Akira Yoshiasa

Cation distribution and crystal chemistry of Y3Al5−xGaxO12 (0 ≤ x ≤ 5) garnet solid solutions

Five single crystals of Y(3)Al(5-x)Ga(x)O(12) (0 </= x </= 5) garnet solid solutions with the compositions x = 0.0, 1.0, 2.0, 3.0 and 4.0 were synthesized using a flux method. The compositional dependence of the lattice constants of the garnet solid solutions shows a large deviation from Vegards law. Investigation of the cation distributions of these garnets using single-crystal X-ray diffraction shows that Ga(3+), which is larger than Al(3+), preferentially occupies the tetrahedral (four-coordinate) site rather than the octahedral (six-coordinate) site. On the basis of the results obtained from structure refinements, geometric analyses of the polyhedral distortions were carried out. The results imply that the cation-cation repulsive force across the polyhedral shared edges decreases with increasing substitution of Ga(3+). Moreover, the proportion of covalent bonding in the cation-oxygen bonds was estimated from the bond strength; the results indicate that the covalency of the Ga-O bond is greater than that of the Al-O bond. The peculiar cation distributions observed in the Y(3)Al(5-x)Ga(x)O(12) garnet solid solutions are most probably caused by the strong covalency of the Ga-O bond and also simultaneously induced by the need to decrease the cation-cation repulsive force. Crystal data: cubic, Ia3;d, Z = 8, Mo Kalpha, lambda = 0.71069 Å; at x = 0.0 (triyttrium pentaaluminium dodecaoxide): a(0) = 12.0062 (5) Å, V = 1730.7 (2) Å(3), D(x) = 4.56 Mg m(-3), M(r) = 593.613, µ = 21.21 mm(-1), F(000) = 2224, R = 0.029 for 294 reflections; at x = 1.0 (triyttrium tetraaluminium gallium dodecaoxide): a(0) = 12.0432 (7) Å, V = 1746.7 (3) Å(3), D(x) = 4.84 Mg m(-3), M(r) = 636.351, µ = 24.09 mm(-1), F(000) = 2368, R = 0.022 for 124 reflections; at x = 2.0 (triyttrium trialuminium digallium dodecaoxide): a(0) = 12.0926 (9) Å, V = 1768.3 (4) Å(3), D(x) = 5.10 Mg m(-3), M(r) = 679.089, µ = 26.85 mm(-1), F(000) = 2512, R = 0.018 for 144 reflections; at x = 3.0 (triyttrium dialuminium trigallium dodecaoxide): a(0) = 12.1552 (6) Å, V = 1795.9 (3) Å(3), D(x) = 5.34 Mg m(-3), M(r) = 721.827, µ = 29.43 mm(-1), F(000) = 2656, R = 0.018 for 184 reflections; at x = 4.0 (triyttrium aluminium tetragallium dodecaoxide): a(0) = 12.2123 (8) Å, V = 1821.3 (4) Å(3), D(x) = 5.58 Mg m(-3), M(r) = 764.565, µ = 31.97 mm(-1), F(000) = 2800, R = 0.014 for 159 reflections.

Crystal structure analyses of the pyrochlore and fluorite-type Zr2Gd2O7 and anti-phase domain structure

Abstract The detailed structure investigations of Zr2Gd2O7 with pyrochlore- and fluorite-type structures have been carried out by X-ray single crystal method at room temperature. The bond distances between cations and oxide ions versus the site population for cations reasonably support that the fluorite phase is made up of the structure with microdomains of the pyrochlore. Sharp fundamental and diffuse superstructure reflections of the pyrochlore are interpreted assuming that anti-phase domain boundaries lie parallel to the &{211} and that domain sizes are not uniform. It should be proposed that the layers composed of the 48f site oxide ions are anti-phase boundaries. The probability density function maps and effective one-particle potentials of the pyrochlore structure reveal that the magnitudes of anharmonic thermal motions for the 48f site oxide ions are large toward the unoccupied 8b site. Compared to the s-pyrochlore phase (with sharp superstructure reflections), the 48f site oxide ions of the d-pyrochlore (with diffuse superstructure reflections) and fluorite phases have larger temperature factors and gentler potential curves. This is due to that the anti-phase domains are coherent each other and that their electron density distributions are the averages of individual domains.

