Monoclinic-to-orthorhombic phase transition in cu2(aso4)(oh) olivenite at high temperature: Strain and mode decomposition analyses

A natural olivenite single crystal was submitted to in situ high-temperature single-crystal X-ray diffraction from room temperature (RT) to 500 degrees C. Unit-cell parameters were measured at regular intervals of 25 degrees C, and complete datasets collected at T = 25, 50, 100, 150, 200, 250, 300, 400 and 500 degrees C. Evolution of unit-cell parameters and structure refinements indicates that olivenite undergoes a structural phase transition from P2(1)/n to Pnnm at similar to 200 degrees C, and eventually becomes isostructural with the other members of the olivenite-mineral group. Volume expansion with temperature is larger in the monoclinic phase - where it follows a non-linear trend - than in the orthorhombic one. Axial and volume expansion coefficients of the orthorhombic olivenite phase are positive and linear and similar to those of the other Cu-bearing member of the mineral family, namely libethenite, but rather different from those of the Zn-analogue arsenate adamite. Distortion of Cu polyhedra is quite high in the olivenite monoclinic phase at RT and goes towards a relative regularization with increasing T until the phase transition occurs. In the orthorhombic phase, no significant variation of the polyhedral distortion parameters is observed with increasing temperature, and maximum expansion is along the b direction and governed by corner-sharing. Landau potential provides a good representation of the macroscopic changes associated with the phase transition, coupling between the strains and the order parameter is responsible for the nearly tricritical character of the transition.