The prediction of starting compositions for the flux growth of complex oxide crystals

Abstract

The flux growth of complex oxides from systems which contain a refractory oxide, an acidic or amphoteric oxide, and a basic oxide (or oxide + fluoride) is considered. It is shown that the growth of a desired compound may require an excess of the acidic or amphoteric oxide to be included in the melt, and that this is related to the difference between the melting points of the refractory oxide and the acidic or amphoteric oxide. When the melting-point differences are large, melts with a relatively high concentration of basic oxide/fluoride have produced crystals larger and more nearly equidimensional than those obtained when relatively less basic oxide was present. A correlation is demonstrated between an increase in the number of bond-breaking anions provided by the basic oxide/fluoride component and the growth of more nearly equidimensional crystals. A generalised pseudoternary composition diagram is provided as a model in determining starting compositions likely to produce the desired binary or ternary oxide. This model has been successfully applied to the flux growth of the zircon family compounds (ZrSiO 4, ThSiO 4, ZrGeO 4, ThGeO 4), compounds in the system Dy 2O 3-K 2O-MoO 3 (Dy 6MoO 12, Dy 2MoO 6 and DyKMo 2O 8), and to PrBO 3, DyVO 4, DyPO 4, Co 2SiO 4 and other materials. It is suggested that a good flux for oxides is one which contains the greatest proportion of discrete tetrahedral or triangular complex anions compatible with crystallisation of the desired phase.