Emergence of Superconductivity at 20 K in Th₃P₄‑type In_3–<i>x</i>S₄ Synthesized by Diamond Anvil Cell with Boron-DopedDiamond 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 (<i>T</i>_c). In this study, we report the emergence of novel superconductivity in a metastable phase of Th₃P₄-type cubic In_3–<i>x</i>S₄ with remarkably high <i>T</i>_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–<i>x</i>S₄ 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 <i>x</i> in In_3–<i>x</i>S₄ decreases with increasing annealing temperature. The <i>T</i>_c in In_3–<i>x</i>S₄ is maximized at <i>x</i> = 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 <i>T</i>_c in In_3–<i>x</i>S₄ among superconducting sulfides, excluding high-<i>T</i>_c H₃S, accelerates the further exploration of high-<i>T</i>_c materials within the Th₃P₄-type cubic family by using flexibility in the crystal structure.