Crystal structure of the 6H BaCrO3 polytype

A product from the reaction between CrO₂ and Ba₂CrO₄ at 900°C under 60–65 kbar was found to be the six-layer polytype of BaCrO₃ from powder diffraction studies. A hexagonal black crystal obtained from this reaction was isolated for single crystal studies and structure determination. The crystal was found to possess a six-layer stacking sequence of BaO₃ layers with space group $\text{P6}{}_3\text{mmc}$ and had unit cell parameters a = 5.629(2), c = 13.698(6)Å, and Z = 6. The structure was determined from 936 independent reflections of which 693 were considered observed. Averaging equivalent reflections yielded 163 unique, observed reflections. Refinement of the structure by least-squares methods gave a conventional R value of 4.8% (R_w = 6.2%). The structure consists of a six-layer stacking sequence of close-packed BaO₃ layers containing tetravalent chromium in all the octahedral oxygen interstices. The compound was found to be isostructural with previously reported BaMO₃ phases.     

The preparation and crystal structure of a BaRhO3 polytype

Barium rhodium oxide, BaRhO₃, was prepared at 1175°C and 60–65 kbar by the reaction of BaO₂ and RhO₂. A hexagonal black platelet obtained in the reaction product was found to possess a four-layer stacking sequence in space group $\text{P6}{}_3\text{mmc}$ having hexagonal unit cell parameters a = 5.744(1), c = 9.642(1) Å. The structure was detemined from 707 independent reflections of which 224 were considered observed. Averaging equivalent reflections yielded 132 unique observed reflections. Refinement of the structure by least-squares methods gave a conventional R value of 4.4%. The structure consists of a four-layer stacking sequence of close-packed BaO₃ layers containing tetravalent rhodium in all the octahedral oxygen interstices. The compound was found to be isostructural with previously reported BaMO₃ phases. This is the first single-crystal refinement of the 4H polytype using a four-circle diffractometer.     

Preparation of the ordered perovskite-like compounds Ba4M3LiO12 (M = Ta,Nb): A powder neutron diffraction determination of the structure of Ba4Ta3LiO12

The hexagonal ordered perovskite-like compounds Ba₄Ta₃LiO₁₂ and Ba₄Nb₃LiO₁₂ have been prepared. The structure of Ba₄Ta₃LiO₁₂ has been determined by profile analysis of a powder neutron diffraction pattern. The structure is based on an eight layer (ccch) stacking sequence of the BaO₃ layers (space group $\text{P6}{}_3\text{mmc}$) with the lithium atoms confined to the face-shared octahedral sites. The relationship of this structure to that of Ba₅Ta₄O₁₅ is discussed.     

A single crystal study of eight-layer barium managanese oxide,BaMnO3

A single crystal study of hydrothermally prepared eight-layer BaMnO₃ has been carried out which confirms the (Zhdanov notation) 121121 layer stacking scheme for the BaO₃ layers. The MnO₆ octahedra share faces in strings of four, and these strings are connected to each other by corner sharing. The compound has an hexagonal unit cell of dimensions a = 5.667 ± 0.003 and c = 18.738 ± 0.009 Å, probable space group $\text{P6}{}_3\text{mmc}\text{, Z = 8}$. Its structure has been determined from 352 independent reflections, of which 242 were considered observed, collected manually by a counter technique and refined to a conventional R value of 0.079.     

The structure of ce-doped Bi2(MoO4)3 as determined by neutron profile refinement

The structure of Bi_1.8Ce_0.2(MoO₄)₃ has been refined with powder neutron diffraction data by the Rietveld method. The structure can be derived by severely distorting the scheelite structure (AMO₄) and is perhaps better written $\text{A}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{Ø}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{M}\text{O}{}_4$, where Ø = cation vacancy. Of the two bismuth atom sites, cerium preferentially occupies the more symmetric of the two (Bi(2) in the structure) with some cerium found in the scheelite subcell vacancies also. This site preference is understood by examining the symmetries of the two Bi sites. Crystal data: monoclinic, space group $\text{P2}{}_1\text{c}\text{, Z = 4, a = 7.697(2), b = 11.535(3), c = 11.944(3)}$, β = 115.19.     

