Modification of flux growth compositions to control nucleation

A new mixed-cluster system of the metastable state in flux growth envisages two or more kinds of cluster being present in the fluxed melt, one the substance about to crystallise, the other a compound formed between the most refractory component and either the additive or the flux itself. It leads to a new specification for an additive: a compound which forms one or more kinds of secondary cluster in the fluxed melt and which does not contaminate the desired crystal. This hypothesis offers the possibility of developing new additives to restrict nucleation and of extending their use from systems with lead fluoride to oxide systems. These "additives" may be major components, rather than the few percent hitherto envisaged; and so the term "cluster-forming modifiers" may be more appropriate. An example is given below: Components (Molar) Possible Clusters Present HoVO~, Pb 2V20 7, Pb 3V20 e HoVO~, HoB0 3 , H0 3 B0 6 o K2B~07' Pb 2V207, Pb 3V20 e Composition (1) represents the.method of Feigelson [1], used extensively by ourselves [2]. It results in a large number of thin rods in each crucible. Under similar experimental conditions composition (2) yielded only five crystals. Considering the possible clusters present, composition (2) provides twice as many kinds: HoB03 and H03B0 6 are both more refractory t han Pb 2V207 and Pb 3V20 e in (1) and thus likely to inhibit f ormation of a stable nucleus until the (probably large) critical supercooling is exceeded. New research into cluster-forming modif ierR will be reported. [1] R.S. Feigelson J. Am. Ceram. Soc. ~ (1 968) 538. [2] S.H. Smith and B.M. Wanklyn J. Crystal Growth 21 (1974) 23.