Versatility of Tellurium in Heteroanionic Ln 2 O 2 Te (Ln = La, Ce, Pr) and Tellurate Ln 2 TeO 6 (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 Ln 2 O 2 Te (Ln = La-Pr), whose layered structure features square nets of Te 2- anions. Upon heating in H 2 or air, we find a reversible phase transition between the oxytelluride and tellurate Ln 2 TeO 6 (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 Ce 2 O 2 Te decomposes to form CeO 2 and demonstrate the instability of Ce 2 O 2 Te in ambient conditions by timelapse X-ray diffraction and diffuse-reflectance spectroscopy experiments. Band gaps of Ln 2 O 2 Te (Ln = La-Pr) were estimated from diffuse-reflectance spectroscopy in the semiconducting range ∼2.1-2.7 eV, while band gaps for La 2 TeO 6 and Pr 2 TeO 6 were much larger at ∼4.3 and ∼3.7 eV, respectively.