Preparation, Characterization, and Structure of α-Zirconium Hydrogen Phosphate Hemihydrate

Abstract

A hemihydrate form of layered zirconium phosphate was obtained by reacting ZrO2 with concentrated H3PO4 (17 M) at 230°C for 2 days, Zr(HPO4)2 · 0.5H2O crystallizes in the monoclinic symmetry with cell constants a = 9.1478(5) Å, b = 5.3242(3) Å, c = 15.288(1) Å, β = 103.848(6)°, space group C2/c. It undergoes a reversible phase transition at about 70°C, without losing the lattice water; the interlayer distance is reduced to 7.30 Å and the symmetry changes to the trigonal one (a = 5.3743(5) Å, c = 21.982(2) Å, space group R3̄ ). The crystal structure of the hemihydrate phase at room temperature was determined by using 36 unambiguously indexed reflections, obtained by the decomposition of the X-ray diffraction pattern, in a conventional single-crystal analysis. A geometric model was assumed for the high-temperature phase. Refinement of the crystal structures was performed by the Rietveld method. In the low-temperature phase, the crystallization water forms interlayer hydrogen bonds with the P-OH of the α-layers, which accounts for the very long times or the elevated temperature required for complete dehydration to occur. Accordingly, the hemihydrate does not transform into the monohydrate phase even when dipped into boiling water and it does not seem obtainable from partial dehydration of the monohydrate form.