The monoclinic phase of Y2O3(B-RES) has been synthesized using a Kawai-type multi-anvil apparatus at 20 GPa and 1800 °C. Samples of the cubic Y2O3(C-RES) and monoclinic Y2O3 phases were characterized by synchrotron radiation X-ray diffraction, X-ray absorption near edge structure and Raman spectroscopy. Crystal structures of the cubic and monoclinic phases have been examined using Rietveld refinement of the X-ray diffraction data. The cubic-to-monoclinic transition of Y2O3 was reconstructive and irreversible. The X-ray diffraction results were further confirmed by simulation of the X-ray absorption spectra.

The structural stability of Fe2Ti (P63/mmc) has been investigated by synchrotron radiation X-ray diffraction with diamond anvil cell. At room temperature, Fe2Ti shows a monotonous change of unit-cell volume in present pressure range up to 40.3 GPa. Isothermal pressure–volume relationship of Fe2Ti (P63/mmc) is well described by the second order Birch–Murnaghan equation of state with V0 = 161.8(3) Å3 and B0 = 201(4) GPa. A new polymorph of Fe2Ti indexed to an orthorhombic phase is observed when the sample undergoes the high-temperature annealing using laser heating at 46.6 GPa. This orthorhombic phase can be quenched and preserved on decompressing to ambient pressure.Graphical abstractHighlights► High pressure structural stability of Fe2Ti (P63/mmc) was investigated. ► Fe2Ti is stable up to 40.3 GPa at room temperature. ► Axial compressibility presents obvious anisotropy. ► A new orthorhombic phase of Fe2Ti was observed after laser heating. ► This orthorhombic phase can be quenched and preserved on decompressing to ambient pressure.

X-ray diffraction and Mössbauer spectroscopy were employed to investigate structural stability of Fe2TiO4 under high pressure. Measurements were performed up to about 24 GPa at room temperature using diamond anvil cell. Experimental results demonstrate that Fe2TiO4 undergoes a series of phase transitions from cubic (Fd3̄m) to tetragonal (I41/amd) at 8.7 GPa, and then to orthorhombic structure (Cmcm) at 16.0 GPa. The high-pressure phase (Cmcm) of Fe2TiO4 is kept on decompression to ambient pressure. In all polymorphs of Fe2TiO4, iron cations present a high-spin ferrous property without electric charge exchange with titanium cations at high pressure supported by Mössbauer evidences.Graphical abstractA series of phase transition of Fe2TiO4 occurs from cubic (a) to tetragonal (b and c) then to orthorhombic phase (d–f) at high pressure.Highlights► High pressure behaviors of Fe2TiO4 were investigated. ► Phase transitions were observed from cubic to tetragonal and then to orthorhombic. ► Orthorhombic phase can be kept on decompression. ► In all polymorphs of Fe2TiO4, iron ions are ferrous with high-spin state.

The structural stability of β-Ti3O5 (C2/m) has been investigated by X-ray diffraction and Raman spectroscopy with diamond anvil cells. β-Ti3O5 is stable up to about 26 GPa at room temperature. Isothermal pressure–volume relationship of β-Ti3O5 is well presented by the third-order Birch–Murnaghan equation of state with V0=348.6(8) Å3 and B0=216(9) GPa. Axial compressibility presents obvious anisotropy. The a-axis and c-axis are more compressible than b-axis due to the different crystal structure arrangement of β-Ti3O5 along b-axis and perpendicular to b-axis direction. The Grüneisen parameters of thirteen observed Raman modes are 0.79–1.74, whose mean is 1.32.Graphical abstractThe crystal structure of β-Ti3O5: the projection of β-Ti3O5 along b-axis (Left) and the projection of β-Ti3O5 along c-axis (Right). Highlights► High pressure structural stability of β-Ti3O5 was investigated. ► β-Ti3O5 is stable up to about 26 GPa. ► Axial compressibility presents obvious anisotropy. ► Pressure–volume relationship is well presented by Birch–Murnaghan equation of state. ► The Grüneisen parameters are 0.79–1.74.

The high-pressure phase transition of Mg2Si was investigated using angel dispersive synchrotron radiation X-ray diffraction with diamond anvil cell. A phase transition of Mg2Si occurs from the anti-fluorite structure to a monoclinic structure instead of the anti-cotunnite structure at about 11.1 GPa, which is reversible. Isothermal pressure-volume relationship of anti-fluorite Mg2Si is well described by the third-order Birch–Murnaghan equation of state with V0=64.0(2) Å3 and B0=58(2) GPa.Highlights► Two rounds of angle dispersive XRD experiments, one with laser-heating. ► New monoclinic high-pressure phase of Mg2Si, instead of anti-cotunnite phase. ► EoS parameters of anti-fluorite phase was fitted.

