Most solid-state materials are composed of p-block anions, only in recent years the introduction of hydride anions (1s2) in oxides (e.g., SrVO2H, BaTi(O,H)3) has allowed the discovery of various interesting properties. Here we exploit the large polarizability of hydride anions (H–) together with chalcogenide (Ch2–) anions to construct a family of antiperovskites with soft anionic sublattices. The M3HCh antiperovskites (M = Li, Na) adopt the ideal cubic structure except orthorhombic Na3HS, despite the large variation in sizes of M and Ch. This unconventional robustness of cubic phase mainly originates from the large size-flexibility of the H– anion. Theoretical and experimental studies reveal low migration barriers for Li+/Na+ transport and high ionic conductivity, possibly promoted by a soft phonon mode associated with the rotational motion of HM6 octahedra in their cubic forms. Aliovalent substitution to create vacancies has further enhanced ionic conductivities of this series of antiperovskites, resulting in Na2.9H(Se0.9I0.1) achieving a high conductivity of ~1 × 10–4 S/cm (100 °C).

The distribution of protons and oxygen vacancies at room temperature at different proton concentrations in 10 mol % Sc-doped BaZrO 3 was investigated to clarify the influence of proton concentration and oxygen vacancies in the trapping of protons caused by the acceptor dopant. To enhance proton conductivity for practical use, it is essential to understand this phenomenon known to limit the long-range transport of protons. In this study, 45 Sc nuclear magnetic resonance (NMR) spectroscopy combined with thermogravimetric analysis (TGA) and 1 H NMR is used to elucidate the protonic defects and oxygen vacancies formed around Sc and Zr. The results reveal that the high protonic defect concentration around Sc with a 9−10 mol % proton concentration is a clear indication for proton trapping and that the high protonic defect concentration around Zr at an intermediate proton concentration of 4 mol % suggests that the protons are residing in the nontrapping sites. The oxygen vacancies that tend to be located around Sc apparently prevent the formation of protonic defects due to the repulsive interaction between the protonic defect and the association of Sc and an oxygen vacancy, both of which have a positive net charge. This study suggests that the formation of oxygen vacancies around the acceptor may inhibit proton trapping and therefore have a positive effect on the long-range transport of protons.

The effect of Ca doping on the Li-ion conductivity and phase stability of the rock-salt-type LiBH phase emerging under high pressures in the range of gigapascals has been investigated. In situ electrochemical measurements under high pressure were performed using a cubic-anvil-type apparatus. Ca doping drastically enhanced the ionic conductivity of the rock-salt-type phase: the ionic conductivity of undoped and 5 mol %Ca-doped LiBH was 2.2 × 10 and 1.4 × 10 S·cm under 4.0 GPa at 220 °C, respectively. The activation volume of LiBH-5 mol %Ca(BH), at 3.2 cm·mol, was comparable to that of other fast ionic conductors, such as lithium titanate and NASICONs. Moreover, Ca-doped LiBH showed lithium plating-stripping behavior in a cyclic voltammogram. These results indicate that the conductivity enhancement by Ca doping can be attributed to the formation of a LiBH-Ca(BH) solid solution; however, the solid solution decomposed into the orthorhombic LiBH phase and the orthorhombic Ca(BH) phase after unloading the high pressure.

It is necessary to elucidate the correlation between hydration properties and proton distributions in electrolytes as proton conductors to allow for further improvements in solid oxide fuel cells. In this study, the hydration properties of Sc-doped BaZrO 3 (BZO) were investigated by means of density functional theory calculations capable of taking both the local structural configurations and the hydration levels into account. At a low hydration level, Sc-doped BZOs gained a negatively larger hydration energy, that is, more exothermic reaction, by incorporating an H 2 O molecule with unstable oxygen vacancies adjacent to Zr. At a high hydration level, the configuration of ScO 4 (OH) 2 , which has a positive net charge as a local structure, was formed with a smaller but negative hydration energy by the reaction of H 2 O with oxygen vacancies adjacent to Sc. This indicates that the stability of the whole system, and not only the local electrostatic interactions of point defects, needs to be taken into account when considering the hydration energy. The characteristic local structure of ScO 4 (OH) 2 was identified using 45 Sc nuclear magnetic resonance (NMR) chemical shift calculations. It is proposed that the resolution of current 45 Sc NMR spectroscopy techniques does not allow for the detection of ScO 4 (OH) 2 in Sc-doped BZOs and that a higher resolution 45 Sc NMR technique will likely reveal the existence of ScO 4 (OH) 2 .

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