Synthesis, Structure, Solid-State NMR Spectroscopy, and Electronic Structures of the Phosphidotrielates Li₃ AlP₂ and...

The lithium phosphidoaluminate Li₉ AlP₄ 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 Li₃ AlP₂ and Li₃ GaP₂ , obtained directly from the elements via ball milling and subsequent annealing, are reported here. It was unexpectedly found through band structure calculations that Li₃ AlP₂ and Li₃ GaP₂ 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 Li₃ AlP₂ , and a=11.5839(2), b=11.7809(2) and c=5.8129(2) Å for Li₃ GaP₂ . The crystal structures feature TrP₄ (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.