Synthesis, Structure, Solid-State NMR Spectroscopy, and Electronic Structures of the Phosphidotrielates Li AlP and Li GaP.

The lithium phosphidoaluminate Li AlP represents a promising new compound with a high lithium ion mobility. This triggered the search for new members in the family of lithium phosphidotrielates, and the novel compounds Li AlP and Li GaP , obtained directly from the elements via ball milling and subsequent annealing, are reported here. It was unexpectedly found through band structure calculations that Li AlP and Li GaP are direct band gap semiconductors with band gaps of 3.1 and 2.8 eV, respectively. Rietveld analyses reveal that both compounds crystallize isotypically in the orthorhombic space group Cmce (no. 64) with lattice parameters of a=11.5138(2), b=11.7634(2) and c=5.8202(1) Å for Li AlP , and a=11.5839(2), b=11.7809(2) and c=5.8129(2) Å for Li GaP . The crystal structures feature TrP (Tr=Al, Ga) corner- and edge-sharing tetrahedra, forming two-dimensional layers. The lithium atoms are located between and inside these layers. The crystal structures were confirmed by MAS-NMR spectroscopy.