Abstract

The lattice parameters extracted from Lebail analysis of neutron powder diffraction data collected between 2 and 300 K have been used to calculate the temperature evolution of the thermal expansion tensor for hopeite, Zn3(PO4)2·2H2O, Pnma, Z = 4 with a = 10.6065(4) Å, b = 18.2977(4) Å, c = 5.0257(2) Å at 275 K. The a lattice parameter shows a negative thermal expansion, the b lattice parameter appears to saturate at 275 K while the c lattice parameter has a more typical positive thermal expansion. At 275 K, the magnitudes of the thermal expansion coefficients are αa = −1.1(4) × 10−5 K−1, αb = 2.4(9) × 10−6 K−1 and αc = 3.6(2) × 10−5 K−1. Under the conditions of these experiments, hopeite begins to dehydrate to the dihydrate between 300 and 325 K, and between 480 and 500 K the monohydrate is formed. The thermal expansion of the dihydrate has been calculated between 335 and 480 and at 480 K the magnitudes of the thermal expansion coefficients are αa = 1(2) × 10−5 K−1, αb = 4(1) × 10−6 K−1, αc = 4(2) × 10−5 K−1, αβ= 1(1) × 10−5 K−1, and αV = 2(2) × 10−5 K−1. The thermal expansion of hopeite is described in terms of its crystal structure and possible dehydration mechanisms for the α and β modifications of hopeite are discussed.

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