Pressure-Induced Phase Transition in La_(1-x)Sm_(x)O_(0.5)F_(0.5)BiS₂

Electrical resistivity measurements on La$_{1-x}$Sm$_{x}$O$_{0.5}$F$_{0.5}$BiS$_{2}$ ($x$ = 0.1, 0.3, 0.6, 0.8) have been performed under applied pressures up to 2.6 GPa from 2 K to room temperature. The superconducting transition temperature $T$$_{c}$ of each sample significantly increases at a Sm-concentration dependent pressure $P$$_{\rm t}$, indicating a pressure-induced phase transition from a low-$T$$_{c}$ to a high-$T$$_{c}$ phase. The compounds that have higher Sm concentrations have higher $T$$_{c}$ values at ambient pressure; however, the $T$$_{c}$ values at $P$ $>$ $P$$_{\rm t}$ decrease with $x$ and $P$$_{\rm t}$ shifts to higher pressures with Sm substitution. In the normal state, semiconducting-like behavior is suppressed and metallic conduction is induced with increasing pressure in all of the samples. These results suggest that the pressure dependence of $T$$_{c}$ for the BiS$_{2}$-based superconductors is related to the lattice parameters at ambient pressure and enable us to estimate the evolution of $T$$_{c}$ for SmO$_{0.5}$F$_{0.5}$BiS$_{2}$ under pressure.