Two Superconducting Phases in the Isovalent Solid Solutions BaTi₂Pn₂O (Pn= As, Sb, and Bi)

We have recently reported two superconductors with the \(d¹\) square lattice, BaTi2Sb2O (\(Tc=1.2\) K) and BaTi2Bi2O (\(Tc=4.6\) K). In order to find a clue behind the superconductivity of these materials, we synthesized isovalent solid solutions of BaTi2(As1-xSbx)2O and BaTi2(Sb1-yBiy)2O. Despite Vegard's law behavior in cell constants, a two-dome structure in \(Tc\) was observed with two distinct superconducting phases ("\(0.9≤ x;y≤ 0.3\)" and "\(0.6≤ y≤ 1\)") separated by a metallic phase. This result implies a multiband nature at the Fermi surface in BaTi2 Pn 2O.