Isofrequency Temperature Anomalies of Raman Scattering Intensity in Quartz Crystals

Isothermal dependences of Raman scattering intensity i ( ω ′; T = const) on frequency Ω = ω 0 − ω ′ and isofrequency temperature dependences of i (Ω = ω 0 − ω ′ = const; T ) in quartz crystals in a wide temperature range, including the α−β phase transition point ( ω 0 and ω ′ are the excitation and Raman frequencies, respectively) have been compared. The isofrequency temperature dependences exhibit an intensity peak (near the phase-transition point), whose spectral position and magnitude depend on the fixed frequency Ω = ω 0 − ω ′. A theory of these dependences is developed for the crystals undergoing structural phase transitions. The theory predicts a soft-mode effect in the form of Raman opalescence: an anomalous increase in the spectral intensity of the isofrequency maximum i 0 (Ω, T ) with a decrease in frequency Ω near the phase-transition temperature ( T ≈ T c ). Agreement is established between the experimentally observed spectral intensity anomalies and the developed theory of isofrequency temperature dependences and processes of interaction of the fundamental soft mode with a low-frequency high- Q oscillator.