Glassy thermal conductivity in Cs3Bi2I6Cl3 single crystal

Abstract

As the periodic atomic arrangement of a crystal is made to a disorder or glassy-amorphous system by destroying the long-range order, lattice thermal conductivity, κL, decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like κL in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (~0.20 W/m·K at room temperature) and glass-like temperature dependence (2–400 K) of κL in a single crystal of layered halide perovskite, Cs3Bi2I6Cl3. Acoustic phonons with low cut-off frequency (20 cm−1) are responsible for the low sound velocity in Cs3Bi2I6Cl3 and make the structure elastically soft. While a strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function evidence a local structural distortion in the Bi-halide octahedra and Cl vacancy. The hierarchical chemical bonding and soft vibrations from selective sublattice leading to low κL is intriguing from lattice dynamical perspective as well as have potential applications.

Strong Antibonding I (p)–Cu (d) States Lead to Intrinsically Low Thermal Conductivity in CuBiI4

“…The symmetry broken tetrahedrally coordinated Cu–I bond, in CuBiI 4 , hence undergoes extended p – d hybridization, and consequently, the presence of a strong antibonding p – d * state weakens the crystal lattice. The strength of the p – d antibonding interaction just below the E f in CuBiI 4 is greater than the general s – p antibonding interaction found in halide perovskites as well as p – d antibonding interactions in other Cu­(I) chalcogenides and halide perovskites, ,, as shown in Figure f. Such strong antibonding interactions below E f make the lattice softer by directly affecting the elastic moduli, which we have verified by measuring very low sound velocity (Table S3, SI) in the material.…”

“…(c) Orbital-resolved electronic band structure, (d) electronic density of states (DOS), and (e) corresponding crystal orbital Hamilton population (COHP) of CuBiI 4. (f) Comparison of maximum antibonding strength below the Fermi level ( E f ) of CuBiI 4 with other halide perovskites ,, and copper chalcogenide samples.…”

“…The studies regarding thermal transport have centered around Pb-based halides until now. ,, Conversely, Pb-free halides have been subjected to investigation in limited cases. ,, The need for a suitable alternative to toxic Pb in perovskite halides has a direct impact on the structures, to reduce the dimension of perovskites, sometimes even adopting a non-perovskite new structure. Recent studies reveal copper­(I) halides to be one of the most promising replacements in optoelectronics due to their lattice softening upon excitation and subsequent self-trapped excitonic broad band emission properties. − However, an in-depth understanding of the influence of chemical bonding on the phonon transport properties is limited, which is important for designing high-performance devices.…”

“…55,56,67 On the other hand, the conduction bands above E f are mainly contributed by the 6p states of Bi that partially overlap with the empty 5p states of I. To determine the nature of p−d interactions, we have further performed crystal orbital Hamilton population (COHP) 68,69 analysis, as shown in Figure 3e halide perovskites, 41,71,72 as shown in Figure 3f. Such strong antibonding interactions below E f make the lattice softer by directly affecting the elastic moduli, which we have verified by measuring very low sound velocity (Table S3, SI) in the material.…”

Pressure-Induced Emission from All-Inorganic Two-Dimensional Vacancy-Ordered Lead-Free Metal Halide Perovskite Nanocrystals

“…Therefore, lead-free perovskites are highly desirable on the basis of environmental and toxicity issues. In particular, the bismuth-based halide perovskites are promising alternatives because of the similar electron configurations of Bi 3+ and Pb 2+ . , Nevertheless, most of them exhibit a low PL efficiency that limits diverse practical applications, which might be caused by the extremely strong electron–phonon interaction . As we know, electron–phonon coupling plays a crucial role in regulating the photophysical properties of bismuth-based perovskites.…”