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 E-A 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 beta-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 SnO, do not noticeably influence the reaction rate. Formation of the intermediate tin oxide Sn2O3 is not observed under high pressure. (c) 2005 Elsevier B.V. All rights reserved.