Phase transformations and elasticity in rutile-structured difluorides and dioxides

Pressure-induced phase transformations in each of the rutile-structured difluorides (NiF2, MgF2, CoF2, ZnF2, FeF2 and MnF2) exhibit unique behavior; however, a general trend is found in the major structural changes: rutile phase → “distorted fluorite” phase → post-“distorted fluorite” phase with volume changes of about 5–10%. For a given phase transformation sequence found commonly in two or more difluorides, the phase transformation pressure is related inversely to the unit cell volume and thus inversely to the mean cation-anion bond length. The relationship in oxides (SnO2, TiO2 and GeO2) is much less systematic. It is therefore not possible to predict without uncertainty the post-stishovite phases in the lower mantle. Velocity-density systematics in the difluorides and oxides are governed, to a large extent, by cationic radius. The pressure dependence of shear elastic constant CS = (C11 − C12)/2 is negative in all of the nine difluorides and oxides. However, the CS mode does not vanish at the initial phase transformation pressure; rather, the ratios of CSKS are 0.10 and 0.04 to 0.10 for transitions of rutile → orthorhombic and of rutile → “distorted fluorite”, respectively, and are in agreement with the approach of Demarest et al.

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