Preparation and Characterization of High‐Purity Metal Fluorides for Photonic Applications

Abstract

We combine chelate-assisted solvent extraction (CASE) and hot hydrogen fluoride gas treatment to enable a general method for the preparation of high-purity binary metal fluorides. The fluorozirconate glass ZBLANI:Yb 3+ (ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF-InF 3 -YbF 3 ), a solid-state laser-cooling material, is used as a test case to quantitatively assess the effectiveness of the purification method. The reduction of transition-metal and oxygen-based impurities is quantified directly by inductively coupled plasma mass spectrometry (ICP-MS) and indirectly by laser-induced cooling, respectively. The concentrations of Cu, Fe, Co, Ni, V, Cr, Mn, and Zn impurities in the ZrCl 2 O precursor solution were measured individually by ICP-MS at various stages of the purification process. CASE was found to reduce the total transition-metal concentration from 72500 tõ 100 ppb. Laser cooling was most efficient in ZBLANI:Yb 3+ glass fabricated from CASE-purified metal fluoride precursors, confirming the results of the ICP-MS analysis and demonstrating the effectiveness of the purification methods in a finished optical material. High-purity metal fluorides prepared by the methods presented herein will enable new high-performance optical materials for solid-state optical refrigerators, crystals for vacuum ultraviolet (VUV) spectroscopy of the Thorium-229 nucleus, VUV optics, fibers, and thin-film coatings.

Infrared fibers

“…The use of high-purity raw materials (both metals and fluorides) is essential to reduce both extrinsic absorption (through a reduction in their water and oxide content) and scattering losses (by preventing the formation of crystals in the glass). Unfortunately, ultrahighpurity, ultradry fluoride raw materials are not commercially available and thus in-house purification is required to fabricate ultralow-loss fluoride glass [204,208]. In addition to raw-material purity, glass processing conditions play a significant role.…”

Identification of parasitic losses in Yb:YLF and prospects for optical refrigeration down to 80K

“…The cooling efficiency can also be improved by material synthesis targeted to reduce the ratio of background absorption to resonant absorption, α b /α r (λ,T). Purification of the starting materials prior to synthesis [18] is one of the direct approaches for reduction of impurities. At the same time, an increase in the doping concentration can improve the ratio, provided heatproducing impurities are not introduced during the doping process.…”

Cryogenic optical refrigeration

“…The chemical inertness of the binary fluorides, however, prevents the use of standard purification methods (e.g., sublimation, recrystallization, ion exchange, solvent extraction) and makes their purification difficult. One approach to creating pure laser cooling materials [131] begins with the respective oxides, carbonates, chlorides, or metals that can be dissolved in acids, purified by solvent extraction, precipitated as fluorides with hydrofluoric acid, and finally converted to high-purity binary fluorides by drying in hot HF gas. This comprehensive process has been demonstrated for each of the ZrF 4 , BaF 2 , LaF 3 , AlF 3 , NaF, InF 3 , and YbF 3 precursors of the Yb 3+ :ZBLANI laser cooling glass [131].…”

“…One approach to creating pure laser cooling materials [131] begins with the respective oxides, carbonates, chlorides, or metals that can be dissolved in acids, purified by solvent extraction, precipitated as fluorides with hydrofluoric acid, and finally converted to high-purity binary fluorides by drying in hot HF gas. This comprehensive process has been demonstrated for each of the ZrF 4 , BaF 2 , LaF 3 , AlF 3 , NaF, InF 3 , and YbF 3 precursors of the Yb 3+ :ZBLANI laser cooling glass [131]. In this study, an ∼725 fold reduction in transition-metal impurities over the initial commercial purity was measured, achieving a final residual transition-metal concentration of ∼100 ppb that was suited for the subsequent synthesis of a high-performance laser cooling material.…”