Reversible Thermal Conductivity Modulation of Non-equilibrium (Sn_1–xPb_x)S by 2D–3D Structural Phase Transition above Room Temperature

We demonstrated a reversible two-dimensional (2D) to three-dimensional (3D) crystal-structure transition and associated thermal conductivity switching (κ) well above room temperature (RT) in (Sn1–xPbx)S bulk polycrystals, solid solutions of 2D-layered SnS and 3D-cubic PbS. The direct phase boundary between the 2D and 3D structures does not exist in (Sn1–xPbx)S under the thermal equilibrium condition due to the solubility limit x = 0.5 in the 2D phase and x = 0.9 in the 3D phase. While, by applying a non-equilibrium synthesis process that combined a high-temperature solid-state reaction at 973 K and subsequent rapid thermal quenching to RT, the Sn solubility limit in the 3D (Sn1–xPbx)S was not changed but the Pb solubility limit was considerably expanded up to x = 0.9 in 2D (Sn1–xPbx)S. As a result, the direct 2D–3D phase boundary was formed in the Pb-rich (Sn0.2Pb0.8)S, which showed the reversible 2D–3D (3D–2D) structural phase transition at ∼573 K (473 K). This transition temperature is much higher than that previously reported for (Sn0.5Pb0.5)Se, which requires temperatures lower than RT for the reversible structure transition. The electronic band structure change from a 2D semiconducting state to a 3D metallic state caused a 20-times increase in electron κ (κele) during the structural phase transition in the (Sn0.2Pb0.8)S. Still, κele negligibly contributes to the total κ. In contrast, the lattice κ (κlat) was largely decreased by the 3D–2D structure transition due to the formation of a 2D-layered structure with strong phonon scattering, resulting in a 1.8-times modulation of the total κ (κ3D phase/κ2D phase) at 463 K. The realization of 2D–3D structural phase transition above RT in (Sn1–xPbx)S would accelerate the development of thermal management materials through a crystal-structure dimensionality switch using non-equilibrium phase boundaries.