Compressibility anomaly and amorphization in the anisotropic negative thermal expansion material NbOPO<SUB>4</SUB> under pressure

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 NbOPO4. 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 B0 and its pressure derivative B10are 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 NbOPO4, the compression is found to be anisotropic