Low Frequency Raman Spectra of Ionic Crystals
Several physical phenomena depend on the normal modes of vibration of the crystal. A ferroelectric phase transition in an ionic crystal is explained as due to a vanishing low frequency transverse optical mode at q → 0 (soft mode). An antiferroelectric transition is due to a vanishing zone boundary phonon. Phase transformations induced by pressure are similarly explained as due to a vanishing transverse acoustic phonon. Order-disorder transformations in alloys are due to coupling of modes which are close in energy and at a phase difference π/2 . These low frequency phonons can be easily observed in Raman spectra, though the far infra-red measurements are very difficult. The ferroelectric soft mode and its frequency variation have been observed by Perry et al in Raman scattering.
KeywordsIonic Crystal Ferroelectric Phase Transition Ammonium Dihydrogen Phosphate Light Scattering Spectrum Force Constant Model
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