Synthesis, Structure, Solid State NMR Spectroscopy, and Electronic Structures of the Phosphidotrielates Li3AlP2 and Li3GaP2

The lithium phosphidoaluminate Li9AlP4 represents a new promising compound with a high lithium ion mobility. This triggered our search for new members in the family of lithium phosphidotrielates, and we report on the novel compounds Li3AlP2 and Li3GaP2, obtained directly from the elements via ball milling and subsequent annealing. 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) A for Li3AlP2, and a = 11.5839(2), b = 11.7809(2) and c = 5.8129(2) A for Li3GaP2. The crystal structures feature TrP4 (Tr = Al, Ga) corner- and edge-sharing tetrahedra, forming two-dimensional Common.EditSubmissionSteps.Transform.EquationText layers. The lithium atoms are located between and inside these layers. The crystal structures were confirmed by MAS-NMR spectroscopy. Band structure calculations reveal that Li3AlP2 and Li3GaP2 are direct band gap semiconductors with band gaps of 3.1 eV and 2.8 eV, respectively.