Lithium selenometallates of triel elements, LiMSe (M = Al and Ga), aliovalent doping and their ionic conductivity.

Ternary selenometallates, LiMSe (M = Al(I) and Ga(II)), have been synthesized for the first time through high temperature solid-state reactions combining elements and LiSe in stoichiometric compositions. LiMSe crystallizes in the 2/ space group, forming a pseudo-2D layer type structure with edge sharing LiSe and MSe tetrahedra along the -axis. These layers are interleaved by octahedrally coordinated Li ions located in the interlayer space. AC impedance spectroscopy measurements yield room temperature ionic conductivities of 0.60 × 10 and 0.58 × 10 S cm with calculated activation energies of 0.51 and 0.48 eV for I and II, respectively. An aliovalent substitution of Sn in LiMSe yields compositions of LiAlSnSe (III) and LiGaSnSe (IV), which crystallize in the 2/ and 3̄1 space groups, respectively. Sn-doped samples show an ∼5-fold increase in ionic conductivity, 3.37 × 10 S cm and 2.4 × 10 S cm with activation energies of 0.54 and 0.28 eV, respectively, for III and IV. The optical band gap values of the compounds are 3.65 and 3.2 eV for I and II, respectively, as measured by diffuse reflectance spectroscopy. Density functional theory (DFT) calculations predicted a major contribution from the Se 4p-states in forming the top of the valence band and strongly hybridized the Se 4p and s orbitals of Al and Ga in forming the bottom of the conduction band with almost no contribution from the Li s-states near the Fermi level indicating their ionic interactions with the ligand.