Structural study of a new hexagonal form of tungsten trioxide

A new form of tungsten trioxide WO₃ has been obtained by dehydration of $\text{WO}{}_3\text{·}\text{1}\text{3}\text{H}{}_2\text{O}$ hydrate. The structural study was carried out from X-ray powder diffraction and selected area electron diffraction data. The crystallographic characteristics are: the hexagonal system; a = 7.298(2) Å, c = 7.798(3) Å; Z = 6. This hexagonal WO₃ is built up of slightly distorted (WO₆) octahedra sharing their corners arranged in six-membered rings in layers normal to the hexagonal axis; stacking of such layers leads to formation of large hexagonal tunnels. Some confirmations of this structure were made by high-resolution electron microscopy. Powder X-ray diffraction allowed us to determine an average structure. Absence of suitable single crystals has not permitted us to perform a complete structural determination. Although the existence of such a hexagonal structure for pure WO₃ had been considered as likely, it had not been hitherto observed.     

Etude cristallochimique des phases de type perovskite du systeme Th0.25 NbO3NaNbO3

The compound Th_0.25 NbO₃ melts congruently at 1390°C. Single crystals obtained by slow cooling from the melt are transparent and show uniaxial optical properties. A single-crystal X-ray analysis confirms the tetragonal cell found by Kovba and Trunov from a powder data and gives a = 3.90 Å and c = 7.85 Å. No systematic absence of the hkl reflections is observed on precession films. The relative intensities of the main reflections are characteristic of a perovskite-like arrangement ABO₃ whose large dodecahedral A sites are only partly occupied. Several domains have been found in the perovskite-type solid solution (1 ? x) Th_0.25NbO₃-x NaNbO₃. For 0 ? x ? 0.5 the phases have a tetragonal cell with $\text{a ? a}{}_0$ and $\text{c ? 2a}{}_0$ as in pure Th_0.25 NbO₃. When 0.6 ? x ? 0.8 the corresponding phases crystallize with a small cubic cell ($\text{a}{}_0\text{? 3.9}$Å), while phases with 0.9 ? x ? 1 have an orthorhombic cell ($\text{a ? 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{a}{}_0\text{, b ? 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{a}{}_0\text{, c ? a}{}_0$).     

Synthèse et caractérisation d'une série de titanates pérowskites isotypes de [CaCu3](Mn4)O12

A series of titanates which have perovskite-like arrangements and are isostructural with [CaCu₃](Mn₄)O₁₂ have been synthesized. The total charge of the A sites can be modified (1) by substituting the Ca²⁺ cations with monovalent ones and the tetravalent manganese cations of the B sites by a mixture of (Ti⁴⁺ + M⁵⁺) in which M = Ta, Nb, Sb, or (2) by substituting the Ca²⁺ cations by a combination of cations plus vacancies. In this case, if the total charge of the A sites is 2, one obtains compounds such as $\text{[}\text{Th}{}^{\mathrm{4+}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{Cu}{}_3\text{](Ti}{}_4\text{)O}{}_{12}$ and $\text{[T}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Cu}{}_3\text{](Ti}{}_4\text{O}{}_{12}\text{(T =}\text{rare earth}\text{)}$; on the contrary, if the charge is less than 2, then one has to compensate it by changing that of the B sites. This leads to compounds such as [□Cu₃](Ti₂M₂)O₁₂ (M = Ta, Nb, Sb).     

The oxygen defect perovskite Ca3Mn1.35Fe1.65O8.02: A highly frustrated antiferromagnet

