Yasuhiko Syono

High pressure transformations in zinc silicates

Phase transformations in Zn2SiO4 and ZnSiO3 have been investigated at high pressure up to 170 kbar and temperature to 1500°C. Crystal structures of high pressure polymorphs have been studied by means of single crystal and powder X-ray diffraction analyses. Chemical compositions have been determined by electron microprobe and wet chemical analyses. Five polymorphs have been identified in Zn2SiO4, designated as I–V in the order of increasing pressure. Coordination numbers of metal ions in the crystal structures of Zn2SiO4 II–IV are four, the same as those in Zn2SiO4 I with the phenacite structure. The crystal structure of Zn2SiO4 II is composed of an approximately body-centered tetragonal arrangement of oxygen ions. Zn2SiO4 III and IV are suggested to be nonstoichiometric. Zn2SiO4 V, appearing above 130 kbar, is identified to be of the modified spinel structure. Zn2+ ions enter the octahedrally coordinated sites in it, accompanied by a large density increase. No olivine-like structures could be found among five polymorphs in Zn2SiO4. The solubility limit of Zn2SiO4 in Mg2SiO4 with the olivine structure is determined to be close to 75% at 90 kbar. Only a clinopyroxene form of ZnSiO3 is found to be stable over a relatively wide region in the pressure-temperature diagram. However, it has anomalous unit cell parameters when compared with more conventional pyroxenes. The extreme instability of the olivine structure in Zn2SiO4, and unusual cell parameters in ZnSiO3 pyroxene are discussed in terms of crystal structures. Stability of the modified spinel structure is also inferred in some detail. It is suggested from the study of phase transformations in Zn2GeO4 and ZnGeO3 that simple analogy in the mode of the high-pressure transformation between silicates and the corresponding germanates should be reexamined carefully.

High-pressure decomposition of the system FeSiO3MgSiO3

Abstract Stability relations of (Mg, Fe)SiO3 solid solutions ranging in composition from pure FeSiO3 to (Mg0.3 Fe0.7)SiO3 have been studied at high temperatures in the pressure range to 96 kb. High-pressure decomposition of (Mg, Fe)SiO3 clinopyroxene into (Mg, Fe)2SiO4 spinel solid solution and stishovite was established. The decomposition curve for FeSiO3, clinoferrosilite, was found to coincide with the coesite—stishovite transformation curve.

A new high pressure phase of MnTiO3 and its magnetic property

Abstract A high pressure phase of MnTiO3 II is newly synthesized at high pressures and temperatures. An equilibrium phase boundary between an atmospheric pressure phase of MnTiO3 I and MnTiO3 II is P(kb) = 110 − 0.044 T(°C). X-ray analysis of powder specimens has revealed that the high pressure phase MnTiO3 II has the disordered ilmenite structure with the hexagonal lattice parameters of a = 5.2051 A, c = 13.699 A, c a = 2.632 and V = 321.4 A[3]. The transformation from MnTiO3 1 to MnTiO3 II causes an expansion of the a axis by 1.3 per cent and a contraction of the c axis by 4.1 per cent, resulting a volume decrease of 1.6 per cent. Both Mossbauer effect and paramagnetic ESR measurements suggest that MnTiO3 II is antiferromagnetic below 24° ± 1°K. The magnetic susceptibility obeys the Curie-Weiss law almost down to the Neel temperature and slightly deviates from it below TN, in marked contrast with that of MnTiO3 I. A possibility that the covalency of Mn2+ ions plays an important role both in the magnetic interactions and the crystal stability of manganese ilmenites is suggested.

High pressure synthesis of ilmenite and perovskite type MnVO3 and their magnetic properties

Abstract Ilmenite and perovskite type Mn 2+ V 4+ O 3 are newly synthesized at high pressures and high temperatures. The low pressure form of MnVO 3 I is supposed to have a trichlinic symmetry and to be isomorphous with the vanadate ilmenites CoVO 3 I and CuVO 3 I. Unit cell dimensions are a = 5.072 (5) A, b = 5.550 (5) A, c = 5.023 (4) A, α = 90°0′ (6′), β = 118°38′ (4′), γ = 63°4′ (4′) and V = 106.3 (1) A 3 . The high pressure form of MnVO 3 II, stabilized above about 45 kbar, has the orthorhombic perovskite structure with the possible space group of D 16 2h -Pbnm or C 9 2v -Pbn2 1 . Unit cell dimensions are a = 5.106 (1) A, b = 5.265 (1) A, c = 7.387 (1) and V = 198.6 (1) A 3 . MnVO 3 II is the first perovskite containing Mn 2+ ions in the larger cationic sites in the perovskite structure. Magnetic and electric properties of both MnVO 3 I and II are studied. Magnetic susceptibilities of both substances obey the Curie-Weiss law. Weak ferromagnetic moments are found in both substances at low temperatures. MnVO 3 I shows semiconducting behaviour, whereas MnVO 3 II relatively low electric resistivity.

Crystal field effect on the olivine-spinel transformation

Abstract The olivine-spinel transformation in silicates and germanates is discussed in terms of ionic radius ratio and crystal field effect and is shown to be understood in a most systematic way. The pressure for the transition to spinel is found to increase linearly with increasing ionic radius ratio r R r M , where R and M are divalent and tetravalent cations, if no contribution from crystal field effect is expected. In case of transition metal ions, the excess crystal field stabilization in spinel over olivine remarkably lowers the transition pressure to cubic spinel below the general trend. It is suggested that without the crystal field stabilization the modified spinel structure becomes more stable than the cubic normal spinel structure at high pressure when the ionic radius ratio has high values.

Stability relations between olivine, spinel and modified spinel

Stability relations between the three polymorphs for Co2SiO4 (olivine, modified spinel and spinel) are studied by the comparison of the thermodynamical properties on the basis of their precise crystal structures and the experimental results for synthesis. In spite of violation of the Paulings electrostatic valency rule, the modified spinel structure is not necessarily unstable with respect to the other polymorphs. The gradients of the transition curves in the P-T diagram and the molar volumes indicate that entropy becomes smaller in the order of olivine, modified spinel, and spinel. These entropy changes between the three polymorphs are qualitatively explained by the modes of connection of the chains of coordination polyhedra within each polymorph.

High-pressure phase transformations in MnAl2S4 and MnGa2S4

Abstract Phase transformations in MnAl2S4 and MnGa2S4 have been investigated at high pressure up to 95 kb and temperature up to 1000°C. The decomposition of MnAl2S4 to MnS with rock salt structure and the spinel phase at moderately high pressure has been observed. The lattice parameter for the spinel phase is shown to decrease with increasing pressure. It is interpreted due to the formation of nonstoichiometric spinel expressed by Mn1−xAl2S4−x. Above about 90 kb, an Al-rich unknown phase appears instead of spinel, which can not be identified on account of its extreme unstableness in air. Six polymorphs have been observed for MnGa2S4, designated as I ∼ VI in the order of increasing pressure. MnGa2S4 IV is identified to be identical with an atmospheric pressure phase of MnAl2S4 with the ZnIn2S4 structure. The crystal structures of MnGa2S4 I, II and III are thought to be due to the polytypism in the ZnIn2S4 structure. The identification of the other two polymorphs can not be made.