Manganese olivine II: Point defect relaxation

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn₂SiO₄, the point defect relaxation time (τ) characterizing the rate of re-equilibration of electrical conductivity following a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The experiments were carried out at temperatures T = 1173-1473 K and oxygen fugacities fo₂ = 10^-6.0-10^3.8 Pa with the MnSiO₃ activity controlled at unity. The value of τ, which ranges from 130 to 1463 s, increases with decreasing temperature. At 1273 K, the value of τ in the high - fo₂ regime is a factor of ∼1.8 smaller than that in the low- fo₂ regime. The point defect relaxation time was used to calculate the chemical diffusivity (D). Values of D̄ lie in the range 2.2× 10^-10- 2.5×10^-9 m²/s. For the high-fo₂ regime, a semi-log plot of D vs l/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn-olivine, it is proposed that the relaxation rate of electrical conduction is determined by the coupled diffusion of manganese vacancies V_Mn″ and electron holes Mn_Mn, which rate-control the relaxation process at low and high temperatures, respectively. Deconvolution of the D̄ - 1/T curve into two straight-line segments yields values for the mobilities and the diffusivities for Mn vacancies and electron holes. These results, combined with the measured electrical conductivity data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mobility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in (Mg_0.9Fe_0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe-olivine crystals and the lowest in Co-olivine crystals, while those in Mn-olivine and (Mg_0.9Fe_0.1)-olivine crystals lie in between. The deviation from stoichiometry in the cation sublattice for a transition-metal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide.