Fluoride bulk crystals growth

Abstract

Fluoride crystals have long presented a challenge for chemists and physicists, because their preparation often requires very specific procedures. Their synthesis usually requires the use of toxic and/or corrosive materials, procedures demanding suitable equipment, and safety operational norms. Additionally, most of the fluoride compounds are moisture sensitive. As a result, a dry working environment is needed. In spite of this, fluoride crystals have found many applications as dosimeters, information storage devices, X-ray monochromators, and, in particular, in the field of optics as laser hosts and other components such as windows, which has encouraged the development of research work on the preparation of large crystals. The successful preparation of bulk fluoride crystals is dependent upon a number of factors directly related to the crystal growth conditions of the most important is the purity of the raw material. The choice of high purity commercial salts will not always assure the growth of a crystal with high quality. First, the manipulation of fluorides without the correct procedures can easily introduce moisture contamination. Fluorides are very sensitive to water and oxygen, which are always present in the work environment. Second, commercial fluorides are rarely of sufficient purity, with respect to oxygen contaminants. Usually, starting materials are ‘five-nines’ (99.999%) pure with respect to the cation. Oxygen-related impurities, such as OH , COH , NO2 , present vibrational modes in the infrared range. Their presence, even in small concentrations, can be detrimental to the quality of infrared optical devices. In general, most simple fluoride crystals, such as alkali halides (LiF, NaF, RbF) or alkaline-earth fluorides (CaF2, SrF2, BaF2), are grown directly from commercial chemicals, after a preliminary treatment to eliminate moisture and oxygen contaminants. However, more complex fluoride compounds (e.g., LiSrAlF6 or BaY2F8) need to be carefully synthesized before the growth process. A preliminary treatment or synthesis of the raw material to be used for the crystal preparation is the first step in the growth of fluoride crystals.