Theoretical and experimental evidence for the post-cotunnite phase transition in zirconia at high pressure
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A post-cotunnite phase transition in zirconia (ZrO2) at high pressure was investigated by synchrotron X-ray diffraction measurements and ab initio calculations based on density functional theory. This study successfully demonstrated a cotunnite- to Fe2P-type phase transition. Static enthalpy difference (ΔH) calculations predicted the appearance of the Fe2P phase at 124 GPa (LDA) and 143 GPa (GGA), and experimental trials demonstrated the coexistence of the Fe2P and cotunnite phases at 175 GPa after heating to 3,000 K. Both phases were quenchable to ambient conditions. The volume of the Fe2P phase was slightly less (~Δ 0.6 %) than that of the cotunnite phase over the experimental pressure range, indicating that the Fe2P phase is the higher pressure phase. The coexistence of both phases in this study may be attributed to the slow kinetics of the phase transition resulting from the close structural relationship of the two phases. An Fe2P-type structural model can be derived by applying a simple operation to the cotunnite-type structure, consisting of a 1/2 shift of several zirconium arrangements parallel to the b-axis of the cotunnite-type unit cell. It is concluded that the high-pressure cotunnite-to-Fe2P phase transition may be a common trend in many dioxides.
KeywordsZrO2 Fe2P Cotunnite Phase relation High pressure
High pressure and high temperature in situ X-ray data were acquired at SPring-8 (Proposal nos. 2011B1449, 2012B1344 and 2013B1141) and KEK (Proposal no. 2013G540). This work was supported by a Young Scientists B Grant (no. 23740389) from the Japan Society for the Promotion of Science.
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