Radiotracer Study of the Preparation of High‐Purity Lanthanum Fluoride

Abstract

The behavior of the impurities iron, cobalt, yttrium, and cerium is determined via radiotracer techniques for the preparation of high‐purity lanthanum fluoride. The behavior of nickel and copper during the coprecipitation of a lanthanum nitrate solution is determined by graphite furnace atomic absorption spectrometric (GFAAS) analysis. There is no commercially available radiotracer for neodymium, a key impurity associated with absorption losses in fluoride glasses. However, the chemical behavior of neodymium and that of yttrium are very similar and, therefore, it is resonable to assume that the behavior of yttrium throughout the processing is indicative of the behavior of neodymium. The concentrations of impurities in lanthanum nitrate, carbonate, and fluoride are estimated using the radiotracer and GFAAS data for each processing step. Results indicate that while high‐purity lanthanum carbonate can be prepared, any impurities present in the lanthanum carbonate will be carried quantitatively into lanthanum fluoride upon hydrofluorination. The data also indicate that lanthanum nitrate can be prepared in higher purity than lanthanum carbonate; however, lanthanum fluoride prepared from the nitrate forms a very wet precipitate which is extremely difficult to filter and dry. The presence of excess water in lanthanum fluoride results in high levels of oxide contamination in lanthanum fluoride as well as the fluoride glass. Preparation of low‐oxide lanthanum fluoride is required because high oxide levels in fluoride glasses cause increased scattering. Therefore, from a fluoride glass point of view, the preferred synthetic route for the preparation of high‐purity lanthanum fluoride is to first purify lanthanum nitrate, then convert this to the carbonate, which is then converted to lanthanum fluoride. The estimated impurity concentrations in lanthanum fluoride derived from lanthanum carbonate are as follows: Fe, 89 ng/g; Co, <3 ng/g; Ni, <12 ng/g; Cu, 6 ng/g; Y, 62 ng/g; Ce, 248 ng/g. The loss contributed to a ZBLAN fiber incorporating this lanthanum fluoride is calculated to be 0.082 dB/km; here the concentration of neodymium is assumed to be equal to that of yttrium. These data indicate that the lanthanum fluoride prepared by the current method will not meet the ultimate requirements for ultra‐low‐loss fluoride glasses.