Pressure induced structural transformation in Gd₂Ti₂O₇ and Gd₂Zr₂O₇.

Ab initio total energy calculations have been performed to study the phase stability of Gd(2)Ti(2)O(7) and Gd(2)Zr(2)O(7) pyrochlores over the pressure range from 0 to 60 GPa. Both compounds are unstable under pressure, and phase transformations to the defect-cotunnite structure are predicted. The phase transformation pressure of 43.6 GPa for Gd(2)Ti(2)O(7) is considerably larger than the value of 13 GPa for Gd(2)Zr(2)O(7), in good agreement with experiments. The decreased structural stability of Gd(2)Zr(2)O(7) under pressure, relative to Gd(2)Ti(2)O(7), is a consequence of the lower compressibility of the ⟨Zr-O⟩ bond and the higher compressibility of the ⟨Gd-O⟩ bond. In addition, the Gd 4f electrons are found to have only a small effect in determining the pressure induced phase transformation.