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Thermal transport phenomena are ubiquitous and play a critical role in the performance of various microelectronic and energy-conversion devices. Binary rocksalt and zinc blende compounds, despite their rather simple crystal structures, exhibit an extraordinary range of lattice thermal conductivity (κL) spanning over 3 orders of magnitude. A comprehensive understanding of the underlying heat transfer mechanism through the development of microscopic theories is therefore of fundamental importance, yet it remains elusive because of the challenges arising from explicitly treating higher-order anharmonicity. Recent theoretical and experimental advances have revealed the essential role of quartic anharmonicity in suppressing heat transfer in zinc blende boron arsenide (BAs) with ultrahighκL. However, critical questions concerning the general effects of higher-order anharmonicity in the broad classes and chemistries of binary solids are still unanswered. Using our recently developed high-throughput phonon framework based on first-principles density functional theory calculations, we systematically investigate the lattice dynamics and thermal transport properties of 37 binary compounds with rocksalt and zinc blende structures at room temperature, with a particular focus on unraveling the impacts of quartic anharmonicity onκL. Our advanced theoretical model for computingκLembraces current state-of-the-art methods, featuring a complete treatment of quartic anharmonicity for both phonon frequencies and lifetimes at finite temperatures, as well as contributions from off-diagonal terms in the heat-flux operator. We find the impacts of quartic anharmonicity onκLto be strikingly different in rocksalt and zinc blende compounds, owing to the countervailing effects on finite-temperature-induced shifts in phonon frequencies and scattering rates. By correlatingκLwith the phonon scattering phase space, we outline a qualitative but efficient route to assess the importance of four-phonon scattering from harmonic phonon calculations. Among notable examples, in zinc blende HgTe, we identify an unprecedented sixfold reduction inκLdue to four-phonon scattering, which dominates over the three-phonon scattering in the acoustic region at room temperature. We also demonstrate a possible breakdown of the phonon gas model in rocksalt AgCl, wherein the phonon states are significantly broadened due to strong intrinsic anharmonicity, inducing off-diagonal contributions toκLcomparable to the diagonal ones. The deep physical insights gained in this work can be used to guide the rational design of thermal management materials.

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr 3 crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties. Neutron and X-ray scattering measurements provide further insight into the anharmonic behaviour of lead halide perovskites, revealing that rotations of PbBr 6 octahedra in CsPbBr 3 crystals occur in a correlated fashion along two-dimensional planes.

SignificanceUnveiling the unusual atomic dynamics in superionic conductors is critical for the design of energy conversion and storage materials, for example to rationalize their thermal transport properties in thermoelectric applications or their fast ionic conductivity in solid-state electrolytes. We combine neutron/X-ray scattering techniques with first-principles calculations to discover hybrid lattice dynamics in AgCrSe2, where the phonon wavelength controls the breakdown of transverse acoustic modes in the superionic phase. We attribute the ultralow thermal conductivity to strong phonon anharmonicity combined with disorder in the Ag sublattice. This understanding provides insights into the thermal conduction mechanism in superionic conductors and sheds light on the role of lattice dynamics in the emergence of superionic behavior. Intrinsically low lattice thermal conductivity (κlat) in superionic conductors is of great interest for energy conversion applications in thermoelectrics. Yet, the complex atomic dynamics leading to superionicity and ultralow thermal conductivity remain poorly understood. Here, we report a comprehensive study of the lattice dynamics and superionic diffusion in AgCrSe2 from energy- and momentum-resolved neutron and X-ray scattering techniques, combined with first-principles calculations. Our results settle unresolved questions about the lattice dynamics and thermal conduction mechanism in AgCrSe2. We find that the heat-carrying long-wavelength transverse acoustic (TA) phonons coexist with the ultrafast diffusion of Ag ions in the superionic phase, while the short-wavelength nondispersive TA phonons break down. Strong scattering of phonon quasiparticles by anharmonicity and Ag disorder are the origin of intrinsically low κlat. The breakdown of short-wavelength TA phonons is directly related to the Ag diffusion, with the vibrational spectral weight associated to Ag oscillations evolving into stochastic decaying fluctuations. Furthermore, the origin of fast ionic diffusion is shown to arise from extended flat basins in the energy landscape and collective hopping behavior facilitated by strong repulsion between Ag ions. These results provide fundamental insights into the complex atomic dynamics of superionic conductors.

