Halide Glasses
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The term "halide glass" refers to glasses in which the anions are from elements in Group VIIA of the periodic table, namely, F, CI, Br and I, as opposed, for example, to "oxide glasses," such as silicates, borates, phosphates, etc. Two known single component halide melts are glassforming, BeF2 and ZnCl2, but the majority of halide glasses are multicomponent. Practical interest in halide glasses has been generated almost entirely by their optical properties, which cannot be duplicated in a more conventional oxide glass. Barriers to the practical deployment of halide glasses have their origin in materials properties in which they can be markedly inferior to oxide glasses, e.g., mechanical strength, resistance of the melt to crystallization, chemical durability, etc. In the past decade there has been considerable and accelerating research activity in the area of halide glass science and engineering. Halide glass research up to 1980 has been reviewed by Baldwin et al.1 and oxide and halide glasses for laser applications have been compared by Weber. 2 Four international symposia on halide glass science and engineering have been held in the period 1982-1987, the proceedings of the last two of which have been or will be shortly published.34 The proceedings of a 1986 NATO-sponsored meeting on halide glasses have also been published in book form.5 The halide glass systems which seem to have the least severe drawbacks of the types mentioned above and which have been given truly serious consideration and developmental effort for real world applications are the fluorides. There are two general categories of these — heavy metal fluoride (HMF) glasses and BeF2-based glasses — and they will be the only halide glasses considered in the remainder of this article. Heavy Metal Fluoride Glasses Background
Heavy metal fluoride (HMF) glasses were discovered quite by accident at Professor Jacques Lucas' laboratory at the University of Rennes on March 15, 1974. One of his co-workers, Michel Poulain, was attempting to prepare a 2ZrF4-BaF2-NaFNdF3 crystal with a large cation in the center of a SmZrF7 structure by heating the components in a sealed Ni tube. Small pieces of glass (3-4 mm), which Poulain initially thought might be single crystals, were obtained. However, x-ray diffraction showed that the material was mostly amorphous (i.e., a glass) with only a few NdF3
crystals present. A batch melted without NdF 3 gave a good glass, and the first report6 on HMF glasses was of a small glass forming region in this system centered about the 50ZrF4-25BaF2-25NaF composition. Interestingly, the unusual HMF glass optical property noted in this first report involved the fluorescence spectra of small amounts of Nd +3 doped into the glass. The time scale for nonradiative de-excitation of the Nd+3 ions was found to be remarkably long. It was not until the second published report7 on ZrF4-based HMF glasses that notice was taken of their extended range of transparency in the infrared compared to oxide glasses. It is this latter feature that is responsi
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