Precise structure analysis by neutron diffraction for<i>R</i>NbO₄and distortion of NbO₄tetrahedra

Abstract

Precise crystal structure analysis by neutron powder diffraction is performed for rare-earth orthoniobates (RNbO4: R = La, Nd, Ho and Yb) and the distortion of NbO4 tetrahedra is estimated with the normal-coordinate method. Displacement of the cubic symmetry mode is large and increases with decreasing ionic radius of VIIIR3+. This displacement produces a regular tetrahedron of NbO4. The mean value of Nb–O distances in the regular tetrahedron of RNbO4 crystals is 1.834 Å. This value is nearly equal to the mean bond length for s = 5/4, \overline R = 1.828 Å, estimated by the bond-valence method. Displacements of the other symmetry modes slightly decrease or are almost constant with decreasing ionic radius.