Negative thermal expansion in orthorhombic NbOPO4

Monoclinic NbOPO4 was found to undergo a readily reversible first-order pha se transition at 292 degreesC. The connectivity of the NbO6 octahedra and P O4 tetrahedra remains the same throughout the transition, but above the tra nsition the symmetry has increased to orthorhombic and Z has decreased by a factor of 2. Rietveld structure refinements utilized neutron diffraction d ata from room temperature to 400 degreesC and X-ray diffraction data up to 700 degreesC. One cell edge shows negative thermal expansion over the entir e temperature range. The other two cell edges show negative thermal expansi on only above the phase transition. The negative thermal expansion behavior is related to transverse thermal motion of oxygen. (C) 2001 Academic Press .