Manganese olivine I: Electrical conductivity

Abstract

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn₂SiO₄, electrical conductivity (’) of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$ atm under both Mn oxide (MO) buffered and MnSiO₃ (MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is ∼1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer.

The electrical conductivity data lie on concave upward curves on a log-log plot of σ vs $f_{{\text{O}}_{\text{2}} }$, giving rise to two $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$ regimes with different oxygen fugacity exponents. In the low-$f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$ regime $\left( {f_{{\text{O}}_{\text{2}} } < 10^{ - 7} {\text{atm}}} \right)$, the $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$ exponent, m, is 0, the MnSiO₃-activity exponent, q, is ∼0, and the activation energy, Q, is 45 kJ/mol. In the high $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$ regime $\left( {f_{{\text{O}}_{\text{2}} } > 10^{ - 7} {\text{atm}}} \right)$, m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T<1100 °C and T>1100 °C, respectively.

Based on a comparison of experimental data with results from point defect chemistry calculations, it is proposed that the change in m with $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$ is induced by a switch in charge neutrality condition. At low $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$s, the charge neutrality condition is [e′]=[Mn ^{_Mn ]; the hopping motion of electron holes h ^. is the dominant conduction mechanism. At high $f_{{\text{O}}_{\text{2}} } = 10^{ - 11} - 10^2$s, the charge neutrality condition is 2[V ^·_Mn ]=[Mn ^· _Mn ]; the hopping motion of electron holes h ^. and the migration of Mn ions associated with a counter flow of divalent Mn vacancies V ^·_Mn control electrical conduction at T<1100 °C and T>1100 °C, respectively.