Anomalous Dimensionally Dependent Thermal Conductivity and Enhanced Thermoelectric Properties of Bulk MS₂ (M = Ba, Sr)

Structural dimensionality plays a pivotal role in the modulation of carrier transport for materials design with the goal of alleviating the inherent coupling of transport parameters to boost thermoelectric performance. This work investigates the impact of structural variation from nonlayered bulk to its two-dimensional (2D) counterpart on the thermoelectric properties of disulfide (MS2, M = Ba, Sr) using first-principles calculations. Contrary to conventional expectations, our findings demonstrate that dimensionality reduction does not invariably lead to improved thermoelectric performance. The results derived from crystal structure analysis, ab initio molecular dynamics modeling, and phonon characteristics indicate that both monolayer and bulk MS2 exhibit inherent stability. The lattice thermal conductivity in bulk BaS2 (SrS2) reaches 1.08 (∼3.00) Wm–1 K–1 at 300 K, which is much lower than the monolayer counterparts of 4.22 (6.23) Wm–1 K–1. This remarkably low thermal conductivity is attributed to the dominant role of weakened Ba–S (Sr–S) bonds in heat conduction for bulk BaS2 (SrS2), as evidenced by the cumulative lattice thermal conductivity and projected phonon density of states, and thus leads to the significant lattice anharmonicity manifested by the smaller interatomic force constants, flatter potential well parameters, and larger Grüneisen parameter. Combined with modest electrical transport properties, we predict the ZT values for n-type and p-type bulk BaS2 (SrS2) to be 1.45 and 1.62 (0.99 and 0.98) at 700 K, respectively. Our work not only deepens our fundamental understanding into the intricate electrical and thermal transport mechanisms governing MS2 across different dimensionalities but also provides an exceptional example for the targeted design and discovery of materials with tailored thermal and thermoelectric properties.