Stability of layered bismuth compounds in relation to the structural mismatch

The relation between the stability and the structural mismatch in layered bismuth compounds, (Bi₂O₂)²⁺(A_n?1B_nO_3n+1)^2?, was formulated on the basis of an elastic model. The pseudo-tetragonal lattice parameter, $\text{a}$, of layered bismuth compounds was estimated from the following equation,

$\text{a}\text{=}\text{[}\text{a}{}_{\mathrm{<mtext>B</mtext>}}\text{′}{}^2\text{ap}\text{′}{}^2\text{(nK+1)}\text{(}\text{ap}\text{′}{}^2\text{+}\text{a}{}_{\mathrm{<mtext>B</mtext>}}\text{′}{}^2\text{nK}\text{)]}$

$\text{1}\text{2}$

where a_B′ is the lattice parameter of the unconstrained Bi₂O₂ unit, a_P′ the lattice parameter of the unconstrained perovskite-like unit, n the number of perovskite like layer in one structural unit, and K a constant. The change of the strain energy for ionic substitutions was estimated from the elastic relationships. It was found that the increase of n in certain component systems causes the increase of the lattice parameter, $\text{a}$, and the increase of the strain energy. This provides an explanation for the existence of maximum of n. New compounds, Pb₃Bi₄Ti₆O₂₁ ($\text{a}\text{=5.476}$, $\text{b}\text{a}\text{=1.000}$ and $\text{c}\text{=58.1}\text{A}\text{?}$) and Pb₄Bi₄Ti₇O₂₄ ($\text{a}\text{=5.485}$, $\text{b}\text{a}\text{=1.000}$ and $\text{c}\text{=66.2}\text{A}\text{?}$) were described.     

Preparation and structure of sodium rare-earth sulfides,NaLnS2 (Ln; rare earth elements)

Ternary sodium rare-earth sulfides, NaLnS₂ (Ln; rare earth elements), have been prepared and characterized. With an x-ray diffraction analysis of their crystal structure, it has been found that the materials containing La, Ce, and Pr crystallize in a cubic structure, including a Th₃P₄-type compound as a second phase, while the other NaLnS₂ sulfides - except for NaNdS₂ and NaSmS₂ - crystallize in a rhombohedral NaHF₂-type structure, which can be described with a hexagonal unit cell. In NaNdS₂ and NaSmS₂ these two phases, the cubic and the rhombohedral, coexist. The following relationships can be obtained from a comparision of the lattice constants of the cubic and hexagonal cells in NaNdS₂; $\text{a}{}_{\mathrm{<mtext>h</mtext>}}\text{=}\text{a}{}_{\mathrm{<mtext>c</mtext>}}\text{√2}\text{2}$, c^h=a_c2 √3. Based on these relationships, the structural type of the cubic phase can be regarded as analogous to that of NaCl.     

Intercroissances des structures de type perovskite et SrO deficitaires en oxygene : Les oxydes Ln2−xSr1+xCu2O6−x/2 (Ln = Sm,Eu, Gd)

New oxides Ln_2?xSr_1+xO_6?x/2 (Ln = Sm, EU, GD), corresponding to oxygen deficient intergrowths of double perovskite and SrO layers have been isolated for 0.70 ≤ x ≤ 0.90. They are characterized by an orthorhombic cell, $\text{a}\text{?}\text{a}{}_{\mathrm{p}}\text{? 3.9}\text{A}\text{?}$, $\text{b}\text{?}\text{3a}{}_{\mathrm{p}}$, and $\text{c}\text{? 20}\text{A}\text{?}$. A structural model has been obtained, showing that this structure, although closely related to that of La_2?xSr_1+xCu₂O_6?x/2+δ exhibits a different distribution of the oxygen vacancies, involving for copper several coordinations. The semi-conductive properties of these compounds, very different from the semi-metallic behaviour of La_2?xSr_1+xCu₂O_6?x/2+δ is explained by the distribution of the oxygen vacancies in the structure.     

