Sb-Triggered β-to-α Transition: Solvothermal Synthesis of Metastable α-Cu₂Se

Control over phase stabilities during synthesis processes is of great importance for both fundamental studies and practical applications. We describe herein a facile strategy for the synthesis of Cu₂Se with phase selectivity through a simple solvothermal method. In the presence and absence of SbCl₃, monoclinic α-Cu₂Se and cubic β-Cu₂Se can be synthesized, respectively. The formation of α-Cu₂Se requires optimization of the Cu/Se molar ratio in the starting reagents, the reaction temperature, as well as the timing for the addition of SbCl₃. Differential scanning calorimetry of the synthesized α-Cu₂Se has shown that a part of it undergoes a phase transition to β-Cu₂Se at 135 °C, and that this phase transition is irreversible on cooling to ambient temperature. Kinetic studies have revealed that in the presence of Sb species the kinetically favored β-Cu₂Se transforms to the thermodynamically favored α-Cu₂Se. In this β-to-α phase transition process, the distribution of Cu ions in β-Cu₂Se, as determined by the Cu/Se ratio and temperature, is likely to play a crucial role.