Electronic and Lattice Thermal Conductivity Switching by 3D−2D Crystal Structure Transition in Nonequilibrium (Pb_1−xSn_x)Se

Abstract

Dynamic control of thermal transport in solid materials is highly desired for thermal management technology. However, the development of a material exhibiting large modulation of thermal conductivity (κ) by external stimuli remains a major challenge. Here, the large κ modulation is reported by the reversible 3D to 2D crystal structure transition in a nonequilibrium solid solution of (Pb1−xSnx)Se, where Pb2+ stabilizes a 3D cubic structure while Sn2+ does a 2D layered structure. The phase boundary of these phases is induced in (Pb0.5Sn0.5)Se bulk polycrystals by thermally quenching the high‐temperature solid solution phase. Through the 3D–2D phase transition, the 1/2.9‐times decrease of lattice κ (κlat.) is achieved by strong phonon scattering in the 2D layered structure, and the electronic κ (κele.) is also decreased by 5 orders of magnitude due to the electronic phase transition from a 3D high conductivity state to a 2D semiconducting state. The total κ (=κlat. + κele.) modulation ratio κ3D phase/κ2D phase = 3.6 is attained at 373 K. The present strategy will lead to a novel concept for designing thermal management materials through crystal‐structure dimensionality switch using nonequilibrium phase boundaries.

Reversible Thermal Conductivity Modulation of Non-equilibrium (Sn1–xPbx)S by 2D–3D Structural Phase Transition above Room Temperature

“…We confirmed that the samples with x = 0.6−0.8 were decomposed to mixtures of the 2D and the 3D phases when heated to T > 700 K and cooled back to RT, substantiating that the Pb-rich (Sn 1−x Pb x )S bulks had been in the non-equilibrium phases. Figure 4 compares the 2D−3D (3D−2D) structural phase transition temperature (T tran ), which is determined as the peak temperature of the first differential of the 2D phase fraction as a function of T, for the (Sn 0.2 Pb 0.8 )S bulk, where those of previously reported (Sn 0.5 Pb 0.5 )Se bulk are also shown for comparison 9 to discuss the difference of T tran for SnS and SnSe system. The 2D−3D structural phase transition has large hysteresis in temperature (different transition temperatures under heating and cooling, respectively) as often observed in first-order phase transitions.…”

“…The T hysteresis of the 2D–3D structural phase transition is characterized by the T window width (Δ T tran ) between the closed and open arrows. The T dependences of the first differential of 2D phase fractions (blue symbols) for (Sn 0.5 Pb 0.5 )Se bulk polycrystals are shown for comparison.…”

“…However, we consider that the solubility limit can be increased at high temperatures because the entropy term has a greater contribution with increasing temperatures. We previously reported that the non-equilibrium synthesis by high-temperature solid-state reaction and subsequent rapid thermal quenching could expand the solubility limit of Pb in 2D-layered (Sn 1– x Pb x )Se bulk polycrystals for x = 0.2–0.5, ,, that of Sn in 3D cubic (Sn 1– x Pb x )Se bulks for x = 0.6–0.5, and that of Te in 2D-layered Sn­(Se 1– x Te x ) bulks for x = 0.2–0.4 . Additionally, non-equilibrium vapor phase thin film deposition stabilized the 3D cubic (Sn 1– x Ca x )Se with x = 0.4–0.8, which cannot be obtained through equilibrium synthesis .…”

“…7 To address this issue, we have developed a new material whose κ is largely modulated by a reversible structural phase transition with different crystal dimensionalities. 8,9 We synthesized a non-equilibrium solid solution between twodimensional (2D)-layered tin selenide (SnSe) and three-dimensional (3D) rock salt-type lead selenide (PbSe) semiconductors, which have different crystal structures. The nonequilibrium (Sn 1−x Pb x )Se solid solution forms a direct phase boundary between the 2D and 3D structures at x = 0.5 and induces a reversible 2D−3D structural phase transition in (Sn 0.5 Pb 0.5 )Se due to varying temperatures.…”

Origins of three‐dimensional charge and two‐dimensional phonon transports in Pnma phase PbSnSe2 thermoelectric crystal

“…50,52 However, phonon conduction in the two distinct phases is largely limited by the alloy disorder arising from randomly distributed Pb and Sn atoms, resulting in low thermal conductivity (<1.0 W m À1 K À1 at 300 K). 52 By controlling the phase and fraction x in a solid solution, the 3D charge and 2D phonon transports could be realized for decoupling electron and phonon transport, which is pivotal for enhancing the ZT of thermoelectric materials. Therefore, it is essential to understand whether a PbSnSe 2 crystal, which is the PbSnSe 2 alloy with the shortest periodicity, can be a good thermoelectric material, particularly near the phase-transition temperature where is of practical relevance to reaching high ZT.…”