Syntheses and magnetic properties of Eu3B2O6 and Sr3B2O6

Europium orthoborate and strontium orthoborate crystallize in the rhombohedral system with two formula units in a cell of dimensions $\text{a}{}_{\mathrm{R}}\text{=6.697}\text{A}\text{?}$, α_R=85.17° for Eu₃B₂O₆, and $\text{a}{}_{\mathrm{R}}\text{=6.695}\text{A}\text{?}$, α_R=85.00° for Sr₃B₂O₆. The equivalent hexagonal lattice parameters are $\text{a}{}_{\mathrm{H}}\text{=9.069}\text{A}\text{?}$, $\text{c}{}_{\mathrm{H}}\text{=12.542}\text{A}\text{?}$, and $\text{a}{}_{\mathrm{H}}\text{=9.046}\text{A}\text{?}$, $\text{c}{}_{\mathrm{H}}\text{=12.566}\text{A}\text{?}$ respectively. Eu₃B₂O₆ appears to be ferromagnetic below 7.5K.     

The SrFeO2.5 | SrFeO3.0 system. Evidence of a new phase Sr4Fe4O11 (SrFeO2.75)

Evidence is presented for a new phase of ideal composition SrFeO_2.75 in the system SrFeO_3?x (0.5 ? x ? 0). Electron diffraction evidence suggests orthorhombic or body-centred tetragonal symmetry with lattice parameters related to the cubic perovskite lattice parameter a_c by $\text{a}\text{? 2√2}\text{a}{}_{\mathrm{c}}$, $\text{b}\text{? 2}\text{a}{}_{\mathrm{c}}$. $\text{c}\text{? 2√2}\text{a}{}_{\mathrm{c}}$. A plausible model for the structure is presented which is related to that of SrFeO_2.5. Powder x-ray diffraction photographs reveal only a simple cubic perovskite cell for SrFeO_2.75, presumably because the ordered domains giving rise to the superstructure observed in electron diffraction are small in extent. At other intermediate compositions slow cooling gives mixtures of the new phase and either SrFeO_2.5 or SrFeO₃. The two-phase regions narrow at high temperatures and thermogravimetric studies at 1 atm pressure of oxygen indicate a single phase between SrFeO_2.5 and SrFeO_2.75 between 500°C and 950°C.     

The crystal structures of Ba2LaRuO6 and Ca2LaRuO6

Neutron diffraction experiments have been performed to determine the structures of Ba₂LaRuO₆ and Ca₂LaRuO₆. Both are ordered, distorted perovskites. Ba₂LaRuO₆ is monoclinic, space group $\text{P}\text{2}{}_{\mathrm{<mtext>1</mtext>}}\text{n}$ with $\text{a}{}_0\text{=6.0285(7)}\text{A}\text{?}$, $\text{b}{}_0\text{=6.0430(7)}\text{A}\text{?}$, $\text{c}{}_0\text{=8.5409(6)}\text{A}\text{?}$, β=90.44(1)^o. The A sites are occupied by barium and the B sites by an ordered arrangement of lanthanum and ruthenium. Ca₂LaRuO₆ is triclinic, space group P1 with a₀=5.6179(5), b₀=5.8350(5), c⁰=8.0667(4), α=90.0^o, β=89.76(1)^o, γ=90.0^o. The A sites are occupied by calcium and lanthanum in a disordered manner, and the B sites are occupied by an ordered arrangement of calcium and ruthenium. The results reported in this paper thus contradict those of previous workers. The low-temperature magnetic structures are discussed briefly.     

The conversion from CrO2 into orthorhombic CrOOH

The reaction, $\text{2}\text{CrO}{}_2\text{+}\text{H}{}_2\text{O}\text{→ 2}\text{CrOOH}\text{+}\text{1}\text{2}\text{O}{}_2$, was confirmed to be a topotactic reaction by X-ray diffraction technique using a single crystal of CrO₂ as starting material. The crystallographic axial relation between CrO₂ and CrOOH was as follows: ^c(CrO₂^//c(CrOOH), ^a(CrO₂)^∧a(CrOOH)^{= a}(CrO₂)^∧b(CrOOH)^{= 1.7°}.     

