Role of lone-pair electrons in determining the thermal transport behavior of LiAsS2: first-principles investigation

Abstract

As³⁺ cations possessing stereochemically active lone pairs (SALPs) lead to high SHG coefficients of nonlinear-optical crystal LiAsS₂, but the role of SALPs in determining the thermal transport behavior of LiAsS₂ is still unknown. In the present work, first-principles based lattice anharmonic dynamic studies are performed to reveal the underlying science of how the SALP influences the thermal conductivity of LiAsS₂. On the one hand, layered-structural LiAsS₂ exhibits anisotropic thermal transport behaviors due to SALPs. The different bond strengths of Li–S and As–S bonds lead to diverse phonon group velocity along the a direction and in the bc plane, which is the source of anisotropic lattice thermal conductivity of LiAsS₂. On the other hand, the existence of SALPs dramatically modifies the phonon anharmonic behavior of LiAsS₂. Lattice vibrational results demonstrate that the rigidity of pyramidal units AsS₃ in LiAsS₂ is weaker than that of tetrahedral units GaS₄ in LiGaS₂, which induces the positive Grüneisen parameters in the low frequency domain of LiAsS₂. As a result, the decrease of phonon lifetime and reduction of lattice thermal conductivity of LiAsS₂ are generated. Nevertheless, high-power nonlinear optical devices can be fabricated along the bc plane of LiAsS₂ due to the remarkably high thermal conductivity in the bc plane and the intrinsic high nonlinear optical coefficients.