Structure of Sr4Fe6O13, a new perovskite-derivative in the SrFeO system

Abstract Crystal structure of Sr 4 Fe 6 O 13 was studied by means of a single-crystal X-ray diffraction method. The crystal structure is orthorhombic and belongs to one of perovskite-derivatives constructed from Fe 2 O 3 - and Sr 2 Fe 2 O 5 -slabs perpendicular to the b-axis. One third of ferric ions occupies a distorted octahedral site and others locates at two different lattice sites with five oxygen-coordination. In the SrFeO system, Sr 4 Fe 6 O 13 with a chemical composition of Sr 3 -slabs.

The Mean-Square Relative Displacement and Displacement Correlation Functions in Tetrahedrally and Octahedrally Coordinated ANB8-N Crystals

The Debye-Waller factors of atoms in Ge, GaN, ZnO, CdS, CdSe, CuBr, CuCl, AgI, CdO, AgBr, KBr and RbCl in order of Phillips ionicity (f i) are studied by both EXAFS and the diffraction method. The displacement correlation functions (DCF) are derived from the mean-square relative displacement (MSRD) and the mean-square displacement (MSD) at room temperature. The magnitudes of the MSRD and MSD correlate well with the coordination number and ionicity. The MSRDs of tetrahedrally coordinated compounds show a gradual approach to those of octahedrally coordinated ones as the ionicity increases. A divergent-like curve toward f i=0.785 is observed for the MSD in the tetrahedrally coordinated compounds, which indicates the behavior of lattice instability. In the tetrahedrally coordinated covalent materials, the MSRD is nearly half the MSD cation or MSD anion and the ratio of the DCF to MSD given by 2DCF/(MSD cation+MSD anion) is about 80%. In the octahedrally coordinated ionic materials, the MSRD is comparable to the MSD and 2DCF/(MSD cation+MSD anion)=50%. An anharmonic contribution to the Debye-Waller factor determined by EXAFS appears pronouncedly when the magnitude of σ(2) is greater than 0.01 A2. The MSRD for the tetrahedrally coordinated compounds of silver and copper halides obeys the systematic ionicity dependence and pronounced specificity of anharmonicity was not observed: AgI, CuBr and CuCl of which the high-temperature forms are typical superionic conductors have a broad interatomic potential which is similar to that of the octahedrally coordinated compounds and strongly correlated displacement in thermal vibration between cation and anion.

Site preference of cations and structural variation in Y3Fe5−xGaxO12 (0 ≤ x ≤ 5) solid solutions with garnet structure

