Structural aspects of the giant oxygen isotope effect in perovskite manganese oxides

The structure of manganites R _1?x A _x MnO₃ (R = La, Sm, or a mixture of rare earth cations; A = Ca, Sr; x = 0.3, 0.5) has been studied by neutron diffraction to find out the reasons for the giant oxygen isotope effect—the transition from a low-temperature metallic state to an insulating state as a result of replacement of ¹⁶O with ¹⁸O. It is shown that this effect is observed in compositions in which the two phases P1 and P2 coexist at low temperatures. They have the same crystal symmetry (sp. gr. Pnma) but different types of Jahn-Teller distortions of oxygen octahedra and different magnetic structure. Phase P1 has a conductivity of the metallic type, weakly distorted MnO₆ octahedra, and a ferromagnetic structure. Phase P2 is insulating, with MnO₆ octahedra extended or compressed in the apical direction and the moments of Mn ions forming an antiferromagnetic structure. The relative volume of phases P1 and P2 in samples depends on the average radius of the A cation and changes occurring upon replacement of ¹⁶O with ¹⁸O. The percolation transition from the metallic to insulating state upon substitution of ¹⁶O with ¹⁸O is caused by the sharp decrease in the volume of the ferromagnetic metallic phase in favor of the insulating antiferromagnetic phase. An effect of the sample microstructure on the formation of the two-phase state is found.     

Correlation between the lattice parameters of crystals with perovskite structure

The correlation \((\bar a_2 + \bar a_3 ) = (\bar a_1 + \bar a_4 )\) has been established for the mean parameters of the reduced perovskite unit cells, \(\bar a_1 ,\bar a_2 ,\bar a_3 \) and \(\bar a_4 \) of four arbitrary compounds with the perovskite structure having the compositions (1) ABX₃, (2) A′BX₃, (3) AB′X₃, and (4) A′B′X₃, where A, A′ and B, B′ are the pairs of metal atoms with equal valences and X is an oxygen or halogen atom. The above correlation is consistent with the model of harmonic quasi-elastic atomic interactions.     

X-ray spectroscopic investigation of the structure of silica,silicates and oxides in the crystalline and vitreous state

The Si Kα_1,2 emission lines and their satellite lines α′, α₃ and α₄ were measured for several samples of vitreous silica (Suprasil W, Infrasil, Suprasil), sodium silicate glasses (8, 15, 20 and 25 wt% Na₂O), and crystalline Mg₂SiO₄ (Forsterite). The observed shifts of the peak positions indicate a systematic increase of the electron density on the silicon atoms with increasing break-up of the SiO₂ network by OH^? or alkali ions. These results are compared with information from the corresponding Si Kβ and O K emission bands and also with the O K emission bands from quartz, MgO and Al₂O₃. They are discussed on the basis of the MO theory and are compared with the characteristic physical properties and structure of silica and silicate glasses. Both the O K and Si Kβ emission bands are closely related to the electronegativities of the relevant metal atoms of the oxides and glasses.     

Group-theoretical study of cationic ordering in perovskite structure

Atomic ordering in the structure of cubic perovskite has been investigated by the group-theoretical method. The possibility of existence of 13 ordered phases in the 1(a) position (including four binary, two ternary, and five quaternary cationic superstructures) and 13 phases with different types of ordering in the 1(b) position (including four binary, two ternary, and five quaternary cationic superstructures) is established. Calculated structures of some types of ordered low-symmetry perovskite modifications are reported.     

Strain of interatomic bonds and multiple-well potential in crystals with perovskite structure

A method for determining the strain characteristics of interatomic bonds in crystals of ABX₃ compounds with perovskite structure is developed. The bond strain energy ΔU _s is estimated. This energy is responsible for the formation of ordered lattice distortions related to the rotation of octahedra and/or the cooperative displacement of cations. It is shown that ΔU _s correlates with the characteristics of ordered distortions and the temperature of the phase transition of compounds to the cubic structure. It is ascertained that the interatomicinteraction potential in crystals with strained bonds is a local multiple-well potential, the form of which depends on the nature of interacting atoms. It is shown that the occurrence of the ferroelectric state in the noted crystals may be due to the presence of bond strains and the anisotropy of the covalent component of the cation-anion interaction.     

