Octahedral Bond Lengths Research Articles

Ion substitutions and structural adjustment in Cr-bearing tourmalines

The geological settings of Cr-bearing tourmaline occurrences are reviewed and the crystal chemistry of Cr-bearing tourmalines is analysed on the basis of new X-ray single-crystal structure refinements ( R 0.019–0.025) of three crystals from the Ural deposits (Cr 2 O 3 content ~6 wt.%) and one from Umba Valley, Tanzania (~1 wt.% Cr 2 O 3 ) and published data (13 structures) on tourmalines from different deposits with Cr content up to 36 oxide wt.%. The boundary between the ordered and the disordered distribution of Cr between Y and Z octahedral sites is determined near 1.03 ± 0.27 Cr apfu and relationships between Cr content and structural parameters (unit-cell parameters, octahedral bond-lengths, charge of Y and Z sites and polyhedral deformation) are established. An increasing Cr content in tourmaline causes an increase of both lattice parameters, of and bond-lengths and a regular variation of the charge of the Y and Z octahedra. In general, the structure becomes less deformed by reducing the distortion of the octahedra and of the tetrahedral ring.

Structural regularities in 2M1 dioctahedral micas: The structure modeling approach

An improved algorithm has been elaborated for computing atomic coordinates in K-dioctahedral micas-2 M 1 from the experimental data on cation composition and unit-cell parameters. The structure modeling procedure is based on regression equations relating the structural features and chemical composition of micas that were obtained from the analysis of published data on 27 refined structures of dioctahedral micas of various compositions including 20 K-dioctahedral micas-2 M 1 , 3 paragonites-2 M 1, 2 margarites-2 M 1 , and 2 celadonites-1 M . The empirical relationships accurately describe the observed structural distortions in dioctahedral micas, such as tetrahedral tilt and rotation, tetrahedral elongation, octahedral flattening, hydroxyl depression, etc. The majority of the regressions have r 2 > 0.8 and p-values M 1 with disordered distribution of tetrahedral and octahedral cations. The estimated standard deviations (e.s.d.) for modeled atomic coordinates vary for different atomic positions and range from 0.0001 to 0.003 (fractional units); the e.s.d. values for structural characteristics obtained from the calculated atomic coordinates are 0.002–0.007 A for mean and individual tetrahedral bond and edge lengths, 0.004–0.013 A for mean and individual octahedral bond and edge lengths, 0.013–0.015 A for K-O distances, and 0.5° for the tetrahedral ditrigonal rotation angle. Computation of atomic coordinates for additional three dioctahedral mica-2 M 1 structures that were not included in the derivation of the empirical structure-composition relationships used in the algorithm yielded close agreement between the modeled and observed structural characteristics. The structure modeling algorithm can be used as an inexpensive and express method for evaluation of fine structural features in large collections of K-dioctahedral mica samples of diverse compositions.

Effect of Mn doping on structural and magnetic susceptibility of C-type rare earth nano oxides Er2−xMnxO3

The manganese doped rare earth oxides Er2−xMnx O3 (0.0≤x≤0.20) were synthesized by a sol–gel process and analyzed by X-ray diffraction using Rietveld refinement methods. A single phase solid solution is formed up to x=0.15 while for x≥0.2 a manganese oxide phase appears in the diffraction pattern. Preferential cationic distribution between the non-equivalent sites 8b and 24d of space group Ia3¯ is found for all samples but to a different extent. The octahedral volume and average bond length of Er1O for 8b site decrease while both octahedral volume and bond length of Er2O for 24d site increase. Magnetization measurements were done in the temperature range 5–300K. The effective magnetic moment μeff is found to decrease with composition parameter x, except for sample x=0.05 where the magnetization is enhanced. The Curie-Weiss paramagnetic temperatures indicate antiferromagnetic interaction.

