Defect structures and nonstoichiometry in lanthanum hexa-aluminate

Abstract

A number of defect structures and nonstoichiometries possible in lanthanum hexa-aluminate (LHA) have been investigated, by using computer-based atomistic simulation techniques. It has been possible to discriminate energetically between alternative models. Our calculations suggest that LHA will not form an ideal beta-alumina-type stoichiometric phase LaAl11O18, but most likely a magnetoplumbite (MP)-type nonstoichiometric phase La(0.83)A(11.83)O(19), which is essentially identical to the "vacancy model" phase proposed by Iyi et al. (J. Solid State Chem., 1984, 54, 70, 123). Our calculations also reveal that the defect complex in this phase is composed of a vacancy pair (V-La and V-Al) on the mirror plane, and a pair of Al Frenkel-like defects formed doubly (rather than singly) above and below the center of the vacancy pair. It is suggested that a nonstoichiometric phase LaAl11.67O19, which has also an MP-type structure with aluminum vacancies, may coexist with the most likely phase, especially when the Al/La ratio of the system is about 13 to 14. It is revealed that, in this nonstoichiometric phase, the aluminum vacancies prefer the central spinel block 2a sites to other sites. It is also quite possible to form solid solutions between these two MP-type LHA phases. Our calculation results indicate that the 'O-La' defect may be improbable in LHA system. It is also revealed that there is a great possibility of existence of metastable phases; it is seen, however, that beta-type or beta/MP-mixed-type structured phases ave less stable than MP-type phases. (C) 1999 Elsevier Science Ltd. All rights reserved.