Ag 2 Se to KAg 3 Se 2 : Suppressing Order–Disorder Transitions via Reduced Dimensionality
Here we report an order–disorder phase transition in the 2D semiconductor KAg 3 Se 2 , which is a dimensionally reduced derivative of 3D Ag 2 Se. At ~695 K, the room temperature β-phase (CsAg 3 S 2 structure type, monoclinic space group C2/m) transforms to the high temperature α-phase (new structure type, hexagonal space group R$ \overline{3}\ $m, a = 4.5638(5) Å, c = 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-KAg 3 Se 2 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-KAg 3 Se 2 yielded n-type behavior with a high electron mobility of ~400 cm 2 V –1 s –1 at 300 K due to a highly disperse conduction band. Finally, 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.