In this chapter, several types of structural relaxation of oxide compounds from the high-pressure phase are systematically introduced in terms of high-pressure comparative crystallography. Structural relaxation of various ABO 3 compounds from the perovskite phase to the lithium niobate phase is explained in detail from rotation of the BO 6 octahedral frameworks. Depressurized amorphization of ASiO 3 perovskites containing large divalent cations (A = Ba 2+ , Sr 2+ , and Ca 2+ ) is elucidated by

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... he characteristics of the hexagonal and cubic perovskite structures. The unquenchable Rh 2 O 3 (II) phases of group-13 sesquioxides, such as Ga 2 O 3 and In 2 O 3 , are confirmed by both experimental and computational studies. Ab initio calculations of Y 2 O 3 show that the unquenchable pressure-induced phase (A-type structure) is not the stable phase under high pressure. Knowledge about the unquenchable and/or metastable phases in recovered high-pressure products is beneficial for advanced computational materials design.