Hydrothermal Synthesis and Spectroscopic and Magnetic Behavior of the Mn7(HOXO3)4(XO4)2(X=As, P) Compounds. Crystal Structure of Mn7(HOAsO3)4(AsO4)2

An in situ Raman spectroscopic study was conducted to investigate the pressure-induced phase transformation in the synthetic ZnCr2O4 spinel up to pressures of 70 GPa at room temperature. Results indicate that ZnCr2O4 spinel starts to transform to the CaFe2O4 (or CaTi2O4) structure at 17.5GPa, and such a phase transformation is complete at 35 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish mechanism upon phase transformation. No experimental evidence was observed to support the theoretical simulation with the dissociation of ZnCr2O4 to ZnO and Cr2O3 at 34 GPa. Moreover, enhancement of the intensity of the Raman peak at 642 cm-1 at either elevated pressures or temperatures is most likely caused by an enhanced order–disorder effect. Upon release of pressure, the recovered phase may exhibit an inverse spinel structure, which differs from the initial normal spinel structure.