Unstrained bond lengths and corresponding cation radii in crystals with perovskite structure

A method for determining average lengths of unstrained bands A-X (l_0AX) and B-X (l_0BX) and the ratio of the rigidity constants of these bonds for ABX₃ compounds with perovskite structure is proposed. The values of l_0AX and l_0BX correspond to the minimum energies of cation-anion interaction of the crystal sublattices. Values of l_0AX and l_0BX are obtained for several groups of halide and oxide compounds: A⁺B²⁺F₃, Cs⁺B²⁺Cl₃, A⁺B⁵⁺O₃, A²⁺B⁴⁺O₃, and A³⁺B³⁺O₃. It is ascertained that, for most compounds studied, the values of l_0AX and l_0BX are equal or close to the interatomic distances in crystals of binary compounds. The values of l_0AX and l_0BX are compared with the sums of the radii of the corresponding cations (R _A , R _B ) and anions (\(^{VI} R_{O^{2 - } } ,^{VI} R_{F^ - } ,^{VI} R_{Cl^ - }\)). It is found that the differences \(l_{0AO^ - } ^{VI} R_{O^{2 - } } (L_{0AF^ - } ^{VI} R_{F^ - } )\) and \(l_{0BO^ - } ^{VI} R_{O^{2 - } } (l_{0BF^ - } ^{VI} R_{F^ - } )\), regarded as the radii of the A and B cations in unstrained bonds, are close to the Shannon radii for a coordination number of six. It is shown that the rigidity constant for A-X bonds is several times smaller than that for B-X bonds.     

Relationship between the rotation angles of octahedra and bond-strength energy in crystals with perovskite structure

The dependence of the strain energy of anion-cation bonds on the sum Ψ² of squares of the angles of ordered rotations of octahedra around the coordinate axes of the reduced cell has been determined for ABX ₃ compounds with perovskite structure in the two-particle approximation. A relation determining the change in the thermodynamic Gibbs potential with allowance for the change in the bond-strain energy in the dependence on Ψ² is proposed. An expression for the equilibrium value of Ψ² as a function of the bond-strain energy and temperature is obtained. Analysis of Ψ² is performed for a homologous series of LnVO₃, LnFeO₃, and LnAlO₃ compounds (Ln are rare earth elements) and a series of CaBO ₃ and SrBO₃ compounds has been performed. It is shown that the calculated values of Ψ² for each of these series of compounds are in agreement with the values experimentally obtained from the structural data. The transition temperatures of the compounds belonging to the series studied into the cubic phase are estimated and compared with the experimental data.     

Combined atomic ordering in the A and B sublattices of perovskite structure

The simultaneous ordering of cations in the A and B sublattices of cubic perovskite structure has been investigated by the group-theoretical method. It is found that 147 ordered phases may exist. Among them, there may be 121 phases with simultaneous cation ordering in the 1(a)- and 1(c) positions of perovskite structure. It is shown that 53 phases are described by improper order parameters related to cation ordering, the formation of 56 phases is related to improper rotations of octahedra, and 15 phases are improper ferroelectrics. Calculated structures of some types of ordered low-symmetry perovskite modifications are presented.     

Optical properties of oxides and fluorides with a rutile-type structure in terms of the point-dipole model

The polarizabilities of ions in the MgF₂, ZnF₂, TiO₂, and SnO₂ compounds have been calculated based on the point-dipole model. It is shown that cation polarizabilities produce a stronger effect on the bire-fringence of AX ₂(X = F, O) compounds than anion polarizabilities.     

Study of cation ordering and magnetic phase transitions in ternary Fe-containing perovskite oxides by Mössbauer spectroscopy

Nuclear gamma-resonance (NGR) spectra were studied in ceramics of ternary perovskite-type oxides Pb(Fe_2/3W_1/3)O₃, Pb(Fe_1/2Ta_1/2)O₃, and Pb(Fe_1/2Nb_1/2)O₃ and in the Pb(Fe_1/2Sb_1/2)O₃ oxide synthesized under pressure. A singlet corresponding to compositional-ordering regions was observed in the NGR spectra for Pb(Fe_1/2Sb_1/2)O₃. Weak doublets associated with paramagnetic regions (in which the degree of cation ordering is higher than the volume average) were observed in the spectra for Pb(Fe_2/3W_1/3)O₃ and Pb(Fe_1/2Nb_1/2)O₃ at temperatures 30–70 K below the antiferromagnetic phase transition point.     

