Angular dispersion of the internal phonon modes in ferroelectric sodium nitrite

Abstract

The angular dispersion of the frequencies of the internal modes in ferroelectric sodium nitrite are determined by measuring the Raman spectra with observation of phonon propagating in the ab, ac and bc crystallographic planes. Using the exciton model and a resonant dipole-dipole interaction mechanism the angular dispersion is calculated resulting in good agreement with the experiment. The k dispersion curves are also calculated for points along the [loo], [OlO], and [OOlj directions.Nous avons Btudik la dispersion angulaire des frequences des phonons internes d'un cristal ferroelectric de NaNO? au moyen de I'analyse des spectres de diffusion Raman. Les spectres que nous avons obtenu correspondent it la propagation des phonons dans le plans cristallographiques ab, ac et bc. L'emploi d'un model d'excitons molbculaires e t de interaction dipole-dipole nous a permis de calculer les curbcs de dispersion angulaire. L'accord entre les valeurs calculkes et le donnbes experirnentaux est excellente. Nous avons calculb aussi les curbes de dispersion des vecteura d'onde k selon les directions [loo], [OlO] et [OOl]. l) Via G. Capponi 9,

Frenkel excitons in : excitation energy transfer and exciton coherence

“…They will be discussed in more detail in later sections of this article. The lower-energy intermolecular modes have been investigated by various experimental techniques such as Raman and infrared spectroscopy [16][17][18][19][20][21][22][23][24][25][26], reflection spectroscopy [27][28][29][30] and inelastic neutron scattering [31,32]. Additional information about the phonons, especially the density of states of the acoustic modes, was obtained by studying the specific heat [33].…”