Precise structure analysis by neutron diffraction for RNbO₄ and distortion of NbO₄ tetrahedra

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, = 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.

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