Thermal behaviour of libethenite from room temperature up to dehydration

The structural modifications with temperature of libethenite, Cu2(PO4)(OH), were determined by single-crystal X-ray diffraction up to dehydration and consequent decomposition of the crystal under investigation. In the temperature range 25-475 °C, libethenite shows positive and linear expansion. The axial thermal expansion coefficients, determined over this temperature range are: αa = 6.6(1)•10-6 K-1, αb = 1.21(2)•10-5 K-1, αc = 9.0(2)•10-6 K-1, αV = 2.78(3)•10-5 K-1. Axial expansion is then anisotropic with αa:αb:αc = 1:1.83:1.33. Structure refinements of X-ray diffraction data collected at different temperatures allowed to characterise 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 P 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 = 500 °C, precursor signs of incoming dehydration are visible: two adjacent OH groups approach each other and cause dramatic changes in the whole structure. Concomitantly, libethenite crystal begins to deteriorate and at T = 600°C broad and weak diffraction effects of polycrystalline material are observed.