Metric for strong intrinsic fourth-order phonon anharmonicity
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Under the framework of Taylor series expansion for potential energy, we propose a simple and robust metric, dubbed ``regular residual analysis,'' to measure the fourth-order phonon anharmonicity in crystals. The method is verified by studying the intrinsic strong higher-order anharmonic effects in ${\mathrm{UO}}_{2}$ and ${\mathrm{CeO}}_{2}$. Comparison of the thermal conductivity results, which calculated by the anharmonic lattice dynamics method coupled with the Boltzmann transport equation and the spectral energy density method coupled with ab initio molecular dynamics simulation further validates our analysis. Analysis of the bulk Si and Ge systems confirms that the fourth-order phonon anharmonicity is enhanced and cannot be neglected at high enough temperatures, which agrees with a previous study where the four-phonon scattering was explicitly determined. This metric will facilitate evaluating and interpreting the lattice thermal conductivity of crystals with strong fourth-order phonon anharmonicity.Keywords:
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Although the quasi-harmonic approximation (QHA) method applies to many materials, it is necessary to study the anharmonic interaction for extremely anharmonic materials. In this work, the unusual negative thermal expansion (NTE) property of CaTiF6 is studied by combing QHA and anharmonic interaction. The improved self-consistent phonon approximation (ISCPA), which treats anharmonic effects in solids nonperturbatively, is employed. The agreement of NTE behavior between the calculation and the experiment can be further promoted from qualitative consistency by QHA to quantitative consistency by the ISCPA. From mode Grüneisen parameters, it is found that the low-frequency phonons, especially acoustic phonons, contribute greatly to the NTE behavior of CaTiF6. The rigid unit modes (RUMs) of low-frequency optical phonons can be identified. The phonon lifetime of CaTiF6 is calculated from three-phonon interactions; thereby, the NTE mechanism can be further explored by phonon lifetimes of phonons with different frequencies on heating. The anomalous lattice thermal conductivity (LTC) is predicted using the Boltzmann transport equation within the relaxation time approximation. The glasslike LTC can occur in crystal CaTiF6.
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Abstract The thermal diffuse scattering (TDS) of X‐rays in the anharmonic one‐phonon and harmonic two‐phonon approximations is considered. The complete expression for the one‐phonon intensity of X‐rays is derived by perturbation theory and presented for a polyatomic crystal in the high‐temperature limit. The method of evaluating the corrections of the X‐rays intensities for the thermal diffuse scattering is given and the importance taking into account the anharmonic effects in the one‐phonon scattering for KZnF 3 , CsCl, and YBa 2 Cu 3 O 7−δ crystals at room temperature is discussed. Anharmonic TDS‐effects achieve considerable magnitudes for soft (acoustic) anharmonic crystals as CsCl.
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The anharmonic frequencies of O-H, C-H, and N-H stretching modes of hydrogen-bonded glycine-H2O complexes are calculated using ab initio classical separable potential approximation. In this approach, ab initio molecular dynamic simulations are used to determine an effective classical potential for each of the normal modes of the system. The frequencies are calculated by solving the time-independent Schrödinger equation for each mode using time-averaged potentials. Three complex structures are studied, which differ in the location of the water molecule on the amino acid. Significant differences are found between the spectra of the three structures, and signatures of individual complexes are established. It is demonstrated that anharmonic effects are essential in the discrimination between different structures, while frequency differences at the harmonic level are much smaller. Intensities are also computed and found to carry information on differences between structures, but the role of anharmonicity in this is small.
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This chapter begins with a discussion of the general theory of transition possibilities. It then discusses anharmonic lattice forces, effects of selection rules, interaction with optical modes, four-phonon processes, elastic anharmonicity, thermal expansion, and the absorption of sound in solids.
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Abstract Group I niobates (KNbO 3 and NaNbO 3 ) are promising lead-free alternatives for high-performance energy storage applications. Despite their potential, their complex phase transitions arising from temperature-dependent phonon softening and anharmonic effects on dielectric properties remain poorly explored. In this study, we employ density-functional theory (DFT) and self-consistent phonon (SCP) calculations to investigate finite-temperature phonons in cubic niobate perovskites. To include explicit anharmonic vibrational effects, SCP frequencies are shifted by the bubble self-energy correction within the quasiparticle (QP) approximation, providing precise descriptions of phonon softening in these strongly anharmonic solids. We further calculate the static dielectric constant of KNbO 3 and NaNbO 3 as a function of temperature using the Lyddane-Sachs-Teller (LST) relation and QP-corrected phonon dispersions. Our theoretical results align with experimental data, offering reliable temperature-dependent phonon dispersions while considering anharmonic self-energies and thermal expansion effects, enhancing our understanding of the complex relations between lattice vibrations and phase transitions in these anharmonic oxides.
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Since last several years, the author has been studying the phonons on functional materials and has established structure-property correlations on flexible framework structure materials, lead free oxides, thin film perovskites, low dimensional 2D materials like graphene nanosheets, TiS3 nanofiber, VSe2 nanosheets, SnO2 quasi nanoparticles etc. In this book chapter, temperature dependent Raman spectroscopic studies on negative thermal expansion framework material H3[Co(CN)6] have been presented in the temperature range 80-300 K to elucidate the phonon anharmonicity of different phonons. No discontinuous or slope changes of phonon mode frequencies, linewidths and their band intensities were noticed suggesting that the compound was stable in the entire temperature range of investigation. Phonon anharmonicity models were used to analyse the temperature dependencies of mode frequencies and their linewidths. It was observed that the three-phonon decay process was dominant over the four-phonon process in this flexible compound. Concisely, the present study demonstrates the anharmonicity of the phonons and their correlation on thermal expansion of H3[Co(CN)6] framework material.
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An approach to compute the anharmonic peaks of the phonon dispersion curves through the ab initio calculated Hellmann-Feynman forces from a series of supercells with realistic atomic displacements of all atoms, which correspond to a given temperature, is reported. Obtained phonon dispersion bands are able to represent the positions and shapes of the anharmonic peaks. As example, the approach to cubic PbTe and perovskite MgSiO3 crystals is applied.
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