DEFECT STRUCTURE AND DISORDER IN LITHIUM NIOBATE. By H. Boysen, F. Frey and N. Zotov^*, Institut für Kristallographie, Universität München, Theresienstr. 41, 80333, München, Germany.

The defect structures of congruent ([Li]:[Nb]=0.947) and, for comparison, stoichiometric LiNbO₃ (space group R3c) have been investigated by Bragg and diffuse scattering of X-rays and neutrons between 38 K and 1515 K. Because the physical properties of Lithium Niobate depend strongly on the non-stoichiometry, a large number of different models for the defect structure has been proposed and discussed in the literature. From the Bragg data the average disorder is characterized by excess Nb⁵⁺ atoms and charge compensating vacancies on the regular Li-sites (Zotov et al. (1994), J.Phys.Chem.Solids 55, 145). With increasing temperature the mobility of Li increases strongly. The para-ferroelectric phase transition (R3c Rc) at T_c ~ 1430 K is of the order-disorder type (of Li) with some displacive components (Boysen and Altorfer (1994), Acta Cryst. B51, 405).

The diffuse scattering is concentrated in three planes perpendicular to the 'pseudo-cubic' directions [21], [241] and [1] (Zotov et al. (1995), Acta Cryst. B51, 961). It is attributed to 1-dim displacive and substitutional disorder along Li-O-Li...-chains running along these directions. The chemical component includes Nb_Li atoms and vacancies. From the observed correlation length the various cluster models discussed are restricted to four atoms. At room temperature 3-dim short-range order correlations are observed. At low temperatures longer range correlations between the (1-dim) defect clusters set in. With increasing temperature the diffuse scattering intensity increases and becomes more homogeneous. This is interpreted either by a temperature dependent variation of the static defect clusters or by dynamical fluctuations similar to those observed in other perovskite structures.

^*Permanent adress: Institute of Applied Mineralogy, Sofia, Bulgaria.