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Oxygen permeability through alumina wafers was evaluated at high temperatures up to 1923 K to elucidate the mass-transfer mechanisms of polycrystalline alumina and serve as a model for protective alumina film formed on heat-resistant alloys. Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher oxygen partial pressure (P) surface side to the lower P surface side, along with the simultaneous GB diffusion of aluminum in the opposite direction to maintain the Gibbs–Duhem relationship. Oxygen GB diffusion coefficients in the vicinity of the P(hi) surface were lower than those of oxygen GB self-diffusion without an oxygen potential gradient (dµ). When dµ was applied to the wafer, the oxygen and aluminum fluxes at the outflow side of the wafer were significantly larger than those at the inflow side. Ln (Y and Lu) and Hf segregation at the GBs selectively reduced the diffusivity of oxygen and aluminum, respectively. Thus, the mesoscopic arrangements of segregating dopants, which were selected by taking into consideration the behavior of the diffusion species and the role of dopants, enabled the alumina film to have enhanced oxygen shielding capability and structural stability at high temperatures. Furthermore, the GB diffusion data derived from the oxygen permeation experiments were compared to those for alumina scale formed by the so-called two-stage oxidation of alumina-forming alloys.

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 compounds from the perovskite phase to the lithium niobate phase is explained in detail from rotation of the BO octahedral frameworks. Depressurized amorphization of ASiO perovskites containing large divalent cations (A = Ba, Sr, and Ca) is elucidated by the characteristics of the hexagonal and cubic perovskite structures. The unquenchable RhO(II) phases of group-13 sesquioxides, such as GaO and InO, are confirmed by both experimental and computational studies. Ab initio calculations of YO 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.

Two classes of new materials possessing ion conductivity have been developed: a lithium ion conductor and a hydride ion conductor. Conventional perovskite and ordered rock-salt structures were adopted as frameworks for lithium migration, and electrochemically stable elements such as Al, Ga, Ta, and Sc were used in the materials to facilitate their use as low-potential negative electrodes. New compositions of (LiSrV)(GaTa)O and LiScZrO were found to be novel oxide-based lithium ion conductors. Oxyhydrides with KNiF-type structures were synthesized via a high-pressure synthesis method and their use in pure hydride ion conduction was demonstrated. The LaSrLiHO oxyhydrides showed wide composition ranges of solid solution formation and the conductivity increased with anion vacancies or the introduction of interstitial hydride ions. The performance of an all-solid-state TiH/-LaLiHO ( =  = 0, : orthorhombic)/Ti cell provided conclusive evidence of pure H conduction.