Superconducting dome and crossover to an insulating state in [Tl₄]Tl_1−xSn_xTe₃

The structural, superconducting, and electronic phase diagram of [Tl₄]Tl_1−xSn_xTe₃ is reported. Magnetization and specific heat measurements show bulk superconductivity exists for 0 ≤ x ≤ 0.4. Resistivity measurements indicate a crossover from a metallic state at x = 0 to a doped insulator at x = 1. Universally, there is a large non-Debye specific heat contribution, characterized by an Einstein temperature of θ_E ≈ 35 K. Density functional theory calculations predict x = 0 to be a topological metal, while x = 1 is a topological crystalline insulator. The disappearance of superconductivity correlates with the transition between these distinct topological states