Anomalous Thermal Conductivity Induced by High Dispersive Optical Phonons in Rubidium and Cesium Halides

Abstract

Knowledge of lattice dynamics is essential for understanding the physical properties of materials and optimizing the performance for applications. Alkali halides (MX, M=Na, K, Rb, and Cs; X=Cl, Br, and I) have been widely recognized for their simple crystal structures and low thermal conductivities. At room temperature, the thermal conductivity (κ) of RbBr is nearly twice as large as that of RbCl and RbI, while the thermal conductivities of three compounds in CsX halides are comparable. These thermal conductivity trends with increased atomic mass are significantly different from the common sense that the thermal conductivity conventionally decreases with the increasing atomic mass and decreasing electronegativity difference. However, little attention has been paid to the microscopic mechanism of these anomalous thermal conductions in RbX and CsX. Here, we report a systematic investigation of the thermal transport properties in alkali halides by the Boltzmann transport equation based on first-principles calculations. The results show that the anomalous thermal conductivity trends of RbX and CsX mainly attribute to the disparity of optical phonons in each compound. The more dispersive the optical phonons, the greater their contribution to the thermal conductivity, and thus the thermal conductivities of compounds with heavier atoms are enhanced. The disparity of optical phonons originates from the mass difference in the compound. Our work offers deeper insight into the unusual phonon thermal transport phenomenon in alkali halides, and provides fundamental reference for rooting the thermal conductivities in related functional materials and finding novel materials with thermal conduction beyond the conventional cognition.

Effect of Atomic Mass Contrast on Lattice Thermal Conductivity: A Case Study for Alkali Halides and Alkaline-Earth Chalcogenides

“…The strength of the longitudinal optical–transverse optical (LO–TO) splitting is notable in metal halides/oxides but diminishes with the size of metal and nonmetal atoms due to a reduction in electronegativity difference of the involved elements (Figures and S1). , Particularly, materials with mass ratio close to unity exhibit a significant overlap in phonon bands between acoustic and optical modes, unlike systems with substantial mass contrast (see Figure ). This pronounced overlap in the low-lying optical phonon modes (see Figures S2–S5) significantly enhances κ L for NaF, KCl, CsI, SrSe, and BaTe in their respective series.…”

“…(a,b) Lattice thermal conductivity as a function of atomic mass ratio for alkali halides and alkaline-earth chalcogenides. The lattice thermal conductivity values are taken from the present work and previous literature. , In panel (a) △ refer to values obtained from the present work, ○ refer to values considered from ref , and ◊ refers to values taken from ref .…”

“…For example, monolayer (ML) BeO has high κ L than ML-MgO and ML-CaO due to strong covalent bonding, zincblend-MgTe has a higher κ L value than rocksalt-MgTe due to distinct bonding nature, and also the interplay between mass ratio and chemical bonding still needs to be investigated thoroughly. The above mentioned studies ,,, raise many fundamental questions: (1) does atomic mass alone play a key role in determining the magnitude of κ L ? (2) what are the dominant factors (group velocities and/or phonon lifetimes) that determine the low or high κ L behavior and what is the role of optical phonon modes in phonon transport in small or large mass ratio compounds?…”

Light Atomic Mass Induces Low Lattice Thermal Conductivity in Janus Transition-Metal Dichalcogenides MSSe (M═Mo, W)

“…Therefore, the phonon–phonon scattering rate of MoSSe is higher than that of WSSe. A similar phenomenon was also observed in refs − .…”

“…However, their atomic radii and electronegativity are different. To account for the effect of electronegativity, − the shared and transferred charges of each atom are calculated and shown in Table . The transferred charge of Mo (0.37| e |) is greater than that of W (0.30| e |), which indicates that MoSSe is more ionic.…”