Polytypism in LaOBiS2-type compounds based on different three-dimensional stacking sequences of two-dimensional BiS2 layers

TL;DR

This study reveals multiple stable stacking sequences (polytypes) in LaOBiS2 compounds due to layer distortions, challenging the previous assumption of a single crystal structure, and shows how pressure and substitution can tune these structures.

Contribution

It demonstrates the existence of multiple energetically close polytypes in LaOBiS2, contradicting the assumption of a unique structure, and explores how external factors influence these phases.

Findings

Multiple polytypes coexist with minimal energy differences (~1 meV/u.c.)

Pressure induces a phase transition to a high-symmetry structure

Substituting atoms can alter the ground state structure

Abstract

LaOBiS2-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 LaOBiS2-type compound has a unique and specific crystallographic structure (with a space group P4/nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the centrosymetric P4/nmm structure to LaOBiS2 is incorrect as a phonon instability renders this structure impossible. 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 LaOBiS2 with different stacking patterns of the distorted BiS2 layers. The energy difference between the polytypes of LaOBiS2 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 P4/nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms (BiS2) could also manifest different ground state structures. One can thus tune the distortion and the ground state by pressure or substituting covalence atoms in LaOBiS2-family.