This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).

Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic ${\mathrm{VO}}_2$ over the 0–19 GPa and 0–14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to $P^{*}\sim 10\mathrm{GPa}$ whereas subtle modifications of V ion arrangements within the monoclinic lattice, together with the onset of a metallization process via band gap filling, are observed for $P>P^{*}$. Differently from $P=0$, where the ${\mathrm{VO}}_2$ metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.

First order Raman spectra in the region 200–800 cm⁻¹ have been collected for AMn₂O₄ (A = Mn, Mg and Zn) tetragonal spinels. A possible correlation between Raman phonons and AO₄ and MnO₆ unit vibrations is proposed. Structural changes have been analyzed following the evolution of the Raman spectra with x for the Mg_1−xMn_xMn₂O₄ solid solution; the effect of the spinel inversion has been also studied on MgMn₂O₄ quenched samples from high temperatures. The results, taking into account also the X-ray diffraction and electron paramagnetic resonance data, show the influence of the Jahn–Teller effect on the Raman scattering for this class of materials.

Raman and infrared transmission and reflectivity measurements were carried out at room temperature and high pressure (0–15 GPa) on ${\text{V}}_{1-x}{\text{Cr}}_x{\text{O}}_2$ compounds. Raman spectra were collected at ambient conditions on the $x=0.007$ and 0.025 materials, which are characterized by different insulating monoclinic phases ($M3$ and $M2$, respectively), while infrared spectra were collected on the $x=0.025$ sample only. The present data were compared with companion results on undoped ${\text{VO}}_2$ [E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 (2007)], which is found at ambient conditions in a different, third insulating monoclinic phase, named $M1$. This comparison allowed us to investigate the effects of different extents of structural distortions (Peierls distortion) on the lattice dynamics and the electronic properties of this family of compounds. The pressure dependence of the Raman spectrum of ${\text{VO}}_2$ and Cr-doped samples shows that all the systems retain the monoclinic structure up to the highest explored pressure, regardless the specific monoclinic structure ($M1$, $M2$, and $M3$) at ambient condition. Moreover, the Raman spectra of the two Cr-doped samples, which exhibit an anomalous behavior over the low-pressure range $\left(P<8\text{GPa}\right)$, merge into that of ${\text{VO}}_2$ in the high-pressure regime and are all found into a common monoclinic phase (a possible fourth kind phase). Combining Raman and infrared results on both the ${\text{VO}}_2$ and the present data, we found that a common metallic monoclinic phase appears accessible in the high-pressure regime at room temperature for both undoped and Cr-doped samples independently of the different extents of Peierls distortion at ambient conditions. This finding differs from the behavior observed at ambient pressure, where the metallic phase is found only in conjunction with the rutile structure. The whole of these results suggests a major role of the electron correlations, rather than of the Peierls distortion, in driving the metal-insulator transition in vanadium dioxide systems, thus opening to new experimental and theoretical queries.

Perovskites are attracting an increasing interest in the wide community of photovoltaics, optoelectronic, and detection, traditionally relying on lead‐based systems. This Minireview provides an overview of the current status of experimental and computational results available on Ge‐containing 3D and low‐dimensional halide perovskites. While stability issues analogous to those of tin‐based materials are present, some strategies to afford this problem in Ge metal halide perovskites (MHPs) for photovoltaics have already been identified and successfully employed, reaching efficiencies of solar devices greater than 7 % at up to 500 h of illumination. Interestingly, some Ge‐containing MHPs showed promising nonlinear optical responses as well as quite broad emissions, which are worthy of further investigation starting from the basic materials chemistry perspective, where a large space for properties modulation through compositions/alloying/fnanostructuring is present.

High-temperature proton conductors are promising as electrolytes for intermediate-temperature solid oxide fuel cells. Among them, BaCeO₃-based materials have high proton conductivity but rather poor chemical stability. In contrast, barium zirconates are rather stable, but have poorly reproducible densities and conductivities. In this study, the investigation of BaCe_1−x−yZr_xY_yO_3−δ solid solutions (x = 0, 0.10, 0.20, 0.30, 0.40; y = 0.15, 0.20) was undertaken, with the final aim of finding a composition having both high conductivity and good stability. The influence of the modified sol–gel Pechini synthetic approach on the powder morphology, and of a barium excess on the densification were demonstrated. Single-phase perovskite powders were prepared and high density pellets were obtained at temperatures lower than those commonly employed. Stability tests demonstrated that the Zr introduction into doped barium cerate greatly enhanced the chemical stability, particularly for Zr ≥ 20%. The proton conductivities, measured in a humidified H₂/Ar atmosphere by impedance spectroscopy, were only slightly influenced by the Zr amount. Overall, BaCe_1−x−yZr_xY_yO_3−δ solid solutions having Zr ≈ 20–40% and Y ≈ 15–20% showed good chemical stability and high conductivity.