Where Are Eu? Structural Diversity from Coloring Variants in Tellurides Eu(Al1– x Ga x )2Te4

The mixed-cation tellurides Eu(Al1–xGax)2Te4 (x = 0–1) were synthesized using two thermal profiles, resulting in subtly different structural features. Samples prepared by one-step annealing at 700 °C exhibit a composition-dependent transformation from a body-centered orthorhombic structure with a larger cell (space group Ibam, x = 0–0.50) to a side-centered orthorhombic structure with a smaller cell (space group Cccm, x = 0.75–1.0) as the Ga content increases. Samples prepared by two-step annealing at 700 and 400 °C retain the Ibam structure throughout the entire composition range, implying that the Ga-richest samples such as EuGa2Te4 undergo a temperature-dependent structural transition, as confirmed from variable-temperature powder X-ray diffraction experiments. The structures consist of a common framework of infinite linear chains of edge-sharing MTe4 (M = Al, Ga) tetrahedra but differ in terms of the site distribution of intervening Eu atoms. These compounds are semiconductors with narrow optical band gaps of 0.6–0.9 eV. Based on calculated transport properties, they may be suitable candidates for thermoelectric applications at high temperatures. They undergo electrocatalysis for water splitting, but with room for improvement in their performance.