Lithium selenometallates of triel elements, Li 5 MSe 4 (M = Al and Ga), aliovalent doping and their ionic conductivity.

Ternary selenometallates, Li 5 MSe 4 (M = Al(I) and Ga(II)), have been synthesized for the first time through high temperature solid-state reactions combining elements and Li 2 Se in stoichiometric compositions. Li 5 MSe 4 crystallizes in the P 2 1 / m space group, forming a pseudo-2D layer type structure with edge sharing LiSe 4 and MSe 4 tetrahedra along the a -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 -7 and 0.58 × 10 -7 S cm -1 with calculated activation energies of 0.51 and 0.48 eV for I and II, respectively. An aliovalent substitution of Sn 4+ in Li 5 MSe 4 yields compositions of Li 4.66 Al 0.82 Sn 0.22 Se 4 (III) and Li 4.37 Ga 0.89 Sn 0.24 Se 4 (IV), which crystallize in the P 2 1 / m and P 3̄ m 1 space groups, respectively. Sn-doped samples show an ∼5-fold increase in ionic conductivity, 3.37 × 10 -7 S cm -1 and 2.4 × 10 -7 S cm -1 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 n 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.