Distortion Of Octahedron Research Articles

Ｌａ２−ｘＳｒｘＮｉＯ４（Ｏ≦ｘ≦１．６）の構造と電気伝導

Structural and electrical properties of the system crystallizing in K2NiF4 structure, La2-xSrxNiO4(O≤x≤1.6) were studied. Powder x-ray diffraction data were analyzed by the Rietveld method to determine the bond lengths. Tetragonality in the lattice parameters, c/a, shows a maximum at x=0.6, while, distortion of a NiO6 octahedron, Ni-0(2)(//c-axis)/ Ni-O(1)(//a-axis), monotonously decreases from 1.17(x=0) to 1.03(x=1.4). A metal-semiconductor transition in observed for O<x<l.2, the transition temperature decreasing from 690K(x=0) to 20K(x=1.2). For x≥1.3, the specimen show metallic behavior down to 10K.

81Br NQR and 119Sn Mössbauer Study for MSnBr3 (M=Cs and CH3NH3)

Abstract Successive phase transitions were observed for CsSnBr3 and CH3NH3SnBr3 by means of 81Br NQR and DTA. These compounds are isomorphous with each other and have typical cubic perovskite structures at room temperature. However, with decreasing temperature both compounds showed successive phase transitions and complex 81Br NQR spectra at 77 K. In the case of CH3NH3SnBr3, both 81Br NQR and 119Sn Mössbauer spectra changed markedly with decreasing temperature from those at room temperature. These findings suggested a large distortion of the SnBr6 octahedron from the regular one and were interpreted on the basis of the three-center-four-electron bond model. In relation to these structural changes, a broad-line 1H NMR experiment on CH3NH3SnBr3 was also observed in order to determine the effect of the dynamic properties of the cation.

C4ν symmetry effects on the electronic structure of BaTiO 3 doped crystals: Local position of the metallic ion (Fe 3+, Co 2+) and distortion of the oxygen octahedron

Owing to the doubtful position along the C 4ν axis of 3 d transition metal impurity Me substituted for the Ti 4+ ion in the ferroelectric barium titanate, molecular orbital calculations are performed by the SCF-Xα method on C 4ν MeO 6 clusters with various positions of the impurity ion Me (Fe 3+ and Co 2+). The electronic structure changes are discussed and the spontaneous polarization is estimated. Similar investigations are carried out for the Me- V 0 pair, where V 0 is an oxygen vacancy.

High temperature X-ray study of single crystal stishovite synthesized with Li2WO4 as flux

Single crystal stishovite with a square prismatic habit and maximum length 0.8 mm was grown from α-quartz at 120 kbar and ∼1,300° C. Li2WO4, chosen as a result of a previous experiment in growing coesite, was also successful as flux for stishovite. Single crystal X-ray structure analysis of the crystals thus obtained has been carried out at high temperatures under ambient pressure. Lattice constant measurements give a larger thermal expansion coefficient along the a-axis than along the c-axis. The bond distances and bond angles show a decreasing distortion of the SiO6 octahedron with increasing temperature. The increasing amplitude of thermal vibrations of oxygen atoms with increasing temperature results in increasing O-O repulsion in the basal plane, which explains the observed crystallographic changes.

EPR detection and study of new phase transitions in Mg(ClO 4) 2·6H 2O

A temperature dependent EPR study of Mn 2+ impurity-doped crystals of Mg(ClO 4) 2·6H 2O has led to the detection of two critical temperatures T 1 ~ 335K and T 2 ~ 324K where the EPR characteristics undergo distinct changes. The monoclinic distortion of the water octahedron around the metal ion shows little temperature dependence between T 1 and T 2 and T 2 and T 3 ~ 272K, while it is found to be comparatively strongly temperature dependent above T 1 and below T 3, decreasing as T approaches T 1 from the high temperature side and as T approaches T 3 from the low temperature side. At room temperature the water-perchlorate symmetry seems to deviate slightly from the perfect P6 3 mc symmetry proposed from previous X-ray measurements by West. Although the space symmetry group of the unit cell appears to remain unchanged at T 1 and T 2, the temperatures T 1 and T 2 are suspected to be related to structural phase transitions in this crystal.

