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Experimental investigation of mode anharmonicity in the phenothiazine single crystal
Abstract
The relationship between vibrