Phosphide-based lithium-ion conductors with potential uses in batteries have been well studied. In contrast, little is known about the corresponding sodium phosphide compounds and their sodium-ion conductivity.

Thomas F. Fässler, Technical University of Munich, Garching, Germany, and colleagues have synthesized the first ternary Na–Ta–P compound, the sodium phosphidotantalate Na₇TaP₄. The team originally wanted to prepare a sodium analogue of the lithium-ion conductor Li₉AlP₄ by reacting the elements Na, Al, and P in tantalum ampules. Instead, they obtained Na₇TaP₄. They then developed a more efficient synthesis of this new compound, where Na₇TaP₄ is prepared from the elements via ball milling and subsequent annealing at up to up to 973 K (ca. 700 °C).

Na₇TaP₄ was characterized using X-ray diffraction. The compound crystallizes in a new structure type in the monoclinic space group P2₁/c. The structure (pictured) contains isolated [TaP₄]^7– tetrahedra—a rarely found structural motif—surrounded by sodium atoms.

• Na₇TaP₄: A Ternary Sodium Phosphidotantalate Containing [TaP₄]^7– Tetrahedra,
  Tassilo M. F. Restle, Jasmin V. Dums, Gabriele Raudaschl-Sieber, Wilhelm Klein, Thomas F. Fässler,
  Inorg. Chem. 2020.
  https://doi.org/10.1021/acs.inorgchem.0c03021