EPR study of a new low-temperature phase transition in Zn(ClO 4) 2·6H 2O

From a temperature variation EPR study of Mn 2+ doped single crystals of Zn(ClO 4) 2·6H 2O phase transition has been detected at T 2 ~290 K. The phase relationships in this crystal are as follows. Phase I transforms at T 2 ~346K to Phase II, which in turn transforms to Phase III at T 2 ~ 290K. The latter exists down to at least 220 K. The space group symmetry of crystal may be the same, i.e. Pmn2 1 both above and below T 2 . The water-perchlorate sublattice symmetry below T 2 is found to be lower than the P6 3 mc symmetry determined previously by X-ray measurements. The onset of a monoclinic or lower symmetry distortion of the water octahedron around a metal ion which starts just below T 2 , is reflected through the observed temperature dependence of the rhombic distortion parameter E. It is felt that during this phase transition a change in the degree of configurational disorder associated with the perchlorate tetrahedra takes place, which in turn modifies the hydrogen bonded interaction in the crystal and consequently results in the onset of temperature dependent displacements of the mean positions of the oxygens of the water molecules.

赤外およびメスバウアー分光法によるチロド閃石―ダンネモル閃石系の陽イオン分布の研究

Cation distribution in seven monoclinic amphiboles of tirodite-dannemo-rite series has been determined by infrared OH streching bands and 57Fe Mössbauer spectra. The results indicate that Mg is concentrated in M2 site rather than in (M1, M3) sites but very few in M4 site, and that Fe2+ is contained more in (M1, M2, M3) sites than in M4 site. This tendency of Fe2+ dis ribution in the tirodite-dannemorite series is considerably different from that in Mg-Fe amphiboles, which is caused by the fact that Mn prefers M4 site rather than (M1, M2, M3) sites. Manganese ion was found to be distributed not only in M4 site but also in (M1, M2, M3) sites except in one sample, in which Mn content was low. The proportion of Mn in M4 site decreases with the increase of total Fe2+, which suggests that there is an intra-crystalline equilibrium of Mn and Fe2+ between (M1, M2, M3) sites and M4 site. The quadrupole splitting of Fe2+ in M4 site becomes smaller with increase of total Fe2+, and this tendency indicates that the distortion of M4 octahedron also increases with the increase of Fe2+.

Character of indirect exchange interactions in lanthanum manganite

AbstractUsing the neutron diffraction and magnetic studies a model for exchange interactions in lanthanum manganite with altering ranges having antiferromagnetic and ferromagnetic ordering in the crystal structure is suggested. The anisotropic exchange interaction of the Mn3+ O2− Mn3+ type, accompanied by the local Jahn‐Teller oxygen octahedron distortions, dominates in antiferromagnetic ranges, while in the ferromagnetic ones there are two types of positive exchange interactions: Mn3+ O2− Mn4+, the mechanism of which has been proposed by Zener, and Mn3+ O2− Mn3+ without the local Jahn‐Teller distortions.

Crystal synthesis and structure refinement of high-pressure ScAlO3 perovskite

Summary Single crystals of ScAlO3 perovskite have been synthesized at 3.5 GPa pressure using lithium fluoride as a high-temperature solvent. The crystal structure has been refined by the least-squares method using 498 three-dimensional reflections to a final R value of 0.028. Unit-cell dimensions at ambient conditions (1 bar, 20° C) are a = 4.9355(3), b = 5.2313(3), c = 7.2007(5) Å, with space group Pbnm. The structure differs slightly from the powder refinement, resulting in less distortion of the aluminium octahedron. The space-group confirmation and the more regular octahedron are used as evidence in support of a centrosymmetric structure for the lower-mantle phase MgSiO3 perovskite.

Infrared spectra of the ammonium ion in crystals: Part V. Tutton's salts: a correlation of vibrational frequency and hydrogen-bonded N⋯O distance for the ammonium ion

The infrared spectra of the isotopically isolated NH 3D + and HDO species have been examined in seven ammonium Tutton salts. The observed spectra are in good agreement with predictions based on the known crystallographic features of these salts. Linear regression of the ND stretching frequencies v 1(NH 3D +) of the isotopically isolated NH 3D + ion on hydrogen-bonded distance d(N ⋯ O) indicated the existence of a correlation ; subsequent fitting of the data to a more plausible empirical function v 1(NH 3D +) = v 1,∞,- k 1 exp(- k 2, d) resulted in a coefficient of determination of 0.94 and a standard deviation of 10 cm −1 for the goodness of fit. The structural differences caused by the distortion of the metal coordination octahedron in the copper(II) Tutton salts are discussed. For this purpose the spectra of isotopically dilute HDO in the salts M 2 i[Cu(H 2O) 6](SO 4) 2 (M i = K, Rb, Cs) have also been measured. No evidence of phase transformations between room and liquid-nitrogen temperatures was detected in the spectra of any of the saltri studied in this work.

