Chemistry of halide window growth

The chemistry of reactive atmosphere processing (RAP) for the growth of crystals of chlorides, bromides, and halides was studied. The purity and freedom from extrinsic absorption obtained by RAP are important in the application of halides as optical elements in high power lasers. The prototype of RAP is CCl4 in the growth of KCl; it reacts directly with water to scavenge out all traces of this contaminant and pyrolyzes with the formation of nascent halogen to react rapidly with hydroxide ions in the melt. The approach with the bromides and fluorides has been to seek RAP agents which simulate the behavior of CCl4 with KCl. On the basis of its hydrolysis behavior, CH2 Br2/He was used as the RAP agent for KBr. This has consistently yielded material with a 10.6 micrometer bulk absorption coefficient of 1/.0002 cm, an order of magnitude below that of non-RAP KBr. Further improvement is possible. In addition, it was found that the minimum-melting solid solution of NaBr and KBr prepared under RAP conditions does not retain phase homogeneity below the melting point, making it impossible to grow single crystals of this material. Work on KCl toward problems associated with the scale up of RAP to produce KCl crystals of greater than 10-cm diameter. Hydrogen fluoride, previously used alone as a RAP agent for fluoride crystal growth, is fast acting but does not achieve a low RAP index, P(H2O)/P(HF). Supplementing it with a fluorocarbon an improved transparency with CF4 admixtures was achieved.