Polytypism in $\mathrm{LaOBi}{\mathrm{S}}_{2}$-type compounds based on different three-dimensional stacking sequences of two-dimensional $\mathrm{Bi}{\mathrm{S}}_{2}$ layers

Polytypism in $LaOBi S_{2}$ -type compounds based on different three-dimensional stacking sequences of two-dimensional $Bi S_{2}$ layers

Qihang Liu, Xiuwen Zhang, and Alex Zunger

Phys. Rev. B 93, 174119 – Published 31 May 2016

Abstract

$LaOBi S_{2}$ -type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each $LaOBi S_{2}$ -type compound has a unique and specific crystallographic structure (with a space group $P 4$ /nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the $P 4$ /nmm structure to $LaOBi S_{2}$ is incorrect. Furthermore, we find that the unstable structure is replaced by a family of energetically closely spaced modifications (polytypes) differing by the layer sequences and orientations. We find that the local Bi-S distortion leads to three polytypes of $LaOBi S_{2}$ with different stacking patterns of the distorted $Bi S_{2}$ layers. The energy difference between the polytypes of $LaOBi S_{2}$ is merely ∼1 meV/u.c., indicating the possible coexistence of all polytypes in the real sample and that the particular distribution of polytypes may be growth induced. The in-plane distortion can be suppressed by pressure, leading to a phase transition from polytypes to the high-symmetry $P 4$ /nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms ( $Bi S_{2}$ ) could also manifest different ground-state structures. One can thus tune the distortion and the ground state by pressure or by substituting covalence atoms in the $LaOBi S_{2}$ family.