A new oxygen defect perovskite Ca₃Mn_1.35Fe_1.65O_8.02 has been isolated. It crystallizes in the orthorhombic system with the following parameters: $\text{a ? a}{}_{\mathrm{<mtext>p</mtext>}}\text{√2; b ? 3a}{}_{\mathrm{<mtext>p</mtext>}}$, and $\text{c ? a}{}_{\mathrm{<mtext>p</mtext>}}\text{√2}$. X-Ray diffraction shows that it corresponds to the second member of the structural series (AMO₃)_m(AMO₂□) and thus consists of double perovskite layers separated by tetrahedral layers. This phase, related to the brownmillerite structure, differs from the latter, in that it exhibits oxygen defects in the perovskite layer and an excess of oxygen in the tetrahedral layer. These results are explained by the ability of Mn^III to adopt pyramidal coordination. Its magnetic properties have been investigated by susceptibility and magnetization measurements and Mössbauer spectroscopy in the temperature range 4–300 K. The dependence of the freezing temperature on the measuring technique (125 K with Mössbauer spectroscopy and 100 K from magnetization), the wide range of temperature where the freezing of the spins occurs, the sensitivity of χ on the cooling magnetic field and the drastic lowering of C_M characterize a highly frustrated behavior due to cationic disorder in the structure.     

Nonstoichiometry and defect structure of the perovskite-type oxides La1−xSrxFeO3−°

In order to elucidate the defect structure of the perovskite-type oxide solid solution La_1?xSr_xFeO_3?δ (x = 0.0, 0.1, 0.25, 0.4, and 0.6), the nonstoichiometry, δ, was measured as a function of oxygen partial pressure, P_O2, at temperatures up to 1200°C by means of the thermogravimetric method. Below 200°C and in an atmosphere of P_O2 ≥ 0.13 atm, δ in La_1?xSr_xFeO_3?δ was found to be close to 0. With decreasing log P_O2, δ increased and asymptotically reached $\text{x}\text{2}$. The value corresponding to $\text{δ =}\text{x}\text{2}$ was about ?10 at 1000°C. With further decrease in log P_O2, δ slightly increased. For LaFeO_3?δ, the observed δ values were as small as <0.015. It was found that the relation between δ and log P_O2 is interpreted on the basis of the defect equilibrium among Sr′_La (or V?_La for the case of LaFeO_3?δ), V^··_O, Fe′_Fe, and Fe^·_Fe. Calculations were made for the equilibrium constants K_ox of the reaction

$\text{1}\text{2}\text{O}{}_2\text{(g) + V}{}^{\mathrm{··}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= O}{}^{\mathrm{x}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe}}$

and K_i for the reaction

$\text{2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= Fe}{}^{\mathrm{\prime }}{}_{\mathrm{Fe}}\text{+ Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe·}}$

Using these constants, the defect concentrations were calculated as functions of P_O2, temperature, and composition x. The present results are discussed with respect to previously reported results of conductivity measurements.     

Etude comparative des structures type K4NiF4 et pérovskite par la méthode des invariants

The study of K₂NiF₄ and perovskite structure type by the “method of invariants” leads to the relationship: $\text{(A-X)}{}_9\text{2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? (A-X)}{}_{12}\text{=}\text{constant}$, where (A-X)₉ and (A-X)₁₂ are the invariant values associated with cation A in coordination number 9 and 12. In the case where A = K⁺ and X = F^?, we propose the relationship:

$\text{(K}{}^{\mathrm{+}}\text{?F)}{}_{\mathrm{R}}\text{= 2.832 R}{}^{\mathrm{<mtext>1</mtext><mtext>11.4</mtext>}}$

where R is the coordination number.     

Decarboxylierung cyclischer und acyclischer dicarboxylato-komplexe des platins

The interaction of (Ph₃P)₂PtO₂ (I) with the dicarboxylic acids HO₂C(CH₂)_nCO₂H (n = 1–3), phthalic acid and maleic acid gives the dicarboxylato complexes (Ph₃P)₂$\text{PtO(O)C(CH}{}_2\text{)}{}_{\mathrm{<mtext>n</mtext>}}\text{C(O)O}$ (II) (n = 1–3), (Ph₃P)₂$\text{PtO(O)CC}{}_6\text{H}{}_4\text{C(O)O}$ (III) and cis-[(Ph₃P)₂Pt(O(O)CCHCHC(O)OH)₂] (IV) in nearly quantitative yield. Thermal and photoinduced decarboxylation of III and IV yields the platina heterocycles (Ph₃P)₂$\text{PtC}{}_6\text{H}{}_4\text{C(O)O}$ (V) and (Ph₃P)₂$\text{PtCHCHC(O)O}$ (VI) with a carbon-platinum σ-bond. Complex VI has been characterized by an X-ray crystal structure determination.     

