We present a computational study of spontaneous polarization and piezoelectricity in Sc _x Al _{1− x} N alloys in the compositional range from x = 0 to x = 0.5, obtained in the context of density functional theory and the Berry-phase theory of electric polarization using large periodic supercells. We report composition-dependent values of piezoelectric coefficients e _ij , piezoelectric moduli d _ij and elastic constants C _ij . The theoretical findings are complemented with experimental measurement of e ₃₃ for a series of sputtered ScAlN films carried out with a piezoelectric resonator. The rapid increase with Sc content of the piezoelectric response reported in previous studies is confirmed for the available data. A detailed description of the full methodology required to calculate the piezoelectric properties of ScAlN, with application to other complex alloys, is presented. In particular, we find that the large amount of internal strain present in ScAlN and its intricate relation with electric polarization make configurational sampling and the use of large supercells at different compositions necessary in order to accurately derive the piezoelectric response of the material.

Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO₂, Ir hydrous oxide contains –Ir^IIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO₂ participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems.

Sc-based III-nitride alloys were studied using density functional theory with special quasi-random structure methodology. ScxAl1−xN and ScxGa1−xN alloys are found to be stable in hexagonal phases up to x ≈ 0.56 and x ≈ 0.66, respectively, above which rock-salt structures are more stable. Epitaxial strain stabilization can prevent spinodal decomposition up to x ≈ 0.4 (ScxAl1−xN on AlN or GaN) and x = 0.27 (ScxGa1−xN on GaN). The increase in Sc content expands the in-plane lattice parameter of ScxAl1−xN and ScxGa1−xN alloys, leads to composition- and strain-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in ScxGa1−xN at x ≈ 0.625. A modified Becke-Johnson exchange-correlation potential was applied to study the electronic structures, which yielded band gaps comparable to those from hybrid functional calculations, yet in a much shorter computational time. The alloys were found to retain wide band gaps, which stay direct up to x = 0.25 (ScxAl1−xN) and x = 0.5 (ScxGa1−xN). The band gaps decrease with increasing x for ScxAl1−xN, in which the Sc-3d states dominate at the conduction band minimum and lead to flat electron dispersion at the Γ point. Conversely, the band gaps increase with increasing x for ScxGa1−xN (up to x = 0.5), in which Sc-3d states do not contribute to the conduction band minimum at the Γ point.