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 C_S = (C₁₁ ? C₁₂)/2 is negative in all of the nine difluorides and oxides. However, the C_S mode does not vanish at the initial phase transformation pressure; rather, the ratios of $\text{C}{}_{\mathrm{<mtext>S</mtext>}}\text{K}{}_{\mathrm{<mtext>S</mtext>}}$ 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.