High-Pressure Effects On Nonfluorinated Bis2-Based Superconductors La1-Xm(X)Obis(2) (M = Ti And Th)

Layered 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 La0.80Ti0.20OBiS2 and La0.85Th0.15OBiS2 superconductors were performed between similar to 1.5 and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature T-c, which is similar to 2.9 K at ambient pressure, gradually increases with pressure to 3.2-3.7 K at similar to 1 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 similar to 0.6 GPa. 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 T-c. 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.