Structural phase diagram of LaO1-xFxBiSSe: suppression of the structural phase transition by partial F substitutions

TL;DR

This study investigates how partial fluorine substitution in LaO1-xFxBiSSe suppresses structural phase transitions, revealing that the observed superconducting anisotropy is not due to structural symmetry changes.

Contribution

It demonstrates that fluorine doping suppresses the tetragonal-to-monoclinic transition in LaO1-xFxBiSSe, clarifying the origin of superconducting anisotropy.

Findings

Structural transition suppressed by 3% F substitution.

Tetragonal structure maintained at 4 K for x=0.5.

Superconducting anisotropy not caused by structural symmetry lowering.

Abstract

We have investigated low-temperature crystal structure of BiCh2-based compounds LaO1-xFxBiSSe (x = 0, 0.01, 0.02, 0.03, and 0.5), in which anomalous two-fold-symmetric in-plane anisotropy of superconducting states has been observed for x = 0.5. From synchrotron X-ray diffraction experiments, a structural transition from tetragonal to monoclinic was observed for x = 0 and 0.01 at 340 and 240 K, respectively. For x = 0.03, a structural transition and broadening of the diffraction peak were not observed down to 100 K. These facts suggest that the structural transition could be suppressed by 3% F substitution in LaO1-xFxBiSSe. Furthermore, the crystal structure for x = 0.5 at 4 K was examined by low-temperature (laboratory) X-ray diffraction, which confirmed that the tetragonal structure is maintained at 4 K for x = 0.5. Our results suggest that the two-fold-symmetric in-plane anisotropy of superconducting states observed for LaO0.5F0.5BiSSe was not originated from structural symmetry lowering.