Strontium metaniobate: Its crystallography,polymorphism, and isomerism

Three polymorphic forms of SrNb₂O₆ are recognized. The monoclinic form has been prepared and studied by x-ray powder diffraction techniques and differential thermal analysis. The results of this study show that SrNb₂O₆ crystallizes in a pseudo-orthorhombic unit cell with $\text{a}{}_{\mathrm{o}}\text{= 10.99}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 7.70}\text{A}\text{?}$ and $\text{c}{}_{\mathrm{o}}\text{= 5.66}\text{A}\text{?}$. Its crystal chemical relations to the other meta-niobates and its thermodynamic stability are discussed.     

New phase with K2NiF4 structure in the LaLiFeO system

The existence has been proved, for the first time, in the LaLiFeO system of the phase (La_1.75Li_.25) (Fe_.5Li_.5)O_4?x with tetragonal symmetry ($\text{a}{}_{\mathrm{o}}\text{=3.765±0.001}\text{A}\text{?}$; $\text{c}{}_{\mathrm{o}}\text{=12.918±0.01}\text{A}\text{?}$; $\text{c}{}_{\mathrm{o}}\text{a}{}_{\mathrm{o}}\text{=3.43}$) and K₂NiF₄-type structure.This phase gives probably the first example of lithium distribution in both A and B sites of A₂BO₄-type mixed oxides. The high $\text{c}{}_{\mathrm{o}}\text{a}{}_{\mathrm{o}}$ ratio suggests a strong elongation of FeO₆ octahedra induced by a high spin Fe(IV) d⁴ configuration. The results of structural studies and of Mössbauer analysis, confirming the above conclusions, are reported.     

Synthesis and electrical properties of Ln2(V43W23)O7 pyrochlores

Mixed oxides of the type $\text{Ln}{}_2\text{(V}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{W}{}^{\mathrm{6+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{)O}{}_7$, Ln=GdLu, Y have been synthesized in vacuum via solid state reactions of the constituent oxides at 1150–1300°C. The compounds, which crystallize in cubic pyrochlore structure ($\text{a ～ 10.2}\text{A}\text{?}$), have been characterized by X-ray, density and wet chemical analysis. Electrical resistivity and Seebeck coefficient measurements show the phases to be p-type extrinsic semiconductors with low energies of activation possibly exhibiting hopping-type behavior.     

Determination de la structure cristalline de la variete a des hydroxycarbonates de terres rares LnOHCO3 (Ln = Na)

A probable model for the structure of the orthorhombic A - type neodymium hydroxycarbonate is presented. It relies on an optical determination of the site symmetry of OH^?, CO^2?₃, and Nd³⁺ atoms from infrared and visible absorption spectra, together with a computation of 50 X-ray diffraction lines intensities of a high resolution Guinier orthorhombic powder pattern ($\text{a}\text{= 4,953}\text{A}\text{?}$, $\text{b}\text{= 8,477}\text{A}\text{?}$, $\text{c}\text{= 7,210}\text{A}\text{?}$; space group Pmcn (n^o62)). The CO^2?₃ groups lies flat between mettalic planes as in the aragonite type of structure and are linked to OH^? by hydrogen bond ($\text{d(CO}{}^{\mathrm{2?}}{}_3\text{?}\text{OH}{}^{\mathrm{?}}\text{) = 2,52}\text{A}\text{?}$). The rare earth coordination is nine fold: two OH^? groups being closer (2,58 Å) than the seven oxygen from the CO^2?₃ groups (2,58 to 2,70 Å).     

On the pyrochlore type Ln2V2O7 (Ln: Rare-earth elements)

Pyrochlore type rare-earth vanadites (Ln₂V₂O₇) were prepared and some physical properties were investigated. Ln₂V₂O₇ (Ln:Tm, Yb or Lu) crystallized in the cubic space group $\text{O}{}_{\mathrm{<mtext>h</mtext>}}\text{=}\text{Fd}{}_3\text{m}\text{,}\text{a}{}_0\text{= 9.9575}\text{A}\text{?}\text{, 9.9346}\text{A}\text{?}\text{and}\text{9.9231}\text{A}\text{?}$ for Tm₂V₂O₇, Yb₂V₂O₇ and Lu₂V₂O₇, respectively. These compounds were all n-type semiconductors and paramagnets in the temperature range 90–300K.     

