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

Highlights: • Three new Ln₂O₂SeO₄ compounds have been prepared (Ln = La, Pr, Nd). • The new compounds are isostructural to Ln₂O₂SO₄. • Double-exchange solid-state synthesis is an only approach to these compounds. -- 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.