Low-pressure ferroelastic phase transition in rutile-type AX2 minerals: cassiterite (SnO2), pyrolusite (MnO2) and sellaite (MgF2)

The structural behaviour of cassiterite (SnO2), pyrolusite (MnO2) and sellaite (MgF2), i.e. AX(2)-minerals, has been investigated at room temperature by in situ high-pressure single-crystal diffraction, up to 14 GPa, using a diamond anvil cell. Such minerals undergo a ferroelastic phase transition, from rutile-like structure (SG: P4(2)/mnm) to CaCl2-like structure (SG: Pnnm), at approximate to 10.25, 4.05 and 4.80 GPa, respectively. The structural evolution under pressure has been described by the trends of some structure parameters that are other than zero in the region of the low-symmetry phase's stability. In particular, three tilting-angles (omega, omega ', ABS) and the metric distortion of the cation-centred octahedron (quantified via the difference between apical-anion and equatorial-anion distances Delta|X-ax-X-eq|) are used to express the atoms' readjustment, i.e. relaxation, taking place in the CaCl2-like structures under pressure. The crystallographic investigation presented is complemented with an analysis of the energy involved in the phase transition using the Landau formalism and adopting the following definition for the order parameter: Q = eta(11)-eta(22), eta(ij) being the spontaneous strain tensor. The dependence of omega, omega ', ABS and Delta|X-ax-X-eq| on Q allows determination of a correlation between geometrical deformation parameter and energy. Lastly, the relaxation mechanisms that exploits omega, omega ', ABS and Delta|X-ax-X-eq| may be related to the ionic degree of bonding, the latter modelled via quantum mechanics and Bader theory. Sellaite, the mineral exhibiting the highest degree of ionic bonding among those investigated, tends to accomplish relaxation through pure rotation of the octahedron, rather than a metric distortion (Delta|X-ax-X-eq|), which would shorten inter-atomic distances thus increasing repulsion between anions.