Unveiling the unusual atomic dynamics in superionic conductors is critical for the design of energy conversion and storage materials, for example to rationalize their thermal transport properties in thermoelectric applications or their fast ionic conductivity in solid-state electrolytes. We combine neutron/X-ray scattering techniques with first-principles calculations to discover hybrid lattice dynamics in A g C r S e 2 , where the phonon wavelength controls the breakdown of transverse acoustic modes in the superionic phase. We attribute the ultralow thermal conductivity to strong phonon anharmonicity combined with disorder in the Ag sublattice. This understanding provides insights into the thermal conduction mechanism in superionic conductors and sheds light on the role of lattice dynamics in the emergence of superionic behavior. Intrinsically low lattice thermal conductivity ( κ l a t ) in superionic conductors is of great interest for energy conversion applications in thermoelectrics. Yet, the complex atomic dynamics leading to superionicity and ultralow thermal conductivity remain poorly understood. Here, we report a comprehensive study of the lattice dynamics and superionic diffusion in A g C r S e 2 from energy- and momentum-resolved neutron and X-ray scattering techniques, combined with first-principles calculations. Our results settle unresolved questions about the lattice dynamics and thermal conduction mechanism in A g C r S e 2 . We find that the heat-carrying long-wavelength transverse acoustic (TA) phonons coexist with the ultrafast diffusion of Ag ions in the superionic phase, while the short-wavelength nondispersive TA phonons break down. Strong scattering of phonon quasiparticles by anharmonicity and Ag disorder are the origin of intrinsically low κ l a t . The breakdown of short-wavelength TA phonons is directly related to the Ag diffusion, with the vibrational spectral weight associated to Ag oscillations evolving into stochastic decaying fluctuations. Furthermore, the origin of fast ionic diffusion is shown to arise from extended flat basins in the energy landscape and collective hopping behavior facilitated by strong repulsion between Ag ions. These results provide fundamental insights into the complex atomic dynamics of superionic conductors.

Intrinsically low lattice thermal conductivity (κ_(lat)) in superionic conductors is of great interest for energy conversion applications in thermoelectrics. Yet, the complex atomic dynamics leading to superionicity and ultralow thermal conductivity remain poorly understood. In this paper, we report a comprehensive study of the lattice dynamics and superionic diffusion in AgCrSe2 from energy- and momentum-resolved neutron and X-ray scattering techniques, combined with first-principles calculations. Our results settle unresolved questions about the lattice dynamics and thermal conduction mechanism in AgCrSe2. We find that the heat-carrying long-wavelength transverse acoustic (TA) phonons coexist with the ultrafast diffusion of Ag ions in the superionic phase, while the short-wavelength nondispersive TA phonons break down. Strong scattering of phonon quasiparticles by anharmonicity and Ag disorder are the origin of intrinsically low κ_(lat). The breakdown of short-wavelength TA phonons is directly related to the Ag diffusion, with the vibrational spectral weight associated to Ag oscillations evolving into stochastic decaying fluctuations. Furthermore, the origin of fast ionic diffusion is shown to arise from extended flat basins in the energy landscape and collective hopping behavior facilitated by strong repulsion between Ag ions. These results provide fundamental insights into the complex atomic dynamics of superionic conductors.

Here, using self-consistent ab initio lattice dynamical calculations that go beyond the quasiharmonic approximation, we show that the high-pressure high-temperature bcc-Fe phase is dynamically stable. In this treatment the temperature-dependent phonon spectra are derived by exciting all the lattice vibrations, in which the phonon-phonon interactions are considered. The high-pressure and high-temperature bcc-Fe phase shows standard bcc-type phonon dispersion curves except for the transverse branch, which is overdamped along the high symmetry direction Γ-N, at temperatures below 4,500 K. When lowering the temperature down to a critical value TC, the lattice instability of the bcc structure is reached. The pressure dependence of this critical temperature is studied at conditions relevant for the Earth's core.