EFFECT OF JUMP LENGTH OF ELECTRON AND CATION DISTRIBUTION STUDY OF Co1-xZnxFe2-xAlxO4

The zinc and aluminum co-substituted cobalt ferrite of the series Co 1-x Zn x Fe 2-x Al x O 4 were prepared by wet chemical co-precipitation method. The variation of lattice constant and jump length of electron of wet and annealed samples was studied. The variation of lattice constant and jump length of electron of wet and annealed wet samples shows that it increases up to x=0.2 thereafter x>0.2 it decreases in both the cases. In the present work the values of tetrahedral and octahedral bond length (dAx) and (dBx), shared tetrahedral and octahedral edge was also studied. The cationic distribution is calculated on the basis X-ray diffraction method. The cation distribution estimated from X-ray intensity ratio calculations suggest that, Zn 2+ ions occupies only tetrahedral (A) site whereas Al 3+ ions occupies both tetrahedral (A) and octahedral [B] sites.

Zn-O tetrahedral bond length variations in normal spinel oxides

Six synthetic single crystals of spinel phases with different compositions along the ZnAl 2 O 4 -ZnCr 2 O 4 solid solution were structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, unit-cell parameters and octahedral bond lengths (M-O) increase with increasing replacement of Al 3+ by Cr 3+ . Despite the constant occupancy of the T site by Zn, also the tetrahedral bond length T Zn-O shows significant variations along this binary. These variations are positively correlated with variations in M-O bond lengths. The present data in conjunction with data from literature provide a basis for quantitative analyses of the variation in T Zn-O in normal spinel structures. A negative correlation between T Zn-O and the ionic potential at M ( M IP ) suggests that increasing M IP is related to a stronger electrostatic cation-cation repulsion across the shared octahedral edge M (O-O) shared of the structure. An observed negative correlation between M IP and M (O-O) shared suggests that a decrease of M (O-O) shared provides a more efficient shielding effect to reduce the octahedral cation interactions. In normal Zn B 2 O 4 spinels (where B = Al 3+ , Cr 3+ , Ga 3+ , V 3+ , Fe 3+ , and Mn 3+ ) cations with a smaller size provides a higher charge density. Increasing charge density at the M site causes shortening of M (O-O) shared , which in turn results in shorter T Zn-O bond length. In general, variations in T Zn-O are required by the structure to better provide an oxygen shielding effect to the octahedral cation-cation repulsion.

Crystal chemistry of the MgAl2O4-MgMn2O4-MnMn2O4 system: Analysis of structural distortion in spinel- and hausmannite-type structures

Single crystals of spinel and hausmannite having seven different compositions in the MgAl2O4-MgMn2O4-MnMn2O4 system were synthesized and structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, tetrahedral and octahedral bond lengths increase with increasing substitutions of Mn2+ for Mg and Mn3+ for Al, respectively. A transition from cubic to tetragonal symmetry occurs at a critical concentration of Mn3+ > 1.4 atoms per formula unit as a result of the Jahn-Teller distortion around octahedrally coordinated Mn3+. The present data in conjunction with data from the literature provide a basis for quantitative analyses of the cation polyhedral-distortion parameters and their variations in spinel- and hausmannite-type structures ( Fd 3 m and I 41/ amd , respectively). In contrast to the linear correlation between (octahedral quadratic elongation) and σ2M (octahedral bond-angle variance) observed for many silicates and isomorphic structures, these two distortion parameters are not correlated in multiple oxides with spinel- and hausmannite-type structures. By using a model of multiple linear regression, it is demonstrated that varies as a function of both σ2M and ΔM (octahedral bond-length distortion). The degree of octahedral distortion is significant in the spinel structures and is in fact comparable with that calculated for the hausmannite-type structures. The degree of octahedral distortion is related to steric effects in both spinel- and hausmannite-type structures, whereas the electronic effects caused by Mn3+ account for the transition from cubic to tetragonal symmetry.

