Journal
PHYSICAL REVIEW B
Volume 93, Issue 17, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.93.174119
Keywords
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Funding
- National Science Foundation (NSF) [DMREF-13-34170]
- NSF [ACI-1053575]
- Division Of Materials Research
- Direct For Mathematical & Physical Scien [1334170] Funding Source: National Science Foundation
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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 P4/nmm structure to LaOBiS2 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 LaOBiS2 with different stacking patterns of the distorted BiS2 layers. The energy difference between the polytypes of LaOBiS2 is merely similar to 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 by substituting covalence atoms in the LaOBiS2 family.
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