Geometrical and structural relations in the rhombohedral perovskites

The rhombohedral perovskites are of interest in lattice dynamics (e.g. LaAlO3, PrAlO3) and for their ferroelectric properties (e.g. LiNbO3, PbZr/TiO3). In this paper, data scattered through the literature are correlated, with correction of some misleading mistakes of calculation. Geometrical descriptions are put in a form allowing comparisons. The structures, classified by their space groups, are described in terms of four structural parameters, the octahedron tilt ω, octahedron distortion d, and A- and B-cation displacements s and t, together with an elongation or flattening ζ of the octahedron, deducible with the help of ω from the interaxial angle αrh. Attention is drawn to the variety of physical causes underlying these departures from ideal perovskite. In the R{\bar 3}c structures a correlation not previously noted in the literature is found between the tilt angle ω and a flattening of the octahedron, and some tentative suggestions are made as to its cause. In the R3m and R3c structures, a lack of general correlation between B-cation displacement and the other parameters is noted, in contradiction to earlier reports.

Influence of the Host Lattice upon EPR Coupling Parameters and d-d Transitions of Planar Copper (II) Complexes

The EPR and optical spectra of two similar square planar CuO2N2 chelates, copper hydroxyquinolinate (CuOx2) and copper picolinate (CuPic2) have been investigated. By studying both molecules in a series of several paramagnetic and diamagnetic single crystal matrices, powders and solutions, the dependence of the g tensor, metal hyperfine (hfs) tensor, nitrogen hfs tensor and the optical d-d transitions on the host lattice was determined. The strong interactions of polar matrix groups with the substituted metal complex cause the major change in the magnetic parameters and the d-d optical transitions of these centrosymmetric planar d9 complexes. In both series of host metal complexes, the copper ligands changed stepwise from a nearly octahedral arrangement (strong axial coordination) to nearly square planar arrangement (negligible axial coordination). This change is accompanied by a total decrease in the Δg parameters of 30%–35%, a total increase in the ligand spin density of 15%, and a total increase in the d-d transition frequencies of 25%. The observed increase in covalency of the singly occupied orbital with increasing axial distortion of the ligand octahedron is in qualitative agreement with results of semiempirical MO calculations. The orthorhombic distortion of the g and A tensors shows significant coorelation with the Fermi contact parameter κ for the copper nucleus. A 2p/2s hybridization ratio of 5.8 for the nitrogen lone pair component of the singly occupied σ orbital has been found for CuOx2, as well as for CuPic2. The nitrogen hfs principal axis (Bx) coincides with one of the in plane g axes (gx) for CuPic2, while the gx and Bx axes differ by 12°–17° in the case of CuOx2. The nitrogen spin density is significantly smaller for CuOx2 than for CuPic2 at equal g shifts, indicating a stronger interaction of the phenolic oxygen in CuOx2 relative to the carboxylate oxygen in CuPic2 with the metal ion. These experimental results show the shortcomings of the simple LCAO model generally used to describe the electronic structure of metal complexes. The use of free ion (or atom) wavefunctions for example leads to large overestimations of the π delocalization parameters and of the amount of g dependence on covalency. In order to obtain agreement with experiment, a scaling parameter (as proposed by K. Ruedenberg in 1962) for the metal 3d part of the involved MO's should be introduced to account for the virtual expansion of the atomic orbitals in antibonding orbitals. Variations of the effective charge appear to be negligible. This expansion results in a smaller spin-orbit constant and a smaller &amp;lt;r−3&amp;gt;3d parameter. This effect contributes substantially to the generally observed reduced spin-orbit and metal hfs interactions in covalent transition metal complexes.

