Metastable cubic Cu₃SbS₃: a facile solution-phase access to a kinetic polymorph

Polymorphism in inorganic solids is central to controlling their structure-property relationships, yet stabilising high-energy phases remains challenging. Cu₃SbS₃, a ternary copper chalcogenide, exhibits rich structural diversity, but its metastable cubic polymorph (space group I4̄3m) has thus far eluded stabilisation in pristine form. Here we report the room-temperature stabilisation of cubic Cu₃SbS₃ nanocrystals via a simple low-temperature solution-phase synthesis. Structural and spectroscopic analysis confirm the formation of phase-pure cubic nanocrystals with a band gap of ∼1.86 eV. Thermal studies, including high-temperature powder X-ray diffraction and positron annihilation spectroscopy, reveal that the cubic phase is kinetically stabilised up to ∼623 K, beyond which it undergoes an irreversible transformation into orthorhombic and tetrahedrite-type phases, assisted by Sb and S-vacancy formation. The metastable cubic Cu₃SbS₃ nanocrystals exhibit p-type semiconducting behaviour and ultralow thermal conductivity (∼0.77 W m^-1 K^-1 at 303 K). The low thermal transport originates from strong anharmonicity induced by stereochemically active Sb 5s² lone pairs and local Cu coordination distortions. These findings demonstrate that nanoscale synthesis enables access to otherwise inaccessible metastable polymorphs and uncover the thermally driven pathways governing their structural evolution. This study extends the accessible phase space of chalcogenides, offering new possibilities for tailoring their optoelectronic and thermoelectric properties.