Orientation, oxygen activity and temperature dependencies of the diffusion of manganese in manganese orthosilicate, Mn2SiO4
Abstract The diffusion of manganese in manganese silicate, Mn 2 SiO 4 , in equilibrium with MnSiO 3 was experimentally studied as a function of crystallographic orientation, oxygen activity and temperature using high purity synthetic single crystals of manganese silicate. The oxygen activity dependence of the manganese cation diffusion in Mn 2 SiO 4 along the three principle orientations at 1200 °C suggests that, at high oxygen activities, manganese cations move via different types of vacancies, most likely isolated manganese vacancies, ( V Mn 2 + )″, and possibly neutral associates, {2(Mn Mn 2 + 3 + )  ⋅ ( V Mn 2 + )″} × , the latter being minority defects. At lower oxygen activities, the oxygen activity dependence of the manganese tracer diffusion becomes smaller than that at higher oxygen activities and eventually becomes negative at very low oxygen activities. The latter is attributed to an increased concentration of manganese interstitials at lower oxygen activities. The temperature dependence of the manganese tracer diffusion along the three principle orientations was experimentally investigated between 1150 and 1250 °C at log a O 2  = − 2.7 ( a O 2  =  P O 2 / P O 2 o with P O 2 o  = 1 atm) and between 1150 and 1300 °C at log a O 2  = − 9.8. The results obtained suggest that the orientation dependence of the diffusion of manganese in Mn 2 SiO 4 does not vary very significantly with temperature. When using the space group Pbnm to describe crystal orientations, the ratio found for the manganese tracer diffusion coefficients at log a O 2  = − 2.7 is approximately D Mn[001] ∗ :D Mn[010] ∗ :D Mn[100] ∗  = 5.3:2.2:1.