Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties