Emergence of Superconductivity at 20 K in Th₃P₄-type In_3–<i>x</i>S₄ 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 (Tc). In this study, we report the emergence of novel superconductivity in a metastable phase of Th3P4-type cubic In3–xS4 with remarkably high Tc 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 In3–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 In3–xS4 decreases with increasing annealing temperature. The Tc in In3–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 Tc in In3–xS4 among superconducting sulfides, excluding high-Tc H3S, accelerates the further exploration of high-Tc materials within the Th3P4-type cubic family by using flexibility in the crystal structure.