Synthesis, crystal structure, spectroscopic properties, and thermal behavior of rare-earth oxide selenates, Ln2O2SeO4 (Ln = La, Pr, Nd): The new perspectives of solid-state double-exchange synthesis

Abstract Three rare-earth oxide selenates Ln2O2SeO4 (Ln = La, Pr, Nd) have been prepared via double-exchange solid-state reactions between respective LnOCl oxyhalides and potassium selenate. This approach succeeded to obtain single-phase specimens of La2O2SeO4 and Nd2O2SeO4, previously known as transients upon thermal decomposition of the corresponding selenates, as well as a new compound Pr2O2SeO4. Refinement of their crystal structures from powder X-ray diffraction data confirmed previous attributions to the grandreefite (Pb2F2SO4) structure type observed also for the Ln2O2SO4 oxide sulfates. According to polythermic X-ray studies, La2O2SeO4 is stable until at least 700 °C. All compounds were characterized by infrared and X-ray photoelectron spectroscopy.