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.
KeywordsCalcite Nitrite Lanthanum Ferro Aragonite
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