A neutron powder investigation of the high-temperature structure and phase transition in stoichiometric LiNbO3

The structural changes of stoichiometric LiNbO3 at temperatures up to 1470 K have been investigated by neutron powder diffraction. At T-c approximate to 1460 K, LiNbO3 transforms from a paraelectric high-temperature (R (3) over bar c) to a ferroelectric low temperature structure (R3c). 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 rakes 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 till angle of the NbO6 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. Anomalies in the behaviour of the order parameters, a more complicated disorder and a high mobility of Li at high temperatures are explained by the competition of the order-disorder and displacive character of the phase transition.