| Summary: | Co-doped SnO<sub>2</sub> nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group <i>P</i>4<sub>2</sub>/<i>mnm</i>) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 GPa and at an ambient temperature. An analysis of experimental results based on Rietveld refinements suggests that rutile-type Co-doped SnO<sub>2</sub> undergoes a structural phase transition at 14.2 GPa to an orthorhombic CaCl<sub>2</sub>-type phase (space group <i>Pnnm</i>), with no phase coexistence during the phase transition. No further phase transition is observed until 20.9 GPa, which is the highest pressure covered by the experiments. The low-pressure and high-pressure phases are related via a group/subgroup relationship. However, a discontinuous change in the unit-cell volume is detected at the phase transition; thus, the phase transition can be classified as a first-order type. Upon decompression, the transition has been found to be reversible. The results are compared with previous high-pressure studies on doped and un-doped SnO<sub>2</sub>. The compressibility of different phases will be discussed.
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