Emergence of Superconductivity at 20 K in Th3P4-type In3–xS4 Synthesized by Diamond Anvil Cell with Boron-Doped Diamond Electrodes

The exploration of superconductors in metastable phases by manipulating crystal structures through high-pressure techniques has attracted significant interest in materials science to achieve a high critical temperature (T_c). In this study, we report the emergence of novel superconductivity in a metastable phase of Th₃P₄-type cubic In_3–xS₄ with remarkably high T_c at 20 K under 45 GPa by using an originally designed diamond anvil cell equipped with boron-doped diamond electrodes, which can perform a high-pressure synthesis and an in situ electrical transport measurement simultaneously. In situ structural analysis indicates that In_3–xS₄ appears partially above 40 GPa without heating. The high-pressure annealing treatment induces a complete transformation to the Th₃P₄-type structure, and the defected concentration of x in In_3–xS₄ decreases with increasing annealing temperature. The T_c in In_3–xS₄ is maximized at x = 0 and approaches 20 K. Electronic band calculations show that the high density of states composed of sulfur and indium bands is located at the conduction band bottom near Fermi energy. The record high T_c in In_3–xS₄ among superconducting sulfides, excluding high-T_c H₃S, accelerates the further exploration of high-T_c materials within the Th₃P₄-type cubic family by using flexibility in the crystal structure.