THE DEFECT STRUCTURE OF COMPOUNDS WITH THE RUTILE STRUCTURE

Abstract

We present a theoretical study of the defect properties of three compounds, MnF2, MgFz and TiOz. Our results predict a defect structure which is in marked contrast to that charac- teristic of fluorite compounds. We propose that Schottky disorder is intrinsic to all three crystals, with ion transport effected by vacancy mobility in both pure and doped materials. Earlier studies on both fluoride and oxide rutiles have, however, suggested interstitial disorder. Nevertheless, we show that our proposals are compatible with the recent data of Park and Nowick (2) on the conductivity of MgFz and MnFz. We then report a detailed study of vacancy migration and we find that, for both anion and cation vacancies, the dominant mechanism is one which will result in largely isotropic conductivity. This accords with the experimental data on crystals doped with trivalent ions ; but for monovalent doped crystals, high anisotropy has been observed, which may, however, be due to dopant mobility. 1. Introduction. - The defect properties of compounds with the rutile structure (e. g. MnFz, TiO,) have been far less extensively studied than those of corresponding fluorite crystals (e. g. CaF,, UO, (I)). The rutiles are more complex systems, and in particular their non-cubic structure, illustrated in figure 1, leads to anisotropy in defect properties - a problem not encountered for the cubic fluorites. In this paper, we examine the defect structure and transport properties of rutiles using theoretical methods which we have applied with considerable success to the fluorites (6, 7, 111. Our results suggest that there are fundamental differences between the two types of compound, both in the nature of the intrinsic disorder and in the defect transport mechanisms. We concentrate mainly on the rutile fluorides, MnF, and MgF, for which some expe- rimental data are available (2), but the more complex problems presented by the oxide, TiO, are briefly