First-principle calculations using density-functional theory with linearized augmented plane wave method and projector-augmented method have been performed for the high-pressure MnTiO3 polymorphs and their possible dissociation products. Theoretical results demonstrate that ilmenite-type MnTiO3 transforms into perovskite phase at 27 GPa and 0 K. The lithium niobate phase of MnTiO3 is confirmed to be metastable according to its higher Gibbs free energy compared with that of ilmenite at ambient conditions. In ilmenite and lithium niobate phases, MnO6 octahedra become more distorted while TiO6 octahedra become more regular with increasing pressure. In orthorhombic perovskite phase, the structural distortion deviated from the ideal cubic perovskite is enhanced at higher pressure. Based on the non-spin-polarized calculations, perovskite phase MnTiO3 is predicted to dissociate into Fm3̄m-MnO+P21/c-MnTi2O5 at 29 GPa.Highlights► Phase stability of MnTiO3 polymorphs under high pressure is determined. ► Perovskite MnTiO3 dissociates into Fm3̄m-MnO+P21/c-MnTi2O5 at 30 GPa. ► More distorted MnO6 octahedra with increasing pressure in ilmenite phase. ► More regular TiO6 octahedra with increasing pressure in ilmenite phase. ► Enhanced structural distortion in perovskite phase at higher pressure.

We have synthesized the (Mn1−xFex)TiO3 (0.0≤x≤1.0) solid solution compounds by high-temperature sintered methods, and characterized their crystal structures by combining X-ray diffraction, Mössbauer spectroscopy and Raman spectroscopy. Lattice constants and volumes show a linear decrease with increase in FeTiO3 content. All experimental results illustrate a decreasing distortion of TiO6 (or FeO6/MnO6) octahedra with increase in FeTiO3 content. The vibrational frequency of OTiO bending motions presents a direct dependence on the corresponding bond angle (the ∠OTiO).Raman spectra of the (Mn1−xFex)TiO3 (0.0≤x≤1.0) system. Inset: the enlarged view from 310 to 390 cm−1. Eg(3) shifts to high frequency and the remaining shifts to low frequency as xFeTiO3 content increases

•We give a systematic investigation on the crystal chemistry and the compressibility of silicate-carbonate minerals.•Pressure-induced discontinuities in the compressional evolutions are identified.•The structure and compressibility among silicate-carbonate minerals, carbonates and silicates are discussed.•Increasing the [CO3]2− proportion will decrease the bulk modulus.Spurrite Ca5(SiO4)2(CO3), galuskinite Ca7(SiO4)3(CO3) and tilleyite Ca5(Si2O7)(CO3)2 are three representative minerals formed in high-temperature skarns in the silicate-carbonate system. Their crystal chemistry and compressibility have been investigated using first-principles theoretical simulation. These minerals are structurally described as the combination of interwoven layers constituted by Ca polyhedra and Si polyhedra, with the [CO3] triangles being “separators” to depolymerize the Si–Ca aggregations. With the effect of pressure, the Si polyhedra and the [CO3] groups present rigid behaviors whereas the Ca–O bonds undergo considerable compression. Several pressure-induced abnormities in the lattice parameter variations have been identified, revealing the existence of subtle changes in the compression process. Isothermal equations of state parameters are obtained: K0 = 71.1(1) GPa, V0 = 1003.31(4) Å3 and K0′ = 5.4(1) for spurrite; K0 = 75.0(1) GPa, V0 = 1360.30(7) Å3, K0′ = 5.4(1) for galuskinite, and K0 = 69.7(3) GPa, V0 = 1168.90(2) Å3 and K0′ = 4.0(1) for tilleyite. These compounds have similar K0 values to calcite CaCO3 but are much more compressible than larnite β-Ca2SiO4. Generally for these minerals, the bulk modulus exhibits a negative correlation with the [CO3] proportion. The structural and compressional properties of silicate-carbonate minerals compared with silicates and carbonates are expected to be a guide for further investigations on Si polyhedra and [CO3] coexistent phases.Download full-size image

The structural stability of manganese titanate MnTiO3 at high pressure was investigated by X-ray diffraction and Raman spectroscopy with diamond anvil cells. Ilmenite-type MnTiO3 is stable at least to 26.6 GPa, and lithium niobate type MnTiO3 reversibly transforms at room temperature to perovskite at 2.0 GPa. Bulk moduli (K300) of ilmenite, lithium niobate and perovskite are 174(4) GPa, 179(8) GPa, and 208(5) GPa, respectively (at fixed first pressure derivative K′ = 4). The Grüneisen parameter γ has been estimated to be 1.28 for ilmenite and 1.75 for perovskite. In ilmenite phase, TiO6 octahedra become more regular with increasing pressure. In perovskite phase structural distortion increases with pressure increase.

The high-pressure phase transition of Mg2Si was investigated using angel dispersive synchrotron radiation X-ray diffraction with diamond anvil cell. A phase transition of Mg2Si occurs from the anti-fluorite structure to a monoclinic structure instead of the anti-cotunnite structure at about 11.1 GPa, which is reversible. Isothermal pressure-volume relationship of anti-fluorite Mg2Si is well described by the third-order Birch–Murnaghan equation of state with V0=64.0(2) Å3 and B0=58(2) GPa.Highlights► Two rounds of angle dispersive XRD experiments, one with laser-heating. ► New monoclinic high-pressure phase of Mg2Si, instead of anti-cotunnite phase. ► EoS parameters of anti-fluorite phase was fitted.