Magnetic properties and structure of hexakisureavanadium tri-iodide

The magnetic susceptibility and magnetic anisotropy of V(urea)6I3 was measured in the temperature range 80–300 K. The results were interpreted in terms of the 3T1g ground term with the set of parameters λ= 45 cm–1, Δ= 450 cm–1, k= 0·5. The structure of the salt was determined by X-ray means at room temperature and at ca. 90 K. It changes only by a small anisotropic contraction of bond lengths. There is an appreciable distortion of the VO6 octahedron by a ‘twist’ about a three-fold axis, which is not changed by temperature. There is also a very slight distortion of the octahedron along this axis, which changes from zero of a very slight compression at room temperature to a slight elongation at the low temperature.

EPR of Ru3+ in LuAlG and LuGaG

EPR data were determined at 4.2°K for Ru3+ impurities located at the octahedral sites of two lutetium garnets and yield the following g values: g‖ = 3.161, g⊥ = 1.136 in LuGaG and g‖ = 2.907, g⊥ = 1.286 in LuAlG. From these data the dependence of the trigonal field parameter υ on polar angle is derived and represented by a quadratic function of the angular distortion of the ideal octahedron. From this dependence it is concluded that the observed trigonal fields can not be explained as arising from the superposition of a positive point-charge field onto a larger negative field of the ligands.

Pyrochlores. IV. Crystallographic and Mössbauer studies of A2FeSbO7 pyrochlores

The lattice parameters and the quadrupole splittings of the room-temperature Mössbauer spectra of the cubic A2FeSbO7 pyrochlores (A = Sm, Eu, Gd, Tb, Y, Er, Lu) have been found to vary linearly with the Templeton–Dauben radii of the A3+ ions. The variation of the quadrupole splitting can be correlated with the positional parameter of the majority oxygen atom, x(O2), which determines the degree of distortion of the BO6 octahedron in cubic A2B2O7 pyrochlores. An increase of 0.1 Å in r(A3+) corresponds to a decrease of ca. 0.20 mm/s in the quadrupole splitting in the A2FeSbO7 series, and to an estimated increase of ca. 0.005 in x(O2). The isomer shift and quadrupole splitting at room temperature are typical of Fe3+(high spin) in octahedral coordination of relatively high distortion. Y2FeSbO7 at 4.2°K was found to exhibit magnetic order with a hyperfine magnetic field of approximately 400 kOe. An attempt to prepare a cubic La2FeSbO7, pyrochlore by the standard procedure was not successful.

Crystallographic studies of some mixed manganite spinels

A crystallographic study has been made of solid solutions of normal, cubic copper manganite with normal, tetragonally distorted manganites of Mn 2+, Co 2+, Cd 2+, Zn 2+ and Mg 2+. The compositions may be represented by A x 2+ Cu 1− x 2+[ Mn 3+ Mn 3+] O 4, and the minimum amounts of Cu 2+ required in different cases for tetragonal to cubic transition have been determined as 58, 53, 48, 83 and 28 mole per cent, respectively. Considering these values as a measure of the stability of the distorted lattice, it has been concluded that the relative contribution of the A-sited cations in stabilizing tetragonal distortion at the B-site follows the order Zn 2+ > Mn 2+ > Co 2+ > Cd 2+ > Mg 2+. A plot of the cube root of the volume of the unit cell against compositions shows an inflexion at the transition region indicating that in the cubic region the distortion of the individual octahedron is suppressed by the strain energy of the lattice. At the transition point, cooperative elongation and ordering of the C-axis take place overcoming the strain imposed by the surrounding cubic lattice and thus a tetragonal distortion appears.

Spectrum of ReF6

The value of the spin-orbit coupling constant, ζ, for Re6+ in ReF6 is discussed in this paper. Moffitt et al. have tentatively assigned a value 3500 cm-1 to ζ for the series of compounds from ReF6 to PtF6. However, a detailed analysis of the OsF6 and IrF6 spectra indicates that ζ has values near 3200 and 3700 cm-1 for these two compounds. Since ζ increase in the sequence Re, Os, Ir, it is probably smaller than 3000 cm-1 in ReF6. If one assumes that there is a Jahn-Teller distortion of the fluorine octahedron, the optical absorption data on ReF6 can be reconciled with this smaller value of ζ.