Unique LaTaO₄ Polymorph for Multiple Energy Applications

Rare-earth niobate and tantalate (RE-Nb/Ta) materials are of considerable interest in environmental and energy-related applications that include phosphors for solid-state lighting, photocatalysts for both contaminant degeneration and H 2 generation, chemically robust hosts for nuclear materials and wastes, and ion conductors for lithium batteries or solid-oxide fuel cells. However, the chemically inert nature limits the synthetic routes available to obtain these materials, which in turn hampers the discovery and development of new RE-Nb/Ta phases. Of the simple orthotantalate, LaTaO 4, there were three polymorphs known prior. With this paper, we present the structural characterization (from high-resolution X-ray powder diffraction data collected at the APS 11-BM line) of a fourth polymorph. It is obtained only from dehydration of La 2 Ta 2 O 7 (OH) 2, which is in turn synthesized hydrothermally. The structure of the new LaTaO 4 polymorph is distinctive from the others in the arrangement of the alternating La−O polyhedra layers and TaO 6 octahedra layers. Luminescence measurements (Eu-doped) and photocatalysis studies of the new LaTaO 4 polymorph, and comparison to the performance of a previously described LaTaO 4 polymorph reveals enhanced performance of the new polymorph in both applications. This study illustrates the relevance of form-function relationships in solid-state materials, as well as the important role of synthesis in the development of advanced functional materials.