Ferroelectric Rattling Enhances Thermoelectric Efficiency by Suppressing Thermal Transport in Metal Thio- and Selenophosphate Monolayers

High-performance thermoelectric devices need materials with an optimal balance between low thermal conductivity and high electrical transport coefficients. Yet achieving such a balance is difficult. We address this challenge using M I M III P 2 Q 6 (M I = Ag, Cu; M III = Al, Ga, Sb, Bi, In, V, Sc, Cr; Q = S, Se) monolayers, which exploit ferroelectric and temperature-induced dynamical off-centering (i.e., emphanisis) instabilities to enhance thermoelectric efficiency. First-principles simulations show that above the Curie temperature, the rattling of coinage metal cations (Ag, Cu) suppresses lattice thermal conductivity in CuInP 2 S 6 down to the amorphous limit (0.19 W m –1 K –1 ) without compromising the power factor. Even in nonferroelectric materials, rattling happens as emphanisis, causing ultralow lattice thermal transport between 0.07 and 0.3 W m –1 K –1 in AgBiP 2 Se 6, AgInP 2 Se 6, and AgGaP 2 Se 6 . The unprecedented glass-like lattice thermal conductivity in these monolayers, paired with the large electrical transport coefficients, yields predicted zT values surpassing two. These findings show that 2D M I M III P 2 Q 6 materials utilize ferroelectric instabilities to enhance their thermoelectric performance, unlocking an avenue for achieving a high figure of merit ( zT ) on the nanoscale.