High-pressure effects on nonfluorinated BiS2 -based superconductors La1−xMxOBiS2 ( M=Ti and Th)

Abstract

Layered ${\mathrm{LnOBiS}}_{2}$ (where Ln represents a lanthanide element) compounds with Ln = La, Ce, Pr, Nd, and Yb can be rendered conducting and superconducting via two routes, substitution of F for O or the tetravalent ions Ti, Zr, Hf, and Th for trivalent Ln ions. Electrical resistivity measurements on nonfluorinated ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_{2}$ and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_{2}$ superconductors were performed between $\ensuremath{\sim}1.5$ and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature ${T}_{c}$, which is $\ensuremath{\sim}2.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ at ambient pressure, gradually increases with pressure to 3.2--3.7 K at $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, above which it is suppressed and the superconducting transitions become very broad. Measurements of the normal-state electrical resistivity of the two compounds reveal discontinuous changes in the resistivity as a function of pressure at $\ensuremath{\sim}0.6\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Surprisingly, above 1.3 GPa, semiconductinglike behavior reappears in ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_{2}$. This paper reveals a new high-pressure phase of ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{OBiS}}_{2}$ containing the tetravalent ions $M=\mathrm{Ti}$, Th which does not favor superconductivity. In contrast, application of pressure to fluorinated ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ produces an abrupt tetragonal-monoclinic transition to a metallic phase with an enhanced ${T}_{c}$. These results demonstrate that the response of the normal and superconducting properties of ${\mathrm{LaOBiS}}_{2}$-based compounds depends strongly on the atomic site where the electron donor ions are substituted.