X-ray diffraction study of the phase transitions and structural evolution of tin dioxide at high pressure:ffRelationships between structure types and implications for other rutile-type dioxides

${\mathrm{SnO}}_2$ was investigated at pressures up to 49 GPa by angle-dispersive x-ray diffraction using an imaging plate. Three phase transitions were observed on compression. Rutile-type ${\mathrm{SnO}}_2$ underwent a second-order transition to a ${\mathrm{CaCl}}_2$-type phase at 11.8 GPa under hydrostatic conditions, as determined from the pressure dependence of the spontaneous strain. This transition was observed at significantly lower pressures under nonhydrostatic conditions. A second transition to an α-${\mathrm{PbO}}_2$-type phase was observed to begin above 12 GPa under nonhydrostatic conditions; however, only a small amount of this phase was obtained. Both the α-${\mathrm{PbO}}_2$-type and the ${\mathrm{CaCl}}_2$-type phases transformed to a modified fluorite-type phase, space group Pa3-bar, above 21 GPa. Upon decompression, retransformation was observed and the sample recovered under ambient conditions consisted of a mixture of the rutile-type and α-${\mathrm{PbO}}_2$-type phases. The structures of the rutile, ${\mathrm{CaCl}}_2$ and modified fluorite type phases were refined in situ by the Rietveld method allowing the structural evolution of ${\mathrm{SnO}}_2$ to be followed as a function of pressure. The relationships between the high-pressure structures of ${\mathrm{SnO}}_2$ are discussed using group theory and potential transformation pathways identified. The transition sequence observed for tin dioxide has important implications for the high-pressure behavior of other rutile-structured compounds.