The structural modifications with temperature of libethenite, Cu 2 (PO 4 )(OH), were determined by single-crystal X-ray diffraction up to dehydration and consequent decomposition of the crystal under investigation. In the temperature range 25À475ºC, libethenite shows positive and linear expansion. The axial thermal expansion coefficients, determined over this temperature range, are: a a = 6.6(1)·10 À6 K À1 , a b = 1.21(2)·10 À5 K À1 , a c = 9.0(2)·10 À6 K À1 , a V = 2.78(3)·10 À5 K À1 . Axial expansion is then anisotropic with a a :a b :a c = 1:1.83:1.33.Structure refinements of X-ray diffraction data collected at different temperatures allowed us to characterize the mechanisms by which the libethenite structure accommodates variations in temperature. Increasing temperature induces expansion of both Cu polyhedra and no significant variation of the PO 4 tetrahedron, which acts as a rigid unit. Cu(1) octahedra expand mostly as a consequence of the increase of the axial bonds, and become more distorted. Starting from T = 500ºC, precursor signs of incoming dehydration are visible: two adjacent OH groups approach each other and cause dramatic changes in the whole structure. Concomitantly, the libethenite crystal begins to deteriorate and, at T = 600ºC, broad and weak diffraction effects of polycrystalline material are observed.
The high-temperature (HT) behaviour of a sample of natural alunite was investigated by means of in situ HT single-crystal X-ray diffraction from room temperature up to the dehydroxylation temperature and consequent collapse of the crystal structure. In the temperature range 25À500ºC, alunite expands anisotropically, with most of the contribution to volume dilatation being produced by expansion in the c direction. The thermal expansion coefficients determined over the temperature range investigated are: a a = 0.61(2)610 À5 K À1 (R 2 = 0.988), a c = 4.20(7)610 À5 K À1 (R 2 = 0.996), a c /a a = 6.89, a V = 5.45(7)610 À5 K À1 (R 2 = 0.998). At~275À300ºC, a minor discontinuity in the variation of unit-cell parameters with temperature is observed and interpreted on the basis of loss of H 3 O + that partially substitutes for K + at the monovalent A site in the alunite structure. Increasing temperature causes the Al(O,OH) 6 sheets, which remain almost unaltered along the basal plane, to move further apart, and this results in an expansion of the coordination polyhedron around the intercalated potassium cation. Sulfate tetrahedra act as nearly rigid units, they contract a little in the lower temperature range to accommodate the elongation of the Al octahedra.
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