A new tungsten trioxide hydrate, WO3 · 13H2O: Preparation,characterization, and crystallographic study

A new hydrate of tungsten trioxide, $\text{WO}{}_3\text{·}\text{1}\text{3}\text{H}{}_2\text{O}$ has been obtained by hydrothermal treatment at 120°C of an aqueous suspension of either tungstic acid gel or crystallized dihydrate. This hydrate has been characterized by different methods. A crystallographic study was carried out from X-ray powder diffraction. The hydrate crystallizes in the orthorhombic system: a = 7.359(3) Å, b = 12.513(6) Å, c = 7.704(5) Å, Z = 12. The existence of structural relationships between the hydrate, $\text{WO}{}_3\text{·}\text{1}\text{3}\text{H}{}_2\text{O}$, and the product of dehydration, hexagonal WO₃, has permitted us to propose a structural model in agreement with the experimental data. $\text{WO}{}_3\text{·}\text{1}\text{3}\text{H}{}_2\text{O}$ must be regarded as an interesting compound because its dehydration leads to a new anhydrous tungsten trioxide, hexagonal WO₃.     

A single-crystal study of eight-layer barium niobium lithium oxide,Ba4Nb3LiO12

A single-crystal study of a sample of Ba₄Nb₃LiO₁₂ provided by Dr. T. Negas has been carried out and confirms the |(4)|(4)| layer stacking scheme (Zhdanov notation) for the eight BaO₃ layers per unit cell. Of the eight MO₆ octahedra per cell (M = Nb or Li), four share faces in pairs, and these pairs are linked by pairs of corner-sharing MO₆ octahedra. The compound has an hexagonal cell of dimensions a = 5.777 ± 0.006 Å and c = 18.95 ± 0.03 Å, probable space group $\text{P6}{}_3\text{mmc}\text{, Z = 2}$. The theoretical density is 6.22 g/cm³; within the limit of error of the pycnometrically measured density, 6.08 ± 0.06 g/cm³. The study was carried out with 620 independent reflections, of which 437 were considered observed, collected by automated counter methods and refined by least-squares to a conventional R value of 0.076.     

A contribution to the structural chemistry of A-type rare earth sesquioxides

La₂O₃ and Nd₂O₃ have been annealed at temperatures between 1000 and 1600°C for decontamination. Only products annealed at least at these temperatures appeared to be monophasic. Both X-ray powder and electron single-crystal diffraction revealed the A-type sesquioxide structure. No difference could be found in samples either quenched or slowly cooled to room temperature. Two space groups, $\text{P}\text{3}\text{m1}$ and $\text{P6}{}_3\text{mmc}$, are reported for the A-type structure. X-Ray powder studies seemed to support the latter on the evidence of extinctions. Electron diffraction from single crystals, however, indicated the space group $\text{P}\text{3}\text{m1}$, confirming the so-called Pauling structure, while $\text{P6}{}_3\text{mmc}$ can now be excluded unequivocally.     

Cyclopentadienyl-ruthenium and -osmium chemistry: XXIV. Some complexes containing the olefinic tertiary phosphine 2-CH2CHC6H4PPh2 (sp): X-ray structures of one isomer of OsBr(sp)(η-C5H5), and of Ru(η3-S2CCHMeC6H4PPh2-2)(η-C5H5), containing an η3-dithiocarboxylato group

