Compressibility anomaly and amorphization in the anisotropic negative thermal expansion material [formula omitted] under pressure

Abstract

We report the results of the high-pressure angle dispersive powder X-ray diffraction measurements obtained up to 31 GPa on the monoclinic and the tetragonal phases of NbOPO 4 . The tetrgonal phase was obtained by quenching the monoclinic phase from 5 GPa and 833 K. The lattice parameters of the monoclinic phase show an anomaly at 0.6 GPa, but the unit cell volume follow an almost linear compressional behavior. The bulk modulus B 0 and its pressure derivative B 0 ′ are estimated to be 66(3) GPa and 1.6(8), respectively, for the monoclinic phase. The irreversible pressure induced amorphization observed at ambient temperature in the monoclinic phase is due to the kinetic hindrance to bond reconstruction that is required for transformation form the monoclinic to the tetragonal phase. In the case of tetragonal NbOPO 4 , the compression is found to be anisotropic.