Structural Transition of MnNb2O6 under Quasi-Hydrostatic Pressure

Abstract

High-pressure Raman scattering and X-ray diffraction studies on manganese niobate (MnNb2O6) have been carried out in a diamond anvil cell at room temperature up to 41 GPa. A pressure-induced phase transition was observed at 12 GPa accompanied by the softening of two internal vibration modes ν10(B2g) and ν8(Ag). Disappearing of most Raman vibration peaks under high pressure reveals that the NbO6 octahedra distort heavily through the transition; this is evidenced by the X-ray diffraction results, but the octahedral units still exist, as the internal Nb–O strengthening vibration is observable even at the highest pressure in this study. The equation of state for MnNb2O6 in low pressure phase I was obtained with third-order Birch–Murnaghan method, yielding a zero-pressure bulk modulus B0 and its pressure derivative B0′ of 154.4 ± 3.5 GPa and 4.1, respectively. The volume dependencies of the optical lattice mode frequencies and their respective Gruneisen parameters were extracted; it is suggested that the insta...