Raman scattering is performed on Fe3GeTe2 (FGT) at temperatures from 8 to 300 K and under pressures from the ambient pressure to 9.43 GPa. Temperature‐dependent and pressure‐dependent Raman spectra are reported. The results reveal respective anomalous softening and moderate stiffening of the two Raman active modes as a result of the increase of pressure. The anomalous softening suggests anharmonic phonon dynamics and strong spin–phonon coupling. Pressure‐dependent density functional theory and phonon calculations are conducted and used to study the magnetic properties of FGT and assign the observed Raman modes, E2g2 $E_{2{\\rm{g}}}^2$and A1g1 $A_{1{\\rm{g}}}^1$ . The calculations proved the strong spin–phonon coupling for the E2g2 $E_{2{\\rm{g}}}^2$mode. In addition, a synergistic interplay of pressure‐induced reduction of spin exchange interactions and spin–orbit coupling effect accounts for the softening of the E2g2 $E_{2{\\rm{g}}}^2$mode as pressure increases. Temperature‐ and pressure‐dependent Raman scattering measurements on Fe3GeTe2 reveal anomalous softening and moderate stiffening of the E2g2 ${E}_{2g}^{2}$and A1g1 ${A}_{1g}^{1}$Raman active modes, respectively, as pressure increases. The anomalous softening of the E2g2 ${E}_{2g}^{2}$mode suggests anharmonic phonon dynamics and strong spin–phonon coupling, which is approved by density functional theory and phonon calculations.

We constructed computer-based simulations of the lattice dynamical properties of plutonium using an electronic structure method, which incorporates correlation effects among the f-shell electrons and calculates phonon spectra at arbitrary wavelengths. Our predicted spectrum for the face-centered cubic δ phase agrees well with experiments in the elastic limit and explains unusually large shear anisotropy of this material. The spectrum of the body-centered cubic phase shows an instability at zero temperature over a broad region of the wave vectors, indicating that this phase is highly anharmonic and can be stabilized at high temperatures by its phonon entropy.

Perovskites ABX 3 with delocalized positions of the X atoms represent a distinct class of dynamically distorted structures with peculiar structural relations and physical properties. The delocalization originates from atoms crossing shallow barriers of the potential energy surface. Quantum mechanically, they can be treated similar to light atoms in diffusive states. Many of these perovskite structures are widely used functional materials thanks to their particular physical properties, such as superconductivity, ferroelectricity and photo-activity. A number of these properties are related to static or dynamic motion of octahedral units. Yet, a full understanding of the relationships between perovskite crystal structure, chemical bonding and physical properties is currently missing. Several studies indicate the existence of dynamic disorder generated by anharmonic motion of octahedral units, e.g. in halide perovskite structures. To simplify structural analysis of such systems we derive a set of space groups for simple perovskites ABX 3 with dynamical octahedral tilting. The derived space groups extend the well established space group tables for static tiltings by Glazer [ Acta Cryst. B (1972). 28 , 3384–3392], Aleksandrov [ Ferroelectrics (1976). 24 , 801–805] and Howard & Stokes [ Acta Cryst. B (1998). 54 , 782–789]. Ubiquity of dynamical tilting is demonstrated by an analysis of the structural data for perovskites reported in recent scientific publications and the signature of dynamic tilting in the corresponding structures is discussed, which can be summarized as follows: ( a ) volume increase upon a lowering of temperature, ( b ) apparent distortion of octahedra (where Jahn–Teller distortions can be ruled out), ( c ) mismatch between observed instantaneous symmetry and average symmetry, ( d ) deviation of the experimental space group from the theoretically predicted structures for static tilting, ( e ) inconsistency of lattice parameters with those suggested by the theory of static tilts, and ( f ) large displacement parameters for atoms at the X and B sites. Finally, the possible influence of dynamic disorder on the physical properties of halide perovskites is discussed.

To quantify the effects of hydration on anharmonicity of olivine thermodynamics, we have measured in situ Raman spectra of an extremely hydrous forsterite with 4500 ppm (wt) H2O at temperatures up to 1273 K. All the Raman modes in hydrous forsterite shift linearly to lower wavenumbers with increasing temperature. The calculated isobaric mode Grunesien parameters related to SiO4 internal stretching and bending vibrations are much lower than lattice vibrations. Additionally, compared with anhydrous forsterite, except for the modes at 919, 858, and 227 cm-1, water greatly reduces the isobaric mode Gruneisen parameters of the Raman modes in forsterite. Water also has a large effect on the anharmonic parameters related to lattice vibrations, whereas it has little effect on the anharmonic parameters related to SiO4 internal stretching and bending vibrations. Those results have the implications to the variations of local structure with temperature and estimation of water effects on the thermodynamics of forsterite.