Ordering of interstitial nitrogen and oxygen in vanadium near to V8N and V8O

The ordered structure of the V₈N subnitride was studied by X-rays, electron diffraction and electron microscopy. V₈N exists in two different modifications (α′ and α″). The vanadium sublattice of both phases is pseudo-tetragonal, in reality triclinic, with lattice parameters: $\text{α}{}_{\mathrm{<mtext>o</mtext>}}\text{=}\text{b}{}_{\mathrm{o}}\text{= 3.114}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 2.994}\text{A}\text{?}$, α_o = β_o = 90.5^o ± 0.1^o, γ_o = 90^o. The proposed unit cell of α′-V₈N has dimensions: a = 2√2 a_o, c = 2c_o and corresponds to V₃₂N₄. Doublets of nitrogen atoms occupy two sets of octahedral cavities whose shortest axes are aligned along the x and y directions of the sublattice in an ordered fashion. The α″-V₈N phase is a periodically twinned modification of the α′-V₈N, the twin plane being of the (001) type. The V₈O suboxide has the same structure as the α″-V₈N, the sublattice parameters being: $\text{a}{}_{\mathrm{o}}\text{=}\text{b}{}_{\mathrm{o}}\text{= 3.11}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 2.994}\text{A}\text{?}$, α_o = β_o = 90.3^o ± 0.1^o, γ_o = 90^o.     

Crystal structures and crystal chemistry in the system Na1+xZr2SixP3−xO12

As part of a search for skeleton structures for fast alkali-ion transport, the system Na_1+xZr₂Si_xP_3?xO₁₂ has been prepared, analyzed structurally and ion exchanged reversibly with Li⁺, Ag⁺, and K⁺ ions. Single-crystal x-ray analysis was used to identify the composition NaZr₂P₃O₁₂ and to refine its structure, which has rhombohedral space group R3?c with cell parameters $\text{a}{}_{\mathrm{<mtext>r</mtext>}}\text{= 8.815(1)}\text{A}\text{?}$ and $\text{c}{}_{\mathrm{<mtext>r</mtext>}}\text{= 22.746(7)}\text{A}\text{?}$. A small distortion to monoclinic symmetry occurs in the interval 1.8 ≤x≤ 2.2. The structure for Na₃Zr₂Si₂PO₁₂, proposed from powder data, has space group $\text{C}\text{2}\text{c}$ with $\text{a}{}_{\mathrm{<mtext>m</mtext>}}\text{= 15.586(9)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{<mtext>m</mtext>}}\text{= 9.029(4)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{<mtext>m</mtext>}}\text{= 9.205(5)}\text{A}\text{?}$, and β = 123.70(5)° Both structures contain a rigid, three-dimensional network of PO₄ or (SiO₄) tetrahedra sharing corners with ZrO₆ octahedra and a three-dimensionally linked interstitial space. Of the two distinguishable alkali-ion sites in the rhombohedral structure, one is completely occupied in both end members, the occupancy of the other varies across the system from 0 to 100 percent. Several properties are compared with the fast Na⁺-ion conductor β-alumina.     

A series of rare earth silicates RE3+2M3+[SiO4]2 (OH)

A series of isotypic silicates of composition RE₂M[SiO₄]₂ (OH) with RE = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, and M = Al³⁺, Fe³⁺ has been synthesized under hydrothermal conditions. Lattice constants of two members as determined from single crystal X-ray diffraction data are: La₂Al[SiO₄]₂ (OH) (La₂Fe[SiO₄]₂ (OH)) $\text{a}{}_{\mathrm{o}}\text{= 7.401 (7.346)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 5.702 (5.862)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 17.072 (97.196)}\text{A}\text{?}$, gb = 112.4 (112.5°), $\text{P}\text{2}{}_1\text{c}$, Z=4.     

High pressure synthesis of Gd3InO6, Tb3InO6 and Lu3InO6

Three new compounds, X₃InO₆ (X = Gd, Tb, Lu) were prepared at 1–4 GPa, ～1050°C. They are monoclinic, space group $\text{P}\text{2}{}_1\text{n}$ with $\text{a}\text{? 8.9}\text{A}\text{?}$, $\text{b}\text{? 9.8}\text{A}\text{?}$, $\text{c}\text{? 5.9}\text{A}\text{?}$, β ～ 95° and Z = 4.