Crystal chemistry of the magnetite-ulvospinel series

Spinel single crystals of 19 compositions along the magnetite-ulvöspinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations, the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mössbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate, and tetrahedral bond length increase with increasing ulvöspinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation, the Akimoto model T (Fe 3+ 1-X Fe 2+ x )M(Fe 2+ Fe 3+ 1-X Ti X )O 4 describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction T Fe 2+ + M Fe 3+ = T Fe 3+ + M Fe 2+ , which causes M Fe 2+ ≠ 1 and T Fe 2+ /Ti ≠ 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti ≈ 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the T Fe 3+ + M Fe 3+ = T Fe 2+ + M Ti 4+ chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvöspinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs.

Spectroscopic studies of the effect of addition of Y3+ on structural characteristics of Ni–Zn ferrites

AbstractThree groups of Ni–Zn ferrite samples of the general formula; Ni1–xZnxYyFe2–iyO4, were prepared by a double‐sintering ceramic method, in an attempt to examine the solubility of yttrium ions on Ni–Zn ferrite. The theoretical values of the tetrahedral and the octahedral bond lengths, the oxygen parameters and the lattice parameters were calculated. X‐ray diffraction was used to analyze the relationship between structural characterization and Zn2+ ion concentration, indicating good agreement with theoretical calculations. It was observed that the lattice parameters of cubic spinel of the investigated samples are not functions of Y3+ ion concentration, i.e. the process of dissolution of any ratio of yttrium in the lattice never occurs. It is proved that the nonstoichiometric system can redistribute itself to produce a single phase of cubic spinel under a suitable sintering process. The IR spectra obtained at room temperature in the range 1000–300 cm–1 for all compositions show splitting of the two bands υ2 and υ3 and an excessive number of bands besides three fundamental absorption bands υ1 (601–549 cm–1), υ2 (512–406 cm–1), and υ3 (380–305 cm–1). Most of the bands were assigned on the basis of the analysis of structural characterization properties. The splitting of the two bands υ2 and υ3 was discussed as being due to the nature of the octahedral site in the ferrite in general. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Geometric crystal chemical models for structural analysis of micas and their stacking polytypes

A sequence of progressively more realistic geometric crystal chemical models for TOT layers in mica is developed, starting from the usual main uniform distortions (octahedral flattening, tetrahedral rotation, octahedral counter-rotation) toward additional features as they are shown to be required. These additional features include tetrahedral basal flattening and apical bond adjustment, geometric meso-octahedral sheets (having unequal M1 and M2/M3 site bond lengths), and geometric heterooctahedral sheets (having unequal M1, M2, and M3 site bond lengths). A crystal chemical model for the unit cell of a 1M polytype with C2/m space group is developed from geometric homo-octahedral sheets (having equal M1, M2, and M3 site bond lengths) and is described using a minimal number of independent crystal chemical parameters: octahedral, tetrahedral basal, tetrahedral apical, and interlayer metal-anion bond lengths, and flattening angles of octahedral and tetrahedral sheets. The monoclinic lattice parameters (a, b, c, and β) and the tetrahedral rotation angle (α) follow from these assumed parameters. These models are designed to allow analyses (that are reported elsewhere) of both structural and lattice-parameter refinement data in terms of deviations from various predictions based on specified sets of crystal chemical assumptions. Fractional atomic coordinates are derived in terms of the atomic positions for the 1M unit cell of C2/m symmetry for each known homogeneous mica polytype with highest space group symmetry (polytype, space group = 2M 1 , C2/c; 2M 2 , C2/c; 2O, Ccmm; 3T, P3 1 12). These coordinates allow a structural analysis of diffraction data for different stacking polytype structures using the same 1M-type TOT layer as a modular unit.

Structural changes upon the temperature dependent C2/c → P21/c phase transition in LiMe3+Si2O6 clinopyroxenes, Me = Cr, Ga, Fe, V, Sc and In

