Anomalous Lattice Thermal Conductivity in Rocksalt IIA-VIA Compounds

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 (rho) and a large Gruneisen parameter (gamma) 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.

Automated summary

The study investigates the microscopic origins of low thermal conductivity behavior in BaO, BaS, and MgTe, proposing factors such as high mass contrast, softening of phonon modes, and strong anharmonicity as responsible mechanisms. Tensile strain is also found to contribute to further reduction in thermal conductivity in the mentioned compounds. This comprehensive research provides insights for designing materials with (ultra)low thermal conductivity through phonon engineering even in simple crystal systems.