Specific features of growth of single crystals of complex oxides with an oxygen deficit

Some methods for estimating the concentration of vacancies in crystals and procedures for decreasing their number are considered by the example of LiNbO₃, SrTiO₃, YAlO₃, and PbWO₄ crystals. In LiNbO₃ crystals, nonstoichiometry was determined by comparing the density determined via X-ray diffraction with the experimental value. In SrTiO₃ and YAlO₃ crystals, the vacancy concentration was estimated with methods based on additivity of refractions. The vacancy concentration in the LiNbO₃ and PbWO₄ crystals was decreased by doping them with elements of lower valence and annealing in air and vacuum. Some specific features of the growth of PbWO₄ crystals affecting their quality are discussed. Peculiarities of the formation of lithium niobate, strontium titanate, yttrium orthoaluminate, and lead tungstate crystals are considered.     

Molecular and crystal structure of the echinochrome complex with dioxane

The molecular and crystal structures of a complex of echinochrome (2,3,5,6,8-pentahydroxy-7-ethyl-1,4-naphthoquinone) with dioxane are determined. The molecular complex 2(C₁₂H₁₀O₇) · C₄H₈O₂ crystallizes in the form of dark red transparent thin plates. The crystal data of the compound are as follows: a = 4.563(1) Å, b = 6.382(1) Å, c = 22.812(4) Å, α = 91.955(5)°, β = 90.408(5)°, γ = 104.801(4)°, space group $P\overline 1 $ , and Z = 2. The crystal structure is solved by direct methods and refined to R ₁ = 0.0656 and wR ₂ = 0.1196.     

Synthesis and structure of iridium complex with thiocarbamide

A single crystal of the [Ir(Thio)₂Cl₄][Ir(Thio)₄Cl₂] compound synthesized by the reaction of K₃[IrCl₆] with thiocarbamide (Thio, SC(NH₂)₂) in a microwave field is investigated using X-ray diffraction. The compound crystallizes in the monoclinic crystal system with space group Cc(C _s ⁴ ). The unit cell parameters are as follows: a = 13.554(1) Å, b = 8.251(1) Å, c = 24.992(2) Å, β = 92.58(1)°, V = 2791.87(10) ų, and Z = 4. The compound has an island structure with two crystallographically independent iridium atoms. Thiocarbamide is coordinated to the central atom through the sulfur atom. The coordination sphere of the Ir(1) atom involves two Cl atoms and four S atoms, whereas the coordination sphere of the Ir(2) atom consists of four Cl atoms and two S atoms. The assignment of the bands in the IR absorption spectrum of the synthesized compound is presented. The thermal behavior of the compound in air is investigated.     

X-ray crystal structure of the complex of enniatin B with KNCS

The crystal structure of the complex of enniatin B, cyclo[-(L-MeVal-D-Hyi)₃-](C₃₃H₅₇N₃O₉), with KNCS is determined by X-ray diffraction [CuKαradiation, R = 0.0594 for 7925 reflections with I > 2 σ(I)]. The crystals belong to the space group P3, a = 24.448(5) Å, c = 23.578(5) Å, V = 12277(9) ų, and Z = 6. The unit cell contains 12 symmetrically independent molecules of the antibiotic, which are located on crystallographic threefold axes. The K⁺ ions are located on the threefold axes and are coordinated by the carbonyl oxygen atoms of the hydroxy acid or amino acid residues of the enniatin molecules to form prisms, twisted prisms, and antiprisms. All the independent enniatin molecules retain the principal conformational features revealed earlier in the structures of enniatin B and its complexes.     

Ordering of cations in the voids of the anionic framework of the crystal structure of catapleiite

The pseudohexagonal crystal structure of the mineral catapleiite Na_1.5Ca_0.2[ZrSi₃(O,OH)₉] · 2(H₂O,F) from the Zhil’naya Valley in the central part of the Khibiny alkaline massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0346): a = 20.100(4), b = 25.673(5), and c = 14.822(3) Å; space group Fdd2, Z = 32, and ρ_calcd = 2.76 g/cm³. Fluorine atoms substituting part of H₂O molecules in open channels of the crystal structure have been found for the first time in the catapleiite composition by microprobe analysis. The pattern of distribution of Na and Ca atoms over the voids of the mixed anionic framework consisting of Zr-octahedra and three-membered rings of Si-tetrahedra accounts for the pronounced pseudoperiodicity along the a and c axes of the pseudohexagonal unit cell and for the lowering of crystal symmetry to the orthorhombic one. It is shown that part of the hydrogen atoms of water molecules is statistically disordered; their distribution correlates with the pattern of the population of large eight-vertex polyhedra by Na and Ca atoms.     

