LiSi3As6 and Li2SiAs2 with flexible SiAs2 polyanions: synthesis, structure, bonding, and ionic conductivity

Abstract

Two novel ternary phases, LiSi₃As₆ and Li₂SiAs₂, have been synthesized and characterized. Both phases have an identical Si : As ratio of 1 : 2 providing insight on how layers of the parent phase SiAs₂ accommodate excess electrons from Li cations to form Si–As anionic frameworks. LiSi₃As₆ exhibits a variety of bonding schemes involving Si–Si and As–As bonds, as well as corner-sharing SiAs₄ tetrahedra, while Li₂SiAs₂ is isostructural to the previously reported Li₂SiP₂, with adamantane-like Si₄As₁₀ units connected into 3D framework. LiSi₃As₆ and Li₂SiAs₂ are predicted to be indirect semiconductors which was experimentally confirmed by optical properties characterization. Li₂SiAs₂ exhibits low thermal conductivity of 1.20 W m⁻¹ K⁻¹ at 300 K in combination with a room temperature ionic conductivity of 7 × 10⁻⁶ S cm⁻¹, an order of magnitude greater than that of the phosphide and nitride analogues, indicating its potential as a solid-state Li-ion conductor.