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An in-situ Raman spectroscopic study was conducted to explore the pressure induced phase transformation of spinel-type ferrite ZnFe2O4. Results indicate that ferrite ZnFe2O4 initially transforms to an orthorhombic structure phase (CaFe2O4-polymorph) at a pressure of 24.6 GPa. Such a phase transformation is complete at 34.2 GPa, and continuously remains stable to the peak pressure of 61.9 GPa. The coexistence of the two phases over a wide range of pressure implies a sluggish mechanism upon the spinel-to-orthorhombic phase transition. Upon release of pressure, the high pressure ZnFe2O4 polymorph is quenchable at ambient conditions.

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of spinel NiCr 2 O 4 to pressures of 57.1 GPa. Results indicate that NiCr 2 O 4 spinel dissociates to the mixture of Cr 2 O 3 and NiO at 13.1 GPa. Such a dissociation is complete at 25.1 GPa. The coexistence of Cr 2 O 3 and NiO remains up to the peak pressure of 57.1 GPa. Upon release of pressure to 33.3 GPa, the Cr 2 O 3 starts to react with NiO to form the spinel phase, and such a reaction is complete at 18.9 GPa. The recovered spinel phase may exhibit an inverse spinel structure, which differs from the initial normal spinel structure. Combined with the results obtained from other end member chromites, it is suggested that the variation of high-pressure behavior of chromites (ACr 2 O 4 ) significantly depends on the average A–O bond length of the A–O tetrahedron.

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr2O4 spinel up to pressures of 76.4 GPa. Results indicate that MgCr2O4 spinel undergoes a phase transformation to the CaFe2O4 (or CaTi2O4) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr2O4 at 5.7–18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr2O4 polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.

We have investigated the melting temperatures of MgSiO3 and CaSiO3 perovskites at high pressure conditions, using an empirical melting equation, combined with the assumption of a boundary phase exi ...

Pure andradite (Ca 3 Fe 2 Si 3 O 12 ) was synthesized at 1150°C and 3 GPa, and then compressed to pressure above 21 GPa, and heated to about 1000°C by a YAG laser in a Diamond Anvil Cell (DAC). After temperature quenching in situ high pressure X-ray diffraction studies were carried out. The results show that andradite has transformed to a cubic perovskite type polymorph with a =3.460(4) A. Upon decompression, in situ X-ray observations reveal that perovskite phase has transformed to an amorphous phase. The density of the perovskite polymorph of Ca 3 Fe 2 Si 3 O 12 is about 13.6% greater than that of an isochemical composition at 21 GPa, and ferric iron can replace Ca 2+ and Si 4+ in the perovskite structure (2Fe 3+ =Si 4+ +Ca 2+ ), giving a chemical formula: (Ca,Fe)(Si,Fe)O 3 . Combining with the electron hopping mechanism between Fe 2+ and Fe 3+ in semiconductivity materials, this new Fe 3+ -rich perovskite polymorph may provide new data to clarify the electrical conductivity of the lower mantle, based on the mineralogical model with magnesiuwustite (Mw) and perovskite (Pv).