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 P2(1)/c; and lattice parameters a = 11.5604(4), b = 8.1530(3), c = 11.5450(5) angstrom, and beta = 101.602(3) 6 ] 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 P-31 and Na-23 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.