Superconducting Behavior of BaTi 2 (Sb 1- y Bi y ) 2 O under Pressure.

The crystal structure and superconducting properties of a new type of titanium-pnictide superconductor, BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O ( y = 0.2, 0.5, and 0.8), are comprehensively investigated over a wide pressure range to elucidate the effect of substituting Bi for Sb on the superconducting behavior. The behavior of superconducting properties under pressure changes drastically with y , as expected from the double-dome T _c - y phase diagram obtained at ambient pressure. In this study, three BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O samples ( y = 0.2, 0.5, and 0.8) are considered, which correspond to the first superconducting dome, nonsuperconducting part, and second superconducting dome, respectively, in the T _c - y phase diagram. The crystal of BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O with y = 0.2 shows a clear collapse transition, i.e., a drastic shrinkage of the lattice constant c at ca. 5 GPa. Strictly speaking, the collapsed crystal phase coexists with the noncollapsed phase above 5 GPa. On the other hand, BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O with y = 0.8 shows a continuous change in the crystal lattice with pressure, i.e., no collapse transitions. The pressure dependence of T _c for BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O with y = 0.2 shows a drastic increase in T _c at approximately 5 GPa, where the collapse transition occurs, indicating a clear pressure-induced superconducting phase transition related to the collapse transition. The value of T _c for BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O with y = 0.8 increases slightly up to ∼2 GPa and is almost constant at 2-13 GPa. It is found that the superconducting behavior under pressure can be unambiguously classified by y based on the double-dome T _c - y phase diagram, indicative of distinguishable superconducting features at different y values. In this study, we comprehensively discuss the superconducting properties of the exotic material, BaTi ₂ (Sb _{1- y} Bi _y ) ₂ O, with a double-dome T _c - y phase diagram.