Negative Thermal Expansion in Orthorhombic NbOPO 4

Monoclinic NbOPO₄ was found to undergo a readily reversible first-order phase transition at 292°C. The connectivity of the NbO₆ octahedra and PO₄ tetrahedra remains the same throughout the transition, but above the transition the symmetry has increased to orthorhombic and Z has decreased by a factor of 2. Rietveld structure refinements utilized neutron diffraction data from room temperature to 400°C and X-ray diffraction data up to 700°C. One cell edge shows negative thermal expansion over the entire temperature range. The other two cell edges show negative thermal expansion only above the phase transition. The negative thermal expansion behavior is related to transverse thermal motion of oxygen.