Commercial starting materials are presently not pure enough to attain the near-intrinsic attenuation necessary for the use of heavy metal fluoride glass (HMFG) in telecommunication quality optical fibers. The purity requirement provided in the work of Ohishi et al. , and the comprehensive work of France et al.  in which the oxidation state was con-trolled, suggest that cationic impurities must be limited for each element to the levels shown in Table 1. It can be argued that slightly less stringent purities than those shown in Table 1 are adequate, but the purity levels available from one repu-table commercial supplier are far greater than those required for a number of critical elements.
Robert C. Folweiler,
"Purification Of Fluorides By A Vapor Phase Technique", Proc. SPIE 0799, New Materials for Optical Waveguides, (24 August 1987); doi: 10.1117/12.941169; https://doi.org/10.1117/12.941169