Theoretical prediction of the new high-density lithium boride LiB₁₁ with polymorphism and pseudoplasticity

Abstract

We predict the possibility of existence of the new lithium boride LiB₁₁ with polymorphism. For energy reasons, the preferred type is α′-LiB₁₁ (trigonal space group R3m, a _h = 0.4982 nm, c _h = 1.1123 nm, z _h = 3, ρ = 2.63 g/cm³), with a framework built of tetrahedra and one-capped octahedra. α′-LiB₁₁ is pseudoplastic because of twinning via the high-symmetry state of α-LiB₁₁ (cubic space group $F\bar 43m$, a = 0.6810 nm, z = 4, ρ = 2.65 g/cm³) and a bipolaron semiconductor. α → α′ transition is accompanied by the 0.0627-nm displacement of 1/11 B atoms. The β′ polymorph (tetragonal space group $I\bar 4m2$, a = 0.4404 nm, c = 0.7708 nm, ρ = 2.80 g/cm³) is transformation hardened because of the transition to the α′ phase. We infer that LiB₁₁ formation is possible under high pressure.