The crystal structures of Y 3 Fe 5-x Ga x O 12 (0≤ x ≤ 5) solid solutions (x = 0.0, 1.0, 1.6, 2.0, 2.5, 3.0, 3.6, 3.8, 4.6 and 5.0) with garnet structure were refined by single-crystal X-ray diffraction analyses. Site preferences of cations in the crystal structure were examined in detail. The smaller Ga 3+ ion occupies only the four-coordinated site in the composition range x = 0.0-1.6 (region I), whereas the larger Fe 3+ ion occupies only the six-coordinated site from x = 5.0 to 3.8 (region III). Both cations occupy these two sites from x=1.6 to 3.8 (region II). The tendency for site preference of cations changes near x=1.6 and 3.8. When Ga 3+ and Fe 3+ occupy only the four- (region I) and six-coordinated sites (region III), respectively, the enhancement of the cation-cation interaction can be considered as a result of the geometric restriction due to the variation of cation size. The change in tendency for cation site preference is most probably caused by the increased cation-cation interaction. Crystal data: cubic, Ia3d, Z = 8, Mo Kα, λ = 0.71069 A; at x = 0.0: a 0 = 12.375(1)A, V = 1895.3(3)A 3 , D x = 5.17 mg m -3 , m r = 737.938, μ = 26.42 mm -1 , F(000) = 2744, R = 0.020 for 144 reflections; at x = 1.0: a 0 = 12.360(1)A, V = 1888.2(4)A 3 , D x = 5.29 Mg m -3 , M r = 751.811, μ = 27.95 mm -1 , F(000) = 2784, R = 0.019 for 131 reflections; at x = 1.6: a 0 = 12.351(1)A, V = 1883.9(3)A 3 , D x = 5.36 Mg m -3 , M r = 760.135, μ = 28.88 mm -1 , F(000) = 2808, R = 0.021 for 110 reflections; at x = 2.0: a 0 = 12.342(1)A, V = 1880.1(4)A 3 , D x = 5.41 Mg m -3 , M r = 765.684, μ = 29.51 mm -1 , F(000) = 2824, R = 0.018 for 117 reflections; at x = 2.5: a 0 = 12.333(1)A, V = 1875.8(3)A 3 , D x = 5.47 Mg m -3 , M r = 772.621, μ = 30.30 mm -1 , F(000) = 2844, R = 0.015 for 111 reflections; at x = 3.0: a 0 = 12.317(1)A, V = 1868.7(3)A 3 , D x = 5.54 Mg m -3 , M r = 779.557, μ = 31.14 mm -1 , F(000) = 2864, R = 0.017 for 119 reflections; at x = 3.6: a 0 = 12.312(2)A, V = 1866.2(5)A 3 , D x = 5.61 Mg m -3 , M r = 787.881, μ = 32.06 mm -1 , F(000) = 2888, R = 0.016 for 168 reflections; at x = 3.8: a 0 = 12.302(1)A, V = 1861.8(4)A 3 , D x = 5.64 Mg m -3 , M r = 790.655, μ = 32.42 mm -1 , F(000) = 2896, R = 0.020 for 117 reflections; at x = 4.6: a 0 = 12.289(1)A, V = 1856.1(3)A 3 , D x = 5.73 Mg m -3 , M r = 801.754, μ = 33.69 mm -1 , F(000) = 2928, R = 0.013 for 103 reflections; at x = 5.0: a 0 = 12.273(1)A, V = 1848.7(4)A 3 , D x = 5.80 Mg m -3 , M r = 807.303, μ = 34.45 mm -1 , F(000) = 2944, R = 0.021 for 118 reflections.

EXAFS study of the fluorite-type compounds in the systems ( 1 − x ) ZrO2−xYbO1.5 ( x = 0.18 ≦ x ≦ 0.5 ) and Zr2Ln2O7 (Ln = Tb, Dy, Ho, Er, and Yb)

Abstract The local structures of the fluorite-type compounds with a composition of Zr 2 Ln 2 O 7 ( Ln = Gd, Dy, Ho, Er and Yb) have been studied by means of EXAFS spectroscopy. The EXAFS results indicate that Ln 3+ ions are essentially 8-coordinated by oxide ions but Zr 4+ ions are 6-coordinated. The distinct difference in coordination number between Zr 4+ and Ln 3+ ions means that the local structures of the fluorite-type Zr 2 Ln 2 O 7 are similar to the pyrochlore-type structure, even though both Zr 4+ and Ln 3+3 ions are located statistically at the cation sites (4a site) in the fluorite-type lattice. The local structure of the fluorite-type solid solution in the system ( 1 − x )ZrO 2 − x YbO 1.5 (0.18≦ x ≦0.50) has been also studied by EXAFS spectroscopy. The result indicates that the coordination numbers of Zr 4+ and Yb 3+ ions are 6 and 8, respectively, in the solid solution in the compositional range 0.18≦ x ≦0.50, and that the oxygen vacancies are located adjacent to Zr 4+ ions.

Reinvestigation of the MgSiO3 perovskite structure at high pressure

Abstract High-pressure single-crystal X-ray diffraction experiments of MgSiO3 perovskite have been carried out up to 15 GPa in a diamond-anvil cell using synchrotron radiation. Precise crystal structural parameters, including the anisotropic displacement parameters of every atom in MgSiO3, are determined under high pressure. In the pressure range up to 15 GPa, the most important responses of the structure are the compressions of SiO6 and MgO8 polyhedra and an increase in tilting of SiO6 octahedra represented by the decrease in angles between octahedra (both Si-O2-Si angle in the a-b plane and Si-O1-Si angle in the b-c plane decrease). The degree of the change in both angles in the a-b and b-c planes is the same. The amplitude of mean square displacement for the Mg atom has the largest value in the structures and its thermal vibration is significantly anisotropic at ambient pressure. Under high pressure, all atoms in the structure have obvious anisotropy of thermal vibration and the largest amplitudes of thermal vibration for Mg, Si, and O2 atoms are directed toward vacant space in the structure. Anisotropy of the structure increases with pressure.

