The negative phonon confinement effect in nanoscopic sodium nitrite

A nanocomposite of porous glass and a NaNO2 ferroelectric (channels of ∼7 nm diameter) was studied using infrared reflectivity, THz transmission and Raman spectroscopy as a function of temperature in the range of 300-500 K, including the ferroelectric transition. From the infrared and THz response the effective dielectric function was calculated and compared with the dielectric functions calculated from the Bruggeman and Lichtenecker models of the effective medium, using the known data on the polar phonon modes of the NaNO2 single crystals. The results show good qualitative agreement, indicating that the stiffening of the effective modes is due to local depolarization fields on the glass-ferroelectric interfaces. The nonpolar Raman modes show no substantial modification compared to those of the bulk NaNO2. Some signatures of the ferroelectric transition were even seen. The results indicate that the intrinsic size effect (phonon confinement) is negligible in this nanocomposite.