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

Abstract

Layered LnOBiS2 (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 La0.80Ti0.20OBiS2 and La0.85Th0.15OBiS2 superconductors were performed between ∼1.5 and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature Tc, which is ∼2.9K at ambient pressure, gradually increases with pressure to 3.2–3.7 K at ∼1GPa, 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 ∼0.6GPa. Surprisingly, above 1.3 GPa, semiconductinglike behavior reappears in La0.80Ti0.20OBiS2. This paper reveals a new high-pressure phase of La1−xMxOBiS2 containing the tetravalent ions M=Ti, Th which does not favor superconductivity. In contrast, application of pressure to fluorinated LaO0.5F0.5BiS2 produces an abrupt tetragonal-monoclinic transition to a metallic phase with an enhanced Tc. These results demonstrate that the response of the normal and superconducting properties of LaOBiS2-based compounds depends strongly on the atomic site where the electron donor ions are substituted.

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