Detailed structures of hexagonal diamond (lonsdaleite) and Wurtzite-type BN

Hexagonal diamond (hDIA) and wurtzite-type BN (wBN) powders were synthesized using a Kawai-type high-pressure apparatus and the essential details of their structures were examined by Reitveld refinements in order to investigate their thermodynamic stability and transition mechanism. X-ray diffraction profiles of the products were well explained by a mixture of hDIA and cubic diamond (cDIA) with stacking faults. The mass fraction of hDIA and cDIA was 50:50 for the products annealed between 800 and 1400°C and it became 20:80 for the product annealed at 1600°C. Temperatures higher than 1600°C seem to favor the formation of cDIA or to induce the conversion from hDIA to cDIA. Structure refinement revealed that a decrease and an increase in the basal and apical distances of C–C and B–N bonds in hDIA and wBN, respectively, are introduced by lowering the symmetry from cubic to hexagonal. Since the relative stability of wurtzite-type compounds largely depends on the distortion of the tetrahedral bond angle, the deviation from the ideal tetrahedron in both hDIA and wBN was refined to discuss their stability. The transition mechanism from graphite and graphite-like BN to hDIA and wBN is discussed by comparing the present results and those of previous simulation studies. Based on analogous features observed in the synthetic hDIA and lonsdaleite (natural hDIA found in meteorites), the formation mechanism of hDIA in meteorites is proposed.

X-ray and Raman study on coordination states of fluorite- and pyrochlore-type compounds in the system ZrO2-Gd2O3

Raman spectra were measured on the pyrochlore- and fluorite-type Zr2Gd7 (x=0.50) in the system (1−x)ZrO2−xGdO 1.5 and structure refinements were carried out for both structure types with a composition of x=0.43 by means of single crystal X-ray diffraction. Bond distances in the fluorite-type structure with x=0.43 were in good agreement with those calculated from the effective ionic radii, assuming that Gd3+ ion was coordinated by eight oxygens and oxygen vacancies were adjacent to Zr4+ ion. A Raman spectrum of the fluorite-type Zr2Gd2O7 was very similar to that of the pyrochlore-type, and had the line near 220 cm−1 attributed to the pyrochlore-type Gd4O tetrahedra. Coordination states and stability of the pyrochlore- and fluorite-type structures are discussed based on the present results.

Pressure and temperature dependence of EXAFS Debye-Waller factors in diamond-type and white-tin-type germanium

Extended X-ray absorption fine-structure (EXAFS) spectra near the Ge K-edge in diamond- and white-tin-type Ge under high temperature and high pressure were measured using a cubic-anvil-type apparatus (MAX90) with synchrotron radiation from the Photon Factory, Tsukuba, Japan. Pressure values up to 10.6 GPa were estimated on the basis of the isothermal equation of state of the diamond-type Ge within an accuracy of 0.4 GPa. Pressures for the same cell assembly were also determined by X-ray diffraction experiment using the NaCl scale. The diamond-type Ge is of great advantage to the pressure calibrant of EXAFS measurements at elevated temperature because a harmonic approximation can be applied up to 900 K. By the phase transition from diamond- to white-tin-type phases, with an increase in coordination number, Ge—Ge distances increase. A sixfold-coordinated Ge atom in the white-tin-type structure has crystallographically non-equivalent two kinds of nearest-neighbour distances [2.530 (8) A and 2.697 (8) A at 12.8 GPa]. The harmonic effective interatomic potential, V(u) = 1/2αu2, was evaluated from the contribution to the thermal vibration, where u is the deviation of the bond distance from the location of the potential minimum. The potential coefficient, α, at 0.1 MPa is essentially temperature independent and is 9.06 eV A−2. At 9 GPa the potential coefficient is 9.71 eV A−2. The effective interatomic potential is influenced not only by pressure but also by changes in coordination number. The high-pressure white-tin-type phase has a broader potential and a relatively larger mean square amplitude of vibration than the diamond-type phase.