Site Dependency of the High Conductivity of Ga₂In₆Sn₂O₁₆: The Role of the 7‑Coordinate Site

The 6-coordinated cation site is the fundamental building block of the most effective transparent conducting oxides. Ga₂In₆Sn₂O₁₆, however, maintains 4-, 6-, 7-, and 8-coordinated cation sites and still exhibits desirable transparency and high conductivity. To investigate the potential impact of these alternative sites, we partially replace the Sn in Ga₂In₆Sn₂O₁₆ with Ti, Zr, or Hf and use a combined approach of density functional theory-based calculations, X-ray diffraction, and neutron diffraction to establish that the substitution occurs preferentially on the 7-coordinate site. In contrast to Sn, the empty d orbitals of Ti, Zr, and Hf promote spd covalency with the surrounding oxygen, which decreases the conductivity. Pairing the substitutional site preference with the magnitude of this decrease demonstrates that the 7-coordinate site is the major contributor to conductivity. The optical band gaps, in contrast, are shown to be site-independent and composition-dependent. After all 7-coordinate Sn has been replaced, the continued substitution of Sn results in the formation of a 7-coordinate In antisite or replacement of 6-coordinate Sn, depending on the identity of the d⁰ substitute

Synthesis and Characterization of MgCr₂S₄ Thiospinel as a Potential Magnesium Cathode

Magnesium-ion batteries are a promising energy storage technology because of their higher theoretical energy density and lower cost of raw materials. Among the major challenges has been the identification of cathode materials that demonstrate capacities and voltages similar to lithium-ion systems. Thiospinels represent an attractive choice for new Mg-ion cathode materials owing to their interconnected diffusion pathways and demonstrated high cation mobility in numerous systems. Reported magnesium thiospinels, however, contain redox inactive metals such as scandium or indium, or have low voltages, such as MgTi₂S₄. This article describes the direct synthesis and structural and electrochemical characterization of MgCr₂S₄, a new thiospinel containing the redox active metal chromium and discusses its physical properties and potential as a magnesium battery cathode. However, as chromium­(III) is quite stable against oxidation in sulfides, removing magnesium from the material remains a significant challenge. Early attempts at both chemical and electrochemical demagnesiation are discussed