Intrinsically Low Thermal Conductivity in the n-Type Vacancy-Ordered Double Perovskite Cs₂SnI₆: Octahedral Rotation and Anharmonic Rattling

Abstract

Fundamental understanding of the relationship between chemical bonding, lattice dynamics, and thermal transport is not only crucial for thermoelectrics but also essential in photovoltaics and optoelectronics. This leads to a widespread search for low thermally conductive crystalline metal halide perovskites with improved electrical transport and stability. Pb-free all-inorganic Sn-based halide perovskites are particularly compelling because of their degenerate hole doping capability, which generally results in p-type conduction. Herein, we demonstrate an n-type thermoelectric conduction in concurrence with an ultralow lattice thermal conductivity (κlat ∼0.29–0.22 W/m·K) in an air-stable vacancy-ordered double perovskite Cs2SnI6. Phonon dispersion calculated by density functional theory indicates the presence of low-frequency localized optical modes at 8 and 32 cm–1 due to the dynamical rotation of SnI6 octahedra and anharmonic rattling of Cs-atoms, respectively, which are experimentally verified by temperature-dependent Raman spectroscopy and low-temperature heat capacity measurement. Cs2SnI6 exhibits a soft elastic lattice with chemical bonding hierarchy that causes low bulk and shear moduli, which in turn results in a low measured sound velocity of ∼1158 m/s. Low-energy anharmonic optical modes strongly couple with heat-carrying acoustic phonons and, consequently, limit phonon group velocity and phonon lifetime to an ultrashort value, leading to an intrinsically ultralow κlat in n-type Cs2SnI6.

Vacuum-Deposited Cesium Tin Iodide Thin Films with Tunable Thermoelectric Properties

“…In contrast, Figure S11 shows that the XRD diffraction pattern after heating to 150 °C in ambient conditions remains unchanged, revealing the high thermal stability of Cs 2 SnI 6 . This finding was previously confirmed by other authors who performed thermogravimetric analysis (TGA), showing no decomposition until 515 K …”

“…In contrast, Figure S11 shows that the XRD diffraction pattern after heating to 150 °C in ambient conditions remains unchanged, revealing the high thermal stability of Cs 2 SnI 6 . This finding was previously confirmed by other authors who performed thermogravimetric analysis (TGA), showing no decomposition until 515 K. 46 The conversion of B-β CsSnI 3 to Cs 2 SnI 6 leads to a change of the optical properties of the thin films. The shape and the intensity of the optical absorbance are completely different from the B-β CsSnI 3 phase (Figure 3c).…”

“…The n-type character of the compound comes from the presence of iodine vacancies and tin interstitials inside the lattice, defects easily formed in this double perovskite. 46 , 51 With our home-built setup, measurements run on a different day and on a different sample yielded a Seebeck coefficient at 25 °C of −514 μV/K ( Figure S13 ), which is considerably higher in absolute value, although in the same order of magnitude as what was obtained from the analysis using the thin-film analyzer. We hypothesize that the difference of Seebeck values arises from a dissimilar oxidation level of the layers.…”

Simultaneous Enhancement of Thermoelectric Power Factor and Phase Stability of Tin-Based Perovskites by Organic Cation Doping

“…For instance, benefiting from large absorption coefficients, high charge carrier mobility, and long carrier diffusion lengths, HPs have been recognized as promising photovoltaic materials and further applied in various optoelectronic devicesLEDs, lasers, photodetectors, and X-ray detectors . In addition to optoelectronic applications, the cubic perovskite structure of HPs leads to ferroelectric polarization, which is applied in piezoelectric energy harvesting devices. , Just recently, the potential room-temperature thermoelectric (TE) application of HPs has attracted wide attention due to their intrinsic ultralow lattice thermal conductivity originating from the cluster rattling mechanism. − …”

“Breathing” organic cation to stabilize multiple structures in low-dimensional Ge-, Sn-, and Pb-based hybrid iodide perovskites

“…Hybrid halide perovskite materials with small exciton binding energy, high carrier lifetime, high absorption coefficient and other favorable properties have been extensively studied for optoelectronic applications. [1][2][3][4][5][6] Three-dimensional (3D) halide perovskites have a general formula of AMX 3 (A = Cs + , CH 3 NH 3 + , HC(NH 2 ) 2 + ; M = Pb, Sn or Ge; X = halide), 4,7,8 for a limited set of A-site cations. In contrast, low-dimensional perovskites enjoy tremendous chemical flexibility as well as structural diversity, offering the opportunity to use a large variety of functional organic molecules as cations.…”