Reactions between MX(PPh₃)₂(η-C₅H₅) (M = Ru, X = Cl; M = Os, X = Br) and 2-CH₂CHC₆H₄PPh₂ afford $\text{MX(η}{}^2\text{-CH}{}_2\text{CHC}{}_6\text{H}{}_4\text{P}$Ph₂)(η-C₅H₅); the Os complex is obtained in two isomeric forms. The X-ray structure of the major isomer shows the CC double bond (OsC, 2.214, 2.195 Å; CC, 1.57 Å) is almost coplanar with the OsBr vector, with the terminal C cis to Br; the minor isomer is assumed to have the alternative, more sterically congested conformation, with the β-C cis to Br. The chlororuthenium complex reacts with NaOMe/MeOH to give the corresponding hydrido complex, which also exists as two isomers in solution; reaction of this complex with CS₂ gives the expected dithio acid derivative $\text{Ru(S}{}_2\text{CCHMeC}{}_6\text{H}{}_4\text{P}$Ph₂)(η-C₅H₅), together with small amounts of a complex assumed to be $\text{Ru[S}{}_2\text{C(CH}{}_2\text{)}{}_2\text{C}{}_6\text{H}{}_4\text{P}$Ph₂](η-C₅H₅). The X-ray structure of the major product reveals an unusual η³-S₂C mode of coordination of the dithio acid fragment (RuS, 2.418, 2.426(1) Å; RuC 2.175(4) Å). Crystals of OsBr(η²-CH₂CHC₆H₄P)Ph₂)( η-C₅H₅) are monoclinic, space group P2₁/n, with a 12.696(2), b 21.719(6), c 15.929(3) Å, β 79.77(2)°, Z = 8; 2867 data (I > 2.5σ(I)) were refined to R = 0.040, R_w = 0.044. Crystals of $\text{Ru(η}{}^3\text{-S}{}_2\text{CCHMeC}{}_6\text{H}{}_4\text{P}$Ph₂)(η-C₅H₅) are orthorhombic, space group Pbca, with a 8.921(2), b 15.982(9), c 32.216(5) Å, Z = 8; 1685 data (I > 2.5σ(I)) were refined to R = 0.027, R_w = 0.030.     

KxP2W2O16: A bronze with a tunnel structure built up from PO4 tetrahedra and WO6 octahedra

The structure of a $\text{K}{}_{\mathrm{x}}\text{P}{}_2\text{W}{}_4\text{O}{}_{16}\text{(x ? 0.4)}$ crystal was established by X-ray analysis. The solution in the cell of symmetry $\text{P2}{}_1\text{m}$, with a = 6.6702(5), b = 5.3228(8), c = 8.9091(8) Å, β = 100.546(7)°, Z = 1, has led to R = 0.033 and R_w = 0.036 for 2155 reflections with $\text{σ(I)}\text{I}\text{≤ 0.333}$. This structure can be described as two octahedra-wide ReO₃-type slabs connected through “planes” of PO₄ tetrahedra. A new structural family K_xP₂W_2nO_6n+4 can be foreseen which is closely related to the orthorhombic P₄W₈O₃₂ and the monoclinic Rb_xP₈W_8nO_24n+16 series.     

Oxydes de type perovskite du systeme CaNbO

Three perovskite-like phases of the CaNbO system have been isolated, $\text{Ca}{}_{\mathrm{x}}\text{NbO}{}_3\text{(0.9 ? x ? 1),}\text{Ca(Ca}{}_{\mathrm{x}}\text{Nb}{}_{\mathrm{1?x}}\text{)}\text{O}{}_3\text{(0 ? x ?}\text{1}\text{3}\text{)}$, and Ca₃Nb₂O_8?x. X-ray powder diffraction and electron microscopy and diffraction were used to obtain crystallographic data of these phases. The high temperature form of Ca(Ca_xNb_1?x)O₃ perovskites, where calcium ions occupy partially the octahedral sites, presents an orthorhombic symmetry and quasi-periodic twin bands like Ca_xNbO₃ compounds. Two monoclinic forms (for $\text{x =}\text{1}\text{3}$ and $\text{x =}\text{1}\text{4}$) were found by heating Ca(Ca_xNb_1?x)O₃ compounds at lower temperature for long periods; ordering of calcium and niobium ions in octahedral sites is proposed for that structure. The Ca₃Nb₂O_8?x compound presents a quadratic cell: ordering of the cations derived from (NH₄)₃FeF₆ structure is proposed in agreement with the observed space group $\text{P4}\text{nnc}$. A new multiple-phase family, with general formula Ca_nNb_nO_3n+2, was obtained for n = 4.25, 4.5, and 5; electron microscopy shows intergrowth phenomena.