Silicon Self-Diffusion in Stishovite: Calculations of Point Defect Parameters Based on the cBΩ Thermodynamic Model

In the present work we apply the cBΩ thermodynamic model to study the diffusion of Si in stishovite crystal at high pressure and in a wide temperature range. According to this model, the point defect activation Gibbs free energy is expressed as a function of the bulk properties of the material, i.e., gact = cBΩ, where B is the isothermal bulk modulus, Ω is the mean atomic volume, and c is a dimensionless constant. In this way, other important point defect parameters, such as the activation volume vact, the activation entropy sact, and the activation enthalpy hact may be estimated if the thermoelastic properties of the material are known over a wide temperature and pressure range. Our calculations are based on previously reported self-diffusion coefficients in stishovite single crystals measured at 14 GPa and at temperatures from 1400 to 1800 °C, in the [110] and [001] directions, by Shatskiy et al. (Am. Mineral. 2010, 95, 135–43). Furthermore, the EOS of stishovite, proposed by Wang et al. (J. Geophys. Res. 2012, 117, B06209) has been used for the accurate implementation of the cBΩ model. Our results suggest that the aforementioned point defect parameters exhibit considerable temperature dependence over the studied temperature range (1000–2000 °C). The estimated activation volumes (4.4–5.3 cm3/mol, in the range of 1400–1800 °C) are in agreement with reported experimental results. Our study confirms the potential of the cBΩ model for the theoretical investigation of diffusion processes in minerals, in order to overcome the experimental difficulties and the lack of experimental diffusion data in mantle conditions.

Bulk moduli of PbSxSe1−x, PbSxTe1−x and PbSexTe1−x from the combination of the cBΩ model with the modified Born theory compared to generalized gradient approximation

The bulk moduli of PbS[Formula: see text]Se[Formula: see text], PbS[Formula: see text]Te[Formula: see text] and PbSe[Formula: see text]Te[Formula: see text] have been recently determined by employing a thermodynamical model, the so-called [Formula: see text] model, which has been found to give successful results in several applications of defects in solids. Here, we suggest an alternative procedure for this determination which combines the [Formula: see text] model with the modified Born theory. The results are in satisfactory agreement with those deduced independently by the generalized gradient approximation approach.