Anharmonic Grüneisen theory based on self-consistent phonon theory: Impact of phonon-phonon interactions neglected in the quasiharmonic theory

Lattice Dynamics and Electron–Phonon Coupling in Double Perovskite Cs2NaFeCl6

“…In the systems with a large Stokes shift between the ground and excited states, amplitude of the n-phonon resonant Raman scattering becomes proportional to the first order of the electron−phonon interaction, greatly enhancing intensities of the phonon overtones. 37,48 We note, however, that such Raman resonances induced by selftrapped excitons were detected in perovskites when the excitation photon energy was only slightly higher than the bandgap energy. 45,47 In our case, however, the high-order phonon modes are pronounced when hω exc is equal to 3.815 eV, i.e., exceeds the bandgap energy by about 1.3 eV.…”

“…Though the exact origin of the states involved in the Raman resonance cannot be singled out from the present study, we note that strong multiphonon resonant Raman scattering under above bandgap excitation was also previously observed in LaMnO 3 and was attributed to the formation of self-trapped excitons that stem from orbital states (the so-called “orbiton”). 37 , 48 …”

Free energy predictions for crystal stability and synthesisability

“…The QHA often gives reasonable predictions of the anharmonic Grüneisen parameter (γ ν = − V ων ∂ων ∂V ) and the thermal expansion in materials -either as a a result of negligible intrinsic anharmonic effects or due to a fortuitous cancellation of errors. 25,26 The change in unit cell volume can have profound impact on phonon frequencies, and thus the free energy. This can affect the relative stability of different phases or polymorphs and lead to more accurate predictions of stability ranges.…”

“…The QHA often gives reasonable predictions of the anharmonic Grüneisen parameter and the thermal expansion in materials—either as a result of negligible intrinsic anharmonic effects or due to a fortuitous cancellation of errors. 25,26…”

Ab-initio structural optimization at finite temperatures based on anharmonic phonon theory: Application to the structural phase transitions of BaTiO$_3$

“…In the former method (SCP, IFCs from DFT at each 𝑎), the calculation of the IFCs and the free energy is performed at 14 different lattice constants, and the optimum lattice constant at each temperature is obtained by fitting the free energy by the Birch-Murnaghan equation of state [58,59], as in the calculations in Refs. [46,60]. It is more accurate because the IFCs are calculated from DFT calculations at different lattice constants instead of estimating them by the IFC renormalization.…”