ICYS Annual Report 2022

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24(i) Bonding heterogeneity in mixed-anion compoundsSince last year, we have focused on bonding heterogeneity within distorted local structures in mixed-anion compounds resulting in extremely low lattice thermal conductivity. In this context, we discovered that the chalcohalide MnPnS2Cl (Pn = Sb, Bi) exhibits an exceptionally low thermal conductivity of 0.5 W/m K at room temperature1).To further confirm whether this strategy can be applied to a broader range of mixed-anion compounds, we have analyzed the thermal transport in MnPnCh2Br (Ch = S, Se, X = Br) and CdPnS2Cl. MnPnSe2Br and CdPnS2Cl are isostructural to MnPnS2Cl while MnPnS2Br has different crystal structure with similar local environment. We have found that the calculated values of lattice thermal conductivity are similar among MnPnSe2Br and CdPnS2Cl while MnPnS2Br shows a bit higher value. We can attribute this trend to difference in the extent of bonding heterogeneity in which MnPnS2Br has less scattered value of interatomic force constants within the same coordination polyhedra. This result indicates that bonding heterogeneity plays a dominant role for determining lattice thermal conductivity rather than atomic mass.(ii) Glass-like thermal conductivity derived from ferroelectric fluctuation and glass-crystal dualityStructural instability accompanied by phonon softening has been recently exploited to control the lattice thermal conductivity. A softening of low-frequency optical phonon results in a drastic change of phonon coupling strength and scattering phase space with acoustic modes. In this topic, we have focused on a stuffed tridymite-type oxide BaAl2O4, which comprises an AlO4 network with six-membered cavities occupied by Ba atoms, which experiences a ferroelectric transition at TC = 450 K from high-temperature phase (a space group P6322) to low-temperature phase (P63). This transition is driven by the soft mode whose vibration pattern is characterized as tilting AlO4 accompanied by in-plane rotation of O1 atoms that connect the tetrahedra along the c-axis. In addition, the transition is rapidly suppressed by substituting Sr for Ba atoms. TC disappears near x = 0.1, where the structural instability lies without being released. For Sr-substituted system, two soft modes are not condensed at Research Digest low-temperature, which results in dynamical fluctuation. It corresponds to our analysis on the pair distribution function, which demonstrates that AlO4 polyhedra deviates from that of the average structure. In addition, inelastic neutron scattering (Fig. 1(a), (b)) reveals glass-like phonon spectra along with sharp Bragg reflections, indicating that the periodic arrangement of the AlO4 network deforms to be a continuum of a short-range correlation while the robust Ba(Sr)-sublattice is preserved2). This partial glass-like state coexisting with the rigid crystalline sublattice can have significant effects on thermal transport. Fig. 1(c) shows the temperature dependence of experimental thermal conductivity for Ba1−xSrxAlO4 together with that of BaFeO4, which has similar local environment FeO4. Ba1−xSrxAlO4 shows glass-like flat temperature dependence and low absolute value of thermal conductivity above room temperature. It is evident by comparing with BaFeO4, which obeys the normal crystalline behaviour. This work motivates the search for network compounds with glass-crystal duality and their anomalous thermal transport properties.Fig. 1. (a) Q-integrated inelastic spectra over |Q| = 2–4 Å. The inset represents the maximum intensity of the peak at Q = 2.5 Å. (b) Energy-integrated inelastic spectra over the energy transfer E = 1–2.3 meV. (c) The temperature dependence of experimental thermal conductivity for Ba1−xSrxAlO4 (x = 0–0.40) together with that of BaFeO4.References1) N. Sato et al., J. Mater. Chem. A 9, 22660–22669 (2021).2) Y. Ishii, N. Sato et al., Phys. Rev. B 106, 134111 (2022).1. Outline of ResearchManipulating thermal transport is of great importance for various applications based on functional materials ranging from thermoelectrics, thermal insulators, thermal barrier coatings, and phase change memory to optoelectronics. Phonons have a mostly dominant role in the thermal transport of semiconductors and insulators. Thus, phonon engineering, namely designing desired structures whose length scale selectively interacts with phonons with aimed mean free path, is a critical approach to enhance performance of thermal management materials. 2. Research ActivitiesHigh-performance Thermal Management Materials Based on Phonon EngineeringNaoki SATO