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

Abstract

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 [(Al0.5Ta0.5)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. Thesodium 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∞[GaP4/2] tetrahedra. For both compounds band structure calculations predict indirect band gaps of 2.9 eV.