Structural Phase Transformations in Iron-Chalcogen under High Pressures

We report high pressure structural phase transformation sequence in a layered Iron-based superconducting compound FeSe0.3Te0.7 to 31 GPa at room temperature. The ambient pressure PbO type tetragonal phase (Space Group - P4/nmm) transforms to a monoclinic phase (Space group - P21/m) at a pressure of 7.3 \pm 0.9 GPa. This monoclinic phase is similar to the one observed below 100 K at ambient pressure. On further increase of pressure above 12 GPa, a transformation to an amorphous phase is observed that is completed by 20 GPa. The amorphous phase is found to be stable to the highest pressure of 31 GPa. This structural sequence of tetragonal \to monoclinic \to amorphous phase transformation appears to be a common feature of iron-based layered superconductors under compression. The pressure induced structural phase transformations are likely to play a key role in the superconductivity in these materials at high pressures.