Ferroelectric Instability Induced Ultralow Thermal Conductivity and High Thermoelectric Performance in Rhombohedral p-Type GeSe Crystal

Abstract

The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: Pnma), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: R3m), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferro­electric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe2. We show ultralow lattice thermal conductivity (κL) of 0.74–0.47 W/mK in the 300–723 K range and high zT ≈ 1.25 at 723 K in the p-type rhombohedral (GeSe)0.9­(AgBiSe2)0.1 crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferro­electric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferro­electric instability in (GeSe)0.9­(AgBiSe2)0.1 are directly evident in the low temperature heat capacity (C p) and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferro­electric instability induced soft transverse optical phonons significantly reduces the κL and enhances the thermoelectric performance in rhombohedral (GeSe)0.9­(AgBiSe2)0.1 crystals.

Gate‐Tunable Polar Optical Phonon to Piezoelectric Scattering in Few‐Layer Bi2O2Se for High‐Performance Thermoelectrics

“…Ferroelectric lattice instability in a material softens the polar transverse optical phonon modes, and their coupling with strain can increase scattering of heat carrying acoustic phonons, resulting in the decrease of lattice thermal conductivity. [ 11 ] The decrease of lattice thermal conductivity may compensate for the increase of electronic thermal conductivity and suppress the deterioration of thermoelectric properties.…”

“…have been shown to be very effective in lattice thermal conductivity reduction. [11][12][13][14][15] Nevertheless, the role of carrier mobility in independently enhancing the PF (improving electrical conductivity without sacrificing the Seebeck coefficient [16,17] ) is often overlooked due to the difficulties in engineering the carrier scattering mechanisms, despite it is non-trivial. [16] Until recent years, related works on enhancing mobility by manipulating scattering mechanism in thermoelectrics has gradually emerged and mainly focused on bulk materials.…”

Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

“…This leads to a recent surge in the discovery of intrinsically low thermal conducting materials. Various intrinsic effects like stereochemically active lone pairs, [7] inherent bonding heterogeneity, [8] layered 2D structures like SnSe, [9] SnS, [10] BiSe, [11] and BiTe, [12] complex crystal structures, [13] ferroelectric instability, [14, 15] or part crystalline–part liquid states [16] have been employed to achieve an intrinsically low κ lat . Low thermal conductivity has been observed in skutterudites [17] and clathrates [18] wherein a host element rattles independently in an oversized cage.…”

Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

“…[6] Recently,i th as been realized that, rather than extrinsic (nano/meso-structuring) phonon transport manipulation strategies,the deployment of intrinsic material properties to block heat transport is effective to achieve PGEC like properties. [3] This leads to ar ecent surge in the discovery of intrinsically low thermal conducting materials.V arious intrinsic effects like stereochemically active lone pairs, [7] inherent bonding heterogeneity, [8] layered 2D structures like SnSe, [9] SnS, [10] BiSe, [11] and BiTe, [12] complex crystal structures, [13] ferroelectric instability, [14,15] or part crystalline-part liquid states [16] have been employed to achieve an intrinsically low k lat .L ow thermal conductivity has been observed in skutterudites [17] and clathrates [18] wherein ah ost element rattles independently in an oversized cage.…”