Three Phases of Basic Zirconium and Hafnium Hydroxohalides

Aqueous solutions of zirconium and hafnium (M) halides (X) with atomic ratios α = X/M near 1 form glasses on evaporation. Herein, we describe the preparation and properties of these glasses and discuss the nature of the crystal-glass equilibria beyond the pure glass compositions. Small- and wide-angle X-ray scattering (SWAXS) studies reveal increased polymerization as α decreases from 2 to 1. The glasses are found to be much denser than their crystalline counterparts. Crystals forming in contact with glasses retain the well-known Zr-tetrameric hydroxo cluster unit with hydroxide compensating for the lowered halide content. We find that the chemical formulas for all of the solid hydroxohalides may be described by the single parameter α, according to the formula M(OH)_4-αX_α·(4α - 1)H₂O. This description is valid for the crystalline chloride (MOX₂·8H₂O = M(OH)₂X₂·7H₂O), the glassy solids with α < 2, and hydrolyzed products (α ≈ 0.5). The water content is also determined by α with hydroxide-hydrogen bonding replacing halide-hydrogen bonding as α decreases. A Eu³⁺-doped Zr,Cl glass exhibits photoluminescence transitions ⁵D₀ → ⁷F (n = 1, 2, and 4) of Eu³⁺, illustrating the asymmetric nature of the dopant sites in the glass.