Anomalous Lattice Thermal Conductivity in Rocksalt IIA–VIA Compounds

Abstract

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.

Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF

“…The phonon dispersions and densities of states of PbXF (X = Cl, Br, I) in Figure (calculated including the third order IFCs) show no imaginary frequencies, demonstrating dynamical stability at 300 K. The low-(high-)­frequency phonons are mainly due to vibrations of the Pb, I, and Br (F and Cl) atoms. The phonon bands are more dispersive for PbClF than PbBrF and PbIF due to the smaller mass difference between F and Cl as compared to that between F and Br/I, which leads to a phonon band gap in the cases of PbBrF and PbIF as previously reported for MQ (M = Ca, Sr, Ba and Q = S, Se, Te) In the low-frequency region, the phonon density of states (Figure ) is due to overlapping acoustic and low-lying optical phonons mainly involving vibrations of the Pb atoms (with contributions of I only in the case of PbIF).…”

“…The phonon bands are more dispersive for PbClF than PbBrF and PbIF due to the smaller mass difference between F and Cl as compared to that between F and Br/I, which leads to a phonon band gap in the cases of PbBrF and PbIF as previously reported for MQ (M = Ca, Sr, Ba and Q = S, Se, Te). 60 In the low-frequency region, the phonon density of states (Figure 3) is due to overlapping acoustic and low-lying optical phonons mainly involving vibrations of the Pb atoms (with contribu- tions of I only in the case of PbIF). The vibrational frequencies of the Cl, Br, and I atoms show a red shift from PbClF to PbBrF to PbIF due to the increasing mass difference to F. The red shift is associated with an increasing LO−TO splitting that reduces the dispersions of the phonon bands below 3 THz from PbClF to PbBrF to PbIF (Figure 3).…”

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.…”

“…For second and third order IFCs, all allowed interactions within the supercell were included for calculation of the lattice dynamics and phonon transport properties. The detailed methodology can be found in our previous studies. , To further analyze the effect of mass ratio on lattice dynamics and κ L , we also calculate the cophonicity, − which quantifies the overlap between phonon density of states of a given atomic pair in the specific window of phonon energy. The cophonicity is calculated within the entire frequency range for each compound under investigation to explore the role of atomic mass and its contribution to both acoustic and optical phonons.…”

Lattice instability and Raman spectra of BaO under high pressure: A first principles study

“…The phonon dispersion curves of B1 phase show large LO-TO splitting (∼ 312.3 cm −1 ) along Γ-direction with two degenerate TO and one LO modes and this large LO-TO splitting causes a fall of low lying optical (LLO) phonon modes into acoustic mode region, which can increase overlap between acoustic and LLO phonon modes there by enhancing the scattering phase space and eventually this leads to lower the lattice thermal conductivity in B1 phase at ambient conditions. 68 However, the LO-TO splitting decreases under pressure (see Figure 4a & b) for B1 phase. Moreover, the optical phonon modes are hardening with pressure while phonon softening is observed for acoustic phonon modes along X-direction in B1 phase and along M and K-Γ directions in B8 phase.…”