Aliovalent substitution in phosphide-based materials – Crystal structures of Na₁₀AlTaP₆ and Na₃GaP₂ featuring edge-sharing EP₄ tetrahedra (E=Al/Ta and Ga)

Recently, ternary lithium phosphides have been studied intensively owing to their high lithium ion conductivities. Much less is known about the corresponding sodium-containing compounds, and during investigations aiming for sodium phosphidotrielates, two new compounds have been obtained. The sodium phosphidoaluminumtantalate Na₁₀AlTaP₆, at first obtained as a by-product from the reaction with the container material, crystallizes in the monoclinic space group P2₁/n (no. 14) with lattice parameters of a=8.0790(3) Å, b=7.3489(2) Å, c=13.2054(4) Å, and β=90.773(2)°. The crystal structure contains dimers of edge-sharing [(Al_0.5Ta_0.5)P₄] tetrahedra with a mixed Al/Ta site. DFT calculations support the presence of this type of arrangement instead of homonuclear Al₂P₆ or Ta₂P₆ dimers. The ³¹P and ²³Na MAS NMR as well as the Raman spectra confirm the structure model. The assignment of the chemical shifts is confirmed applying the DFT-PBE method on the basis of the ordered structural model with mixed AlTaP₆ dimers. The sodium phosphidogallate Na₃GaP₂ crystallizes in the orthorhombic space group Ibam (no. 72) with lattice parameters of a=13.081(3) Å, b=6.728(1) Å, and c=6.211(1) Å and is isotypic to Na₃AlP₂. Na₃GaP₂ exhibits linear chains of edge-sharing ¹_∞[GaP_4/2] tetrahedra. For both compounds band structure calculations predict indirect band gaps of 2.9 eV.