Superconductivity in Metastable K<i>_1+δ</i>Mo₆Se₈: A Potassium-Intercalated Chevrel Phase

The Chevrel phase (CP), characterized by its unique Mo₆X₈ (X = S, Se, Te) cluster structure, represents a class of promising materials demonstrating exceptional performance in various applications, including battery cathodes, electrocatalysts, and superconductors. However, the exploration of new CP derivatives remains challenging due to the inherent lattice destabilization caused by cation intercalation, particularly evident in selenide and telluride systems. This study reports the successful synthesis of thermodynamically metastable K_1+<i>δ</i>Mo₆Se₈ (<i>δ</i> ∼ 0.37) and the superconducting properties therein. K_1+<i>δ</i>Mo₆Se₈ crystallizes in the triclinic space group <i>P</i>1̅ (No. 2), where potassium cations occupy interstitial sites between the Mo₆Se₈ clusters. Comprehensive characterization through electrical resistivity, magnetization, and specific heat measurements reveals bulk superconductivity at <i>T</i>_c = 8.9 K. Notably, the upper critical field is estimated to be 26.4 T, violating the Pauli paramagnetic limit. Furthermore, low-temperature specific heat analysis indicates possible multigap superconducting behavior. Our findings not only expand the family of high-critical-field superconducting CPs but also demonstrate the potential to synthesize novel CP materials through solid-state reactions at lower temperatures.