Materials with an intrinsic (ultra)low lattice thermal conductivity (k L ) are critically important for the development of efficient energy conversion devices. In the present work, we have investigated microscopic origins of low k L behavior in BaO, BaS, and MgTe by exploring lattice dynamics and phonon transport of 16 isostructural MX (M = Mg, Ca, Sr, and Ba and X = O, S, Se, and Te) compounds in the rocksalt (NaCl)-type structure. Anomalous trends are observed for k L in MX (M = Mg, Ca, Sr, and Ba and X = O, S, Se, and Te) compounds except for the MgX (X = O, S, Se, and Te) series in contrast to the expected trend from their atomic mass. The underlying mechanisms for such low k L behavior in large mismatch atomic mass systems, namely, BaO, BaS, and MgTe, are thoroughly analyzed. We propose the following factors that might be responsible for low k L behavior in these materials: (1) high mass contrast provides a phonon gap between the acoustic and optic branches; (2) softening of transverse acoustic (TA) phonon modes due to the presence of heavy element; (3) low-lying optic (LLO) phonon modes fall into the acoustic mode region and are responsible for softening of the acoustic phonon modes or enhancing the overlap between LLO (TO) and longitudinal acoustic (LA) phonon modes, thereby increasing scattering rates; (4) shorter phonon lifetimes; and (5) a relatively high density (ρ) and a large Gruneisen parameter (γ) leads to strong anharmonicity. Moreover, tensile strain causes a further reduction in k L for BaO, BaS, and MgTe through phonon softening and near ferroelectric instability. Our comprehensive study on 16 binary MX (M = Mg, Ca, Sr, and Ba and X = O, S, Se, and Te) compounds provides a pathway for designing (ultra)low k L materials through phonon engineering even with simple crystal systems.

Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity (κ l ) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb 2+ 6s 2 lone pair, and phonon softening through the mass difference between F and Pb/X. The weak inter-layer van der Waals bonding and the strong intra-layer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction.Large average Grüneisen parameters (≥ 2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1%. The κ l values obtained by the two channel transport model are 20-50% higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow κ l values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low κ l materials for thermal energy applications.

Dibismuth dioxychalcogenides, Bi 2 O 2 Ch (Ch = S, Se, Te), are a promising class of materials for next-generation electronics and thermoelectrics due to their ultrahigh carrier mobility and excellent air stability. An interesting member of this family is Bi 2 O 2 S, which has a stereochemically active 6s 2 lone pair of Bi 3+ cations, heterogeneous bonding, and a high mass contrast between its constituent elements. In the present study, we have used first-principles calculations in combination with Boltzmann transport theory to systematically investigate the effect of hydrostatic pressure on lattice dynamics and phonon transport properties of Bi 2 O 2 S. We found that the ambient Pnmn phase has a low average lattice thermal conductivity (κ l ) of 1.71 W/(m K) at 300 K. We also predicted that Bi 2 O 2 S undergoes a structural phase transition from a low-symmetry (Pnmn) to a high-symmetry (I4/mmm) structure at around 4 GPa due to centering of Bi 3+ cations with pressure. Upon compression, the lone pair activity of Bi 3+ cations is suppressed, which increases κ l by almost 3 times to 4.92 W/ (m K) at 5 GPa for the I4/mmm phase. The computed phonon lifetimes and Gruneisen parameters show that anharmonicity decreases with increasing pressure due to further suppression of the lone pair activity and strengthening of intra-and intermolecular interactions, leading to an average room-temperature κ l of 12.82 W/(m K) at 20 GPa. Overall, this study provides a comprehensive understanding of the effect of hydrostatic pressure on the stereochemical activity of the lone pair of Bi 3+ cations and its implications on the phonon transport properties of Bi 2 O 2 S.