Phonon Coupling and Angular Dispersion in Biaxial NaNO2

Abstract

A detailed Raman investigation of the angular dispersion of the five phonons, propagating in oblique directions in the $\mathrm{ac}$ plane of biaxial NaN${\mathrm{O}}_2$, is reported. It is shown that this dispersion is caused by the coupling of all the vibrational modes through the induced longitudinal electric field. When the polarization configurations used in the Raman measurements are along the principal axes of the crystal, all the coupled modes are then detectable, leading to apparent selection-rule violations. For two dimensions the analysis yields a frequency-gap picture, which accounts quantitatively for the experimental results. An expression for the Raman intensities of the oblique phonons is also given.