Dense yttria phase eclipsing the A-type sesquioxide structure: high-pressure experiments and ab initio calculations.

In situ X-ray diffraction experiments and ab initio calculations elucidated the high-pressure phase transition properties of yttrium sesquioxides. The C-, B-, and A-type sesquioxides structure sequence observed in the room-temperature compression does not coincide with the high-pressure phase sequence of yttrium sesquioxides at high temperature. A reconstructive-type transformation taking place at high temperature yields the Gd(2)S(3) structure around 8 GPa with a drastic change in cation-oxygen coordinations. Ab initio structural optimization suggests that a displacive-type transformation from B- to A-type sesquioxides structure metastably occurs under pressure at room temperature. The calculated density of states indicates that the transition to the Gd(2)S(3) structure causes a significant decrease in the band gap. The Gd(2)S(3) phase was also found to be partially recovered at ambient pressure. We briefly discuss the quenchability of the Gd(2)S(3) structure in sesquioxides on the basis of the enthalpy differences between the ambient phase and the recovered products.