Studies of the fergusonite to scheelite phase transition in LnNbO4 orthoniobates

Abstract

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid orthoniobates, LnNbO4 at room temperature. The observed decrease in the unit cell volume in going from Ln = La to Lu correlates with a decrease in the volume of the LnO8 polyhedra due to the changing size of the Ln3+ cation. In all cases the Nb is in a highly distorted six-coordinate geometry with two long, ~2.5 Å, and four short ~1.9 Å Nb–O distances. The importance of the long Nb–O bonds is evident from Bond Valence Sum Calculations. Variable temperature structural studies of three examples, Ln = La, Sm and Gd, showed each transformed to a high temperature tetragonal scheelite type structure in space group I41/a. In the high temperature structure the Nb are tetrahedrally coordinated and it appears that monoclinic – tetragonal transition is best described as a reconstructive transition in which the long Nb–O(1) bond is broken upon heating