Electrical conductivity jump produced by the α-β-δ transformation in Mn2GeO4

The electrical conductivity of three polymorphs of Mn₂GeO₄ was measured under high pressures in the temperature range of 300–1200 K. It was found that the electrical conductivity increases discontinuously due to the transformation both from olivine structure (α) to modified spinel structure (β) and from β to strontium plumbate structure (δ). The amount of discontinuous change is about one half order of magnitude from α to β and one third order of magnitude from β to δ at 1200 K. In order to see the effect of the presence of iron ions, the electrical conductivity of the solid solution of (Mn_0.9Fe_0.1)₂GeO₄ was also measured. It was found that at low temperatures, where impurity conduction may be dominant, the solid solution is more conductive than the pure Mn₂GeO₄, but at high temperature no significant differences were observed between the solid solution and pure Mn₂GeO₄ in all polymorphs.A phase transformation from modified spinel structure to strontium plumbate structure is considered to be one of the plausible transformations occurring at a depth around 650 km in the earth's mantle. These experiments suggest that if this kind of transformation occurs in the mantle, some degrees of discontinuous change in electrical conductivity may be expected around 650 km.