Na₇TaP₄: A Ternary Sodium Phosphidotantalate Containing [TaP₄]^7- Tetrahedra

While lithium phosphides have been investigated intensively, very little is known about the corresponding sodium-based phosphides. Here, we report on the first ternary Na-Ta-P compound Na₇TaP₄, which is easily accessible via ball milling of the elements and subsequent annealing. The single crystal X-ray structure determination [monoclinic symmetry; space group P2₁/c; and lattice parameters a = 11.5604(4), b = 8.1530(3), c = 11.5450(5) Å, and β = 101.602(3)°] reveals [TaP₄]^7- tetrahedra, which are surrounded by Na⁺ counterions. Na₇TaP₄ crystallizes in a new structure type. The structure can be described as a strongly distorted hexagonal close packing of P atoms, in which the Ta atoms are located in tetrahedral voids, and Na atoms occupy all octahedral voids and additionally 3/8 of the tetrahedral voids. The possibility to increase the ion conductivity by changing the number of charge carriers through aliovalent substitution in compounds containing [SiP₄]^8- and [AlP₄]^9- is considered. The ³¹P and ²³Na MAS NMR as well as the Raman spectra are in accordance with the structure model, and band structure calculations predict a direct band gap of 2.9 eV.