Phase transition and negative thermal expansion in tetragonal NbOPO4

The structure of tetragonal NbOPO4 has been studied from room temperature to 500 degrees C by X-ray powder diffraction and to 327 degrees C by neutron powder diffraction, Rietveld refinements were conducted on neutron data collected at 25, 100, 175, 200, 225, 275, and 327 degrees C, 4 phase transition was found at about 200 degrees C, The space group is P4/n below this transition and P4/nmm above this transition. The c cell edge shows positive thermal expansion over the entire temperature range, The a and b cell 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 NbO6 octahedra and PO4 tetrahedra, (C) 1998 Academic Press.