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

Abstract

Lattice thermal conductivity (κL) is of great scientific interest for the development of efficient energy conversion technologies. Therefore, microscopic understanding of phonon transport is critically important for designing functional materials. In our previous study (Roshan et al., ACS Applied Energy Mater. 2021, 5, 882–896), anomalous κL trends were predicted for rocksalt alkaline-earth chalcogenides (AECs). In the present work, we extended it to alkali halides (AHs) and conducted a thorough investigation to explore the role of atomic mass contrast on lattice dynamics and phonon transport properties of 36 binary compounds (20 AHs + 16 AECs). The calculated spectral and cumulative κL reveal that low-lying optical phonon modes significantly boost κL alongside acoustic phonons in materials where the atomic mass ratio approaches unity and cophonocity nears zero. Phonon scattering rates are relatively low for materials with a mass ratio close to one, and the corresponding phonon lifetimes are higher, which enhances κL. Phonon lifetimes play a critical role, outweighing phonon group velocities, in determining the anomalous trends in κL for both AHs and AECs. To further explore the role of atomic mass contrast in κL, the effect of tensile lattice strain on phonon transport has also been investigated. Under tensile strain, both group velocities and phonon lifetimes decrease in the low frequency range, leading to a decrease in κL. This work provides insights on how atomic mass contrast can tune the contribution of optical phonons to κL and its implications on scattering rates by either enhancing or suppressing κL. These insights would aid in the selection of elements for designing new functional materials with and without atomic mass contrast to achieve relatively high and low κL values, respectively.

Highly Anisotropic to Isotropic Nature and Ultralow Out-of-Plane Lattice Thermal Conductivity of Layered PbClF-Type Materials

“…The contribution of low-lying optical phonons to κ l along the in-plane direction is responsible for the higher κ l in CaIF than in CaBrF. Similarly, SrBrF (BaBrF) has higher κ l than SrClF (BaClF) along the out-of-plane direction due to the relatively higher contribution of low-lying optical phonons to κ l in this direction, as previously reported for a few binary alkali halides and alkaline-earth chalcogenides . CaIF possesses ultralow κ l (0.39 W/m K) along the out-of-plane direction due to the extremely low contribution of acoustic and optical phonons to κ l .…”

“…Similarly, SrBrF (BaBrF) has higher κ l than SrClF (BaClF) along the out-of-plane direction due to the relatively higher contribution of low-lying optical phonons to κ l in this direction, as previously reported for a few binary alkali halides and alkaline-earth chalcogenides. 21 CaIF possesses ultralow κ l (0.39 W/m K) along the out-of-plane direction due to the extremely low contribution of acoustic and optical phonons to κ l . In addition, CaIF has the highest phonon transport anisotropy ratio (in-plane κ l to out-of-plane κ l ) of 10.95 at 300 K (Table 5) due to the highly anisotropic crystal structure with the highest axial ratio (c/a) of 2.281.…”

“…Discovery and design of functional quasi two-dimensional (2D) layered materials with extremely low or high lattice thermal conductivity (κ l ) are crucial for thermal energy management applications. , Ultralow κ l materials find potential applications in thermoelectrics, , thermal insulation, , thermal storage, , and thermal barrier coatings, − while high κ l materials find applications in power electronics and solid state lighting. , A plethora of mechanisms ,− including rattling, , strong quartic anharmonicity, , bonding heterogeneity, and various chemical bonding principles were proposed to achieve low κ l . Among them, bonding heterogeneity is an essential mechanism, and it is an intrinsic property of layered materials.…”