Versatility of Tellurium in Heteroanionic Ln₂O₂Te (Ln = La, Ce, Pr) and Tellurate Ln₂TeO₆ (Ln = La, Pr)

Heteroanionic compounds continue to gain interest in materials design because the expanded composition space provides opportunities to discover new phases and tune physical properties. Among heteroanionic materials, oxytellurides comprised of oxygen and tellurium anions are relatively underexplored despite the significant role of tellurium in emerging technologies. Herein, we present synthetic strategies toward oxytelluride Ln₂O₂Te (Ln = La-Pr), whose layered structure features square nets of Te^2- anions. Upon heating in H₂ or air, we find a reversible phase transition between the oxytelluride and tellurate Ln₂TeO₆ (Ln = La, Pr), wherein Te is octahedrally coordinated and a 6+ oxidation state is corroborated by bond valence analysis. We use X-ray diffraction along with thermogravimetric analyses to confirm the presence of oxytelluride and tellurate phases and emphasize key structural distinctions. In contrast, we find that Ce₂O₂Te decomposes to form CeO₂ and demonstrate the instability of Ce₂O₂Te in ambient conditions by timelapse X-ray diffraction and diffuse-reflectance spectroscopy experiments. Band gaps of Ln₂O₂Te (Ln = La-Pr) were estimated from diffuse-reflectance spectroscopy in the semiconducting range ∼2.1-2.7 eV, while band gaps for La₂TeO₆ and Pr₂TeO₆ were much larger at ∼4.3 and ∼3.7 eV, respectively.