Transport properties of the systems SnS 2-SnI 4 and SnS 2-I 2

Abstract

Mass transport rate studies on SnS 2 in the temperature gradient 648–723 K using SnI 4 as initial transport agent revealed the existence of forward (high → low temperature) and reverse (low → high temperature) transport. The net transport direction was found to be pressure dependent. At higher temperatures, only forward transport was observed for the SnS 2-SnI 4 system. When elemental iodine is employed as initial transport agent in the above gradient, SnS 2 is transported from low to high temperature for pressures between about 0.3 and 150 kPa. Thermodynamic calculations for these systems yielded partial pressures of the major gaseous species SnI 4, SnI 2, I 2, I, S 2, S 3, S 4, S 5, S 6, S 7 and S 8, which led to the formulation of the dominant transport reactions consistent with the observed net transport direction. Based on the hypothetical pressure P ∗ of SnS 2, defined as the “solubility” of SnS 2 in the gas phase, the quantity δP ∗ is given by the equation δP ∗ = P ∗(SnS 2) T 2 - P ∗(SnS 2) T 1, where T 2 < T 1. The solubility of SnS 2 can be expressed in terms of the gaseous sulfur or Sn-iodine species. In the absence of kinetic limitations, for positive values of δP ∗ forward transport (T 2 → T 1) is dominant. When δP ∗ is negative, net reverse transport will occur. The presence of both forward and reverse transport implies the existence of an inversion temperature at which the solubility of the solid is a minimum. Calculations of δP ∗ as a function of pressure reveal that the inversion pressure (change in transport direction) occurs at higher total pressures with increasing temperature. The results of these calculations are consistent with present investigations and previous studies on the SnS 2-SnI 4 and SnS 2-I 2 systems.