Thermal disproportionation of SnO under high pressure

The kinetics of the disproportionation of SnO to SnO2 and Sn was studied in situ under pressures up to 15 GPa by X-ray powder diffraction with synchrotron radiation. From thermal disproportionation of SnO at different temperatures we extracted the reaction rate law and the reaction rate constants k and activation energy EA for different pressures. Under high pressure a drastic change of the disproportionation reaction is found. While the reaction follows a nucleation and growth mechanism at ambient pressure a diffusion controlled mechanism is observed at pressures of 2 GPa and above. Starting at 5 up to 15 GPa a gradual change back to a nucleation and growth mechanism is observed. For a given temperature the reaction is always faster under pressure than at ambient pressure in the complete studied p/T-region (0-15 GPa, 500-700 K). The reaction rate constant k of the disproportionation of SnO depends on the phase of the emerging metallic tin. The value of k is one order of magnitude lower when β-Sn is formed compared to the case when Sn evolves as a liquid or in the high pressure phase Sn-II. In contrast, the different phases of SnO2 do not noticeably influence the reaction rate. Formation of the intermediate tin oxide Sn2O3 is not observed under high pressure.