Where Are Eu? Structural Diversity from Coloring Variants in Tellurides Eu(Al1-xGax)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.