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The structural changes of stoichiometric LiNbO 3  at temperatures up to 1470 K have been investigated by neutron powder diffraction. At  T c   ≈ 1460 K, LiNbO 3  transforms from a paraelectric high-temperature ( R [unk] c ) to a ferroelectric low temperature structure ( R 3 c ). The phase transition is of coupled order-disorder and displacive type. In the high-temperature phase, Li is highly disordered over two off-centre positions, whereas Nb takes a centrosymmetric position within O 6  octahedra. An analysis of anisotropic displacement parameters shows that the probability density function of Li is extended along  c , that of the oxygens is more or less perpendicular to the Nb–O bonds, and that of Nb is slightly extended in the  a – b  plane at high temperatures. An increase of the tilt angle of the NbO 6  octahedra away from an h.c.p. arrangement towards a perovskite structure was observed. The octahedra become almost regular at high temperatures. Comparison with an earlier investigation of congruent lithium niobate shows a quite similar behaviour, however, all parameters of the latter are closer to those of the perovskite structure at same temperatures.

A neutron powder investigation of the high-temperature structure and phase transition in stoichiometric LiNbO 3