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

Abstract

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^IM^IIIP₂Q₆ (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 CuInP₂S₆ 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 AgBiP₂Se₆, AgInP₂Se₆, and AgGaP₂Se₆. 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^IM^IIIP₂Q₆ materials utilize ferroelectric instabilities to enhance their thermoelectric performance, unlocking an avenue for achieving a high figure of merit (zT) on the nanoscale.