is isotypic to WOI₃ and contains one-dimensional strands of edge-sharing double-octahedral W₂O_4/2Cl₆ groups connected via common corners in trans position. A W-W bond of 2.99 Å is present within the planar W₂Cl₆ groups. A series of non-stochiometric, mixed valence W(IV,V) compounds M_1−x[W₂O₂Cl₆] can be obtained from WOCl₃ by reaction with metal halides (TlCl, KCl, PbCl₂) or by reaction of elemental Hg with WOCl₄. All were characterized by single crystal structure determinations and EDX measurements (Tl_0.981(2)[W₂O₂Cl₆]: monoclinic, C2/m, a=12.7050(4), b=3.7797(1), , β=107.656(1)°; K_0.84(2)[W₂O₂Cl₆]: monoclinic, C2/m, a=12.812(3), b=3.7779(6), , β=107.422(8)°; Pb_0.549(3)[W₂O₂Cl₆]: orthorhombic, Immm,a=3.7659(1), b=9.8975(4), ; Hg_0.554(6)[W₂O₂Cl₆]: monoclinic, C2/m, a=12.8361(8), b=3.7622(3), , β=113.645(3)°). Two representatives of this family of compounds have already been reported: Na[W₂O₂Br₆] [Y.-Q. Zhang, K. Peters, H.G. von Schnering, Z. Anorg. Allg. Chem. 624 (1998) 1415-1418] and Ag_0.74[W₂O₂Br₆] [S. Imhaïne, C. Perrin, M. Sergent, Mat. Res. Bull. 33 (1998) 927-933]. The Ag containing compound can be obtained from elemental Ag and WOBr₃. The crystal structure, originally reported in the triclinic system, was redetermined and shown to be monoclinic with space group C2/m (a=13.7338(10), b=3.7769(3), , β=112.401(3)°). The crystal structures of these compounds are in close relationship to the structure of WOCl₃ and all contain W₂O_4/2X₆ (X=Cl, Br) double strands with the mono and divalent cations coordinated by the terminal halogen atoms of the W₂X₆ groups and a short W-W bond (2.85 Å for X=Cl). A cube-shaped coordination environment is present for M=Tl, K and a trigonal-prismatic coordination for M=Ag, Hg. Hg_0.55[W₂O₂Cl₆] is a semiconductor with a non-Arrhenius behaviour, high specific conductivity of 0.05 Ω^-1 cm⁻¹ and a very small activation energy of 0.03 eV. Hg_0.55[W₂O₂Cl₆] and Ag_0.8[W₂O₂Br₆] show a temperature independent paramagnetism with a magnetic moment around 300×10^-6 cm³ mol^-1.     

Studies on magnetic and calorimetric properties of double perovskites Ba2HoRuO6 and Ba2HoIrO6

Magnetic properties of double perovskite compounds Ba₂HoRuO₆ and Ba₂HoIrO₆ have been reported. Powder X-ray and neutron diffraction measurements show that these compounds have a cubic perovskite-type structure with the space group and the 1:1 ordered arrangement of Ho³⁺ and Ru⁵⁺ (or Ir⁵⁺) over the 6-coordinate B sites. Results of the magnetic susceptibility and specific heat measurements show that Ba₂HoRuO₆ exhibits two magnetic anomalies at 22 and 50 K. Analysis of the temperature dependence of magnetic specific heat indicates that the anomaly at 50 K is due to the antiferromagnetic ordering of Ru⁵⁺ ions and that the anomaly at 22 K is ascribable to the magnetic interaction between Ho³⁺ ions. Neutron diffraction data collected at 10 and 35 K show that the Ba₂HoRuO₆ has a long range antiferromagnetic ordering involving both Ho³⁺ and Ru⁵⁺ ions. Each of their magnetic moments orders in a Type I arrangement and these magnetic moments are anti-parallel in the ab-plane with each other. The magnetic moments are aligned along the c-direction. On the other hand, Ba₂HoIrO₆ is paramagnetic down to 1.8 K.     

Neutron diffraction study of phase transitions in perovskite-type strontium molybdate SrMoO3

The structure of a polycrystalline sample of SrMoO₃ has been investigated using powder neutron diffraction from 5 to 300 K, to reveal two structural phase transitions, the first from the cubic structure with a=3.97629(3) Å to a tetragonal structure in I4/mcm near 266 K and the second to an orthorhombic Imma phase below 125 K. The average Mo-O distance is essentially independent of temperature. The temperature dependence of the octahedral tilting appears typical of a tricritical phase transition.