Phase Transition and Negative Thermal Expansion in Tetragonal NbOPO 4

The structure of tetragonal NbOPO₄has been studied from room temperature to 500°C by X-ray powder diffraction and to 327°C by neutron powder diffraction. Rietveld refinements were conducted on neutron data collected at 25, 100, 175, 200, 225, 275, and 327°C. A phase transition was found at about 200°C. The space group isP4/nbelow this transition andP4/nmmabove this transition. Theccell edge shows positive thermal expansion over the entire temperature range. Theaandbcell edges show positive thermal expansion below the transition and negative thermal expansion above the transition. The negative thermal expansion is attributed to rocking motions of the corner-shared NbO₆octahedra and PO₄tetrahedra.