Compositions of melts for growth of functional single crystals of complex oxides and other compounds

The melt compositions (M _c) are calculated for growing crystals with valuable physical properties. The calculation is based on the compositions of the invariant points of the liquidus curves for 33 congruently and 12 incongruently melting solid phases of 42 fusibility diagrams of binary systems. These systems include Na, Ca, Ba, Mg, and Y aluminates; Bi and Pb germanates; Li, K, Ba, and Bi borates; Ba, Fe, Sr, and Bi titanates; Li, K, Cs, Ba, Zn, Ca niobates; Li, Pb, and Gd molibdates; Pb and Nd tungstates; etc. More than 60 studies with data on the experimentally found melt compositions (M _e) for growing the noted crystals are analyzed. It is shown that the melt compositions M _c and M _e for growth of congruently and incongruently melting crystals are similar. Large-size stoichiometric crystals of high optical quality are grown using these melt compositions. Nonstoichiometric crystals of low structural quality are grown from melt compositions either corresponding to the stoichiometric ratio of the components (M _s) or similar to the compositions at invariant points (M _i). In these cases, a large difference is observed between the melt compositions M _c, M _s, and M _e.     

Crystal structure of an yttrium nitrate complex with tetramethylurea

An X-ray diffraction study of Y(NO₃)₃ · 3TMU crystals is performed (heavy-atom method, difference electron-density maps, H atoms in calculated positions, full-matrix anisotropic-isotropic (H) least-squares refinement). The crystals are monoclinic, a = 9.353(1) Å, b = 15.966(3) Å, c = 18.805(8) Å, β = 95.41(2)°, Z = 4, and space group P2₁/c. The structural units of the crystals are molecular complexes. The coordination number of the Y atom is nine due to three bidentate NO₃ groups and three monodentate tetramethylurea molecules. The mean Y-O bond lengths are 2.464 and 2.274 Å for NO₃ and tetramethylurea, respectively.     

Mechanism of formation of barium cuprate-based solid solutions with perovskite-like structure doped with cadmium and yttrium oxides

The influence of Y₂O₃ and CdO content on the phase composition, structure, and electrical properties of Y₂O₃-CdO-BaO-CuO ceramics was studied. The concentration limits of the existence of the orthorhombic perovskite-like Y_xBa_{2 ? x} Cd_yCu_{1 ? y} O_{3 + δ} solid solutions (where 0.20 ≤ x ≤ 0.37 and y ≤ 0.2) and their unit-cell parameters are determined.     

Three-dimensional structure of photosystem II from Thermosynechococcus elongates in complex with terbutryn

Photosystem II is a key component of the photosynthetic pathway producing oxygen at the thylakoid membrane of cyanobacteria, green algae, and plants. The three-dimensional structure of photosystem II from the cyanobacterium Thermosynechococcus elongates in a complex with herbicide terbutryn (a photosynthesis inhibitor) was determined for the first time by X-ray diffraction and refined at 3.2 Å resolution (R _factor = 26.9%, R _free = 29.9%, rmsd for bond lengths is 0.013 Å, and rmsd for bond angles is 2.2°). The terbutryn molecule was located in the binding pocket of the mobile plastoquinone. The atomic coordinates of the refined structure of photosystem II in a complex with terbutryn were deposited in the Protein Data Bank.

     

Molecular structure of diantipyrylphenylmethane and its complex with neodymium nitrate

The structures of diantipyrylphenylmethane (DAPM) and its complex with neodymium nitrate [Nd(NO₃)₃ · DAPM · CH₃OH] · 2CH₃OH are determined by X-ray diffraction. The free ligand adopts a trans conformation with the opposite orientation of the oxygen atoms of the carbonyl groups. In the complex, diantipyrylphenylmethane acts as a bidentate ligand and coordinates the Nd atom through the carbonyl oxygen atoms, thus forming the eight-membered metallocycle. The coordination number of neodymium is nine (six O atoms of the bidentate nitrate groups, two O atoms of diantipyrylphenylmethane, and one O atom of the methanol molecule).