Role of lone-pair electrons in determining thermal transport behavior of LiAsS2: First-principle investigation

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 Gruneisen 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.