Abstract Within the Li-clinopyroxene series LiMe3+Si2O6, a temperature induced C2/c → P21/c phase transition has been observed for Me = Cr, Ga, V, and Sc at temperatures of 330(1), 286(1), 205(3) and 234(1) K, respectively. There is no phase transition for the Al3+ and the pure In3+ compound down to 80 K. Within the LiSc1– x In x Si2O6 solid solution se ries, the transition temperature rapidly decreases with increasing In3+ content and drops below 90 K between x = 0.26 and 0.30. The C → P phase transition is found only in samples with nearly fully extended tetrahedral chains. The phase transition in LiScSi2O6, with its “O-rotated” and distinctly more kinked tetrahedral chains in C2/c (O3—O3—O3 angle = 175.7(1)° at 298 K), exhibits a different character: the decay of the b-type Bragg reflections hkl: h + k ≠ 2n and the structural changes in the vicinity of the phase transition are less rapid. The temperature dependent evolution of the order parameter Q2 (as expressed by the decay of the b-type Bragg reflections) suggests a second order thermodynamic character of the C → P phase transition in the Sc3+ compound, whereas it is close to a tri-critical behaviour in the other ones. The structural changes, taking place at the phase transition and below are similar to those in LiFeSi2O6, i.e. dislinkage and appearance of two differently kinked tetrahedral chains in P21/c, decrease of the coordination number of Li+ from 6 to 5, slight alterations of octahedral bond lengths. The structural changes at the phase transition are of similar magnitude in the Ga, Cr, V and Fe compound. LiScSi2O6 is again exceptional. Different thermal expansion of octahedral and tetrahedral sites and increasing site distortion of the M1 site as a consequence of tetrahedral chain kinking in C2/c are assumed to be factors inducing the C2/c → P21/c phase transition.

The ferriannite KFe(3)(2+)(Al(0.26)Fe(0.76)(3+)Si(3))O(10)(OH)(2) at 100 and 270 K.

Unusually large and good-quality single crystals of the synthetic trioctahedral mica KFe(3)(2+)(Al(0.26)Fe(0.76)(3+)Si(3))O(10)(OH)(2) [potassium triiron(II) aluminasilaferrate(III) decaoxide dihydroxide] have been grown hydrothermally. X-ray diffraction data measured at 270 and 100 K have been used to refine the crystal structure, including the positions of the H atoms. This synthetic mica is similar to annite, KFe(3)AlSi(3)O(10)(OH)(2), and crystallizes with the same monoclinic C2/m symmetry. No phase transition has been observed down to 100 K. At low temperature, the ditrigonal distortion of the mica structure increases markedly, while the octahedral and tetrahedral bond lengths tend to decrease and increase, respectively. A detailed comparison of structural parameters in various Fe-rich micas is presented.

Structural variations in the brownmillerite series Ca2(Fe2−xAlx)O5: Single-crystal X-ray diffraction at 25 °C and high-temperature X-ray powder diffraction (25 °C ≤T≤ 1000 °C)

A total of 30 synthetic samples of the Ca2Fe2-xAlxO5, 0.00 ≤ x ≤ 1.34 solid solution series have been investigated by single crystal X-ray diffraction at 25 °C. Pure Ca2Fe2O5 and samples up to x = 0.56 have space group Pnma, Z = 4, whereas samples with x > 0.56 show I2mb symmetry, Z = 4. The substitution of Fe3+ by the smaller Al3+ cation decreases unit-cell parameters and average octahedral and tetrahedral bond lengths and induces distinct changes in the O-atom coordination of the interstitial Ca atom. Discontinuities in the structural parameters vs. the Al3+tot content and changes in slope of these quantities are associated with the phase transition. The essential difference between the two modifications is the cation-O atom-cation angle within the planes of corner sharing octahedra, which is close to 180° in I2mb, but ≈184° in the Pnma phase, and the existence of two different orientations of the tetrahedral chains in Pnma as opposed to one in I2mb. At low overall Al3+ concentrations Al3+ preferentially enters the tetrahedral site until ≈2/3 of it is filled. Additional Al3+ cations, substituted for Fe3+, are equally distributed over octahedral and tetrahedral sites. At high temperature pure Ca2Fe2O5 transforms to a body-centered structure at 724(4) °C. Substituting Al3+ for Fe3+ linearly decreases the transition temperature by 15 °C per 0.1 Al3+ down to 623(5) °C for x = 0.65.

Single-crystal structure refinements and crystal chemistry of synthetic trioctahedral micas KM3(Al3+,Si4+)4O10(OH)2, where M = Ni2+, Mg2+, Co2+, Fe2+, or Al3+

To study structural changes with changing chemical composition in the octahedral sheet of trioctahedral potassium-rich 1M micas, 3 natural and 12 synthetic micas were examined by singlecrystal X-ray diffraction. Samples with Ni2+, Mg2+, Co2+, Fe2+, and Al3+ in the octahedral sheet and with Si4+, Al3+ and one sample with Ga3+ in the tetrahedral sheet were prepared at high temperatures and pressures, which yielded crystallites ≤200 mm in size. For samples with approximately AlSi3 composition of the tetrahedral sheet and with no octahedral Al3+, mean M1-O and M2-O octahedral bond lengths correlate very well with the mean ionic radius of the octahedral cations. For these samples, the M1 and M2 sites are found to be very similar or identical within experimental error in terms of mean M-O bond lengths, bond angles, and polyhedral distortion parameters. Octahedral distortion is negatively correlated with the size of the octahedral cation. Octahedra in Ni2+-mica show the largest deviation from ideal octahedral geometry, whereas those in annite are closest to ideal octahedral geometry, but they are still significantly flattened. Octahedral Al3+ prefers the M2 site. This causes the mean M1-O bond length to decrease less with increasing Al3+ content as compared to the mean M2-O distance. This Al3+ preference for M2 also causes the M2 site to become smaller and more distorted than the M1 site. Refinement of the Mg-Fe ratios in the octahedral sites along the annite-phlogopite join shows that the two cations are statistically distributed over M1 and M2. Tetrahedra are regular and show only a small elongation along c*. Tetrahedral distortion parameters for the AlSi3 micas show no correlation with chemistry of the octahedral layer. However, the mean T-O bond lengths increase slightly with increasing size of the octahedral cation. With decreasing size of the lateral dimension of the octahedral sheet in the (001) plane, the tetrahedral sheet shows increasing ditrigonal distortion. Largest tetrahedral rotation angles are observed for synthetic near end-member siderophyllite with α = 11.5° and for tetra-gallium-phlogopite KMg3GaSi3O10(OH)2 with α = 10.8°.

Structure and compressibility of synthetic ZnAl 2 O 4 (gahnite) under high-pressure conditions, from synchrotron X-ray powder diffraction

The structural behavior of synthetic gahnite (ZnAl2O4) has been investigated by X-ray powder diffraction at high pressure (0–43 GPa) and room temperature, on the ID9 beamline at ESRF. The equation of state of gahnite has been derived using the models of Birch–Murnaghan, Vinet and Poirier–Tarantola, and the results have been mutually compared (the elastic bulk modulus and its derivatives versus P determined by the third-order Birch–Murnaghan equation of state are K0=201.7(±0.9) GPa, K ′0=7.62(±0.09) and K ″0=−0.1022 GPa−1 (implied value). The compressibilities of the tetrahedral and octahedral bond lengths [0.00188(8) and 0.00142(5) GPa−1 at P=0, respectively], and the␣polyhedral volume compressibilities of the four-␣and␣sixfold coordination sites [0.0057(2) and 0.0041(2) GPa−1 at P=0, respectively] are discussed.

57Fe M�ssbauer, Infrared and X-Ray Studies of the System Zn1?xCuxCr0.8Fe1.2O4

Spectral studies of the cubic spinel ferrites Zn1—xCuxCr0.8Fe1.2O4, 0 ≤ x ≤ 1 are carried out at room temperature by X-ray, Mössbauer and IR spectra. The deduced X-ray parameters have been studied as functions of x. The average and true lattice parameters (ā and a0), the theoretical and bulk densities (DX and D), and the porosity P were determined for all samples. The positional oxygen parameter u, the tetrahedral and octahedral ionic radii (rA and rB), the tetrahedral and octahedral bond lengths (dAX and dBX), the tetrahedral and shared and unshared octahedral edges (dAE, dBE and dBEU), have been calculated and deduced. It has been found that ā decreases and DX increases with x ≤ 0.6 and the opposite behaviour takes place with x > 0.6. Except x = 0, D decreases and P increases with x. The parameters a0, u, rA, dAX, dAE and dBEU decrease with x while rB, dBX and dBE increase. The room temperature Mössbauer spectra showed that the samples with x ≤ 0.6 are paramagnetic while the samples with x > 0.6 are ferrimagnetic. The determined hyperfine interaction parameters as functions of x have been studied and discussed. The cation distributions have been determined. The infrared studies confirmed the presence of four peaks. These peaks were ν1, ν2, ν3 and ν4 and assigned to the tetrahedral A-site, octahedral B-sites, divalent ions at B-sites and lattice vibrations, respectively.

Synthesis and Structure Refinement of the Spinel,γ-Ge3N4

γ-Ge 3 N 4 is formed from α-or β-Ge 3 N 4 at pressures greater than 12 GPa and temperatures above 100°C. It has the spinel structure, symmetry Fc3m, and lattice parameter a = 8.2125(1) A. Germanium has both tetrahedral [Ge-N = 1.879(2) A] and octahedral [Ge-N = 1.996(1) A] coordination to nitrogen. The difference between the octahedral and tetrahedral bond lengths in this nitride is close to that expected from systematics, which were largely derived from oxides.

A neutron powder diffraction study of cation ordering in high-temperature synthetic amphiboles

The structures of synthetic fluoro-edenite and pargasite end-members have been refined by Rietveld analysis of neutron powder diffraction data to /?wp values of 1.4 % and 1.9 %, respectively. The quality of the Rietveld refinements is much higher than has been obtained for synthetic amphiboles by X-ray powder diffraction. The distribution of Al and Si over T(l) and T(2) has been determined from mean T(l)-O bond lengths. Pargasite crystallized at 1 kbar, 932 °C has 1.71 ± 0.11 Al apfu at T( 1) and 0.28 + 0.11 Al apfu at T(2), implying at least 15 % long-range Al-Si disorder. Within experimental error fluoro-edenite crystallized at 2 kbar, 1006°C is long-range-ordered, with all A1 at T(l). The size of the error on the long-range Al-Si order parameter (Ψ) scales approximately with 2/Al. This means that an amphibole with one A1 apfu will have an error on Ψ that is approximately twice that for an amphibole with two A1 apfu. Consequently, Al-Si disorder can be quantified more accurately for pargasites than hornblendes. The Al-Si distribution of synthetic pargasite is similar to that of natural high-temperature pargasites. The results of this study indicate that long-range Al-Si order-disorder in pargasites rather than hornblendes is worth investigating as a geothermometer because of the inherently larger errors on Ψ for hornblendes. Mg-Al ordering on octahedral sites in pargasite has been determined from mean octahedral bond lengths. Al is at M(2) and M(3) sites, with little or none at M(l): M(2) = 1.5Mg + 0.5Al and M(3) = 0.5Mg + 0.5Al. These occupancies accord very well with IR and H MAS NMR spectra of synthetic end-member pargasite and are different from those of high-temperature natural pargasites which have M distributed statistically over M(2) and M(3), with none at M(l): M(2) = 1.33M + 0.67M and M(3) = 0.67M + 0.33M. The difference in M(2) and M(3) site occupancies for natural and synthetic pargasites may reflect fundamentally different growth mechanisms. However, both natural and synthetic pargasites show qualitatively similar ordering behaviour for octahedral cations, pointing to a shared crystal chemistry. Key-words: synthetic amphibole, neutron powder diffraction, cation ordering, geothermometry.

Perovskite at high P‐T conditions: An in situ synchrotron X ray diffraction study of NaMgF3 perovskite

The lattice distortion and structural phase transition of NaMgFa perovskite (Neighborite) have been studied using synchrotron X ray powder diffraction at high pressure and temperature. Changes in the unit cell dimensions of the perovskite are determined by conventional peak indexing and least squares routines. The stress field within the high‐pressure cell assembly is analyzed, and the yield strength of the NaMgF3 perovskite is determined at high P and T. The pressure‐ and temperature‐induced dimensional changes of the NaMgF3 perovskite structure are expressed empirically as a combination of compression/expansion of the [Mg‐F] bond length and tilting of the MgF6 octahedral framework. The linear thermal expansions of the NaMgF3 perovskite observed at different pressures show significant anisotropy with αa > αc > αb which reflects the decrease of structural distortion and the development of a phase transition in the perovskite with increasing temperature. The tilting angle of the MgF6 octahedral framework is observed to decrease rapidly toward zero, in a manner expected for a ferroelastic phase transition, as the temperature approaches the transition point Tc. The apparent [Mg‐F] bond lengths of the MgF6 octahedra experience drastic shrinkage with increasing temperature just prior to the transition. Despite a 12% change in volume due to compression, the experimental results on NaMgF3 perovskite show that the thermal expansivity is independent of pressure, i.e., dα/dP ≈ 0, and, compatibly, that the compressibility is independent of temperature, i.e., dβ/dT 0. However, the dominant compression mechanism is the compression of the octahedral bond length, whereas the dominant mechanism for thermal expansion is the diminishing of octahedral tilting. The Earth's mantle may be isochemical if the thermal expansion of MgSiO3 perovskite at high pressure behaves like NaMgF3, of which the Anderson‐Grüneisen parameter is near zero, i.e., δs ≈ 0. It is observed that crystal structure of the NaMgF3 perovskite transforms directly from the orthorhombic Pbnm phase to the cubic phase at all pressures. The transition temperature is observed to increase with increasing pressure with a positive slope of 45 K/GPa.

Structural study of ellenbergerite. Part II: Effects of high pressure

Compressibility and structural deformations induced by pressure in the high-pressure silicate ellenbergerite [Mg,(Ti,Zr),□] 2 Mg 6 (Al,Mg) 6 (Si,P) 8 O 28 (OH) 10 ; hexagonal, space group P6 3 , have been studied by single-crystal X-ray diffraction in a diamond anvil cell (DAC) in the pressure range 1 bar - 46 kbar. The bulk modulus of the mineral is 1330 kbar. The compressibility pattern is slightly anisotropic, with β a = 2.4 10 −4 and β c = 3.1 10 −4 kbar −1 . Compressibility is constant in the pressure range examined. Limited compositional differences of the samples do not produce significant variations in the compressibility coefficients. The geometrical data vs. pressure fit along the linear trends: a = a 0 - 0.0029 P c = c 0 - 0.0015 P with P in kbar and a and c in A. In agreement with its occurrence in coesite-bearing rocks, ellenbergerite is a very dense phase, with a structure formed by chains of face-sharing octahedra, which represent rigid structural moduli. Therefore the structural response to the high pressure applied in the DAC consists of small deformations in octahedral bond lengths and angles, with only minor interpolyhedral geometrical variations. Comparison between high-pressure and high-temperature data indicates that the unit cell and bond geometry of ellenbergerite at formation conditions, namely 700-800°C and 25-30 kbar, do not differ greatly from those at room conditions

Neutron diffraction study of the cationic distribution in Cu xCo 3− xO 4 (0< x⩽1.0) spinels prepared by thermal decomposition of layered hydroxide nitrate precursors

A neutron powder diffraction refinement of the structure parameters of six samples of copper cobalt oxide spinels Cu x Co 3− x O 4 (0 < x ⩽ 1.0), prepared by thermal decomposition of layered hydroxide nitrate precursors has been carried out. It was found that the Cu 2+ ions are statistically distributed over the tetrahedral ( A) and the octahedral ( B) lattice sites. This particular cationic distribution is responsible for the observed cubic lattice symmetry and the dependences of the unit cell parameters, the oxygen parameter and the tetrahedral and octahedral bond lengths on the copper content, x, in the formula unit. The results can be regarded as the first direct confirmation of the suggestion that the cationic distribution in the spinels is predetermined by the structure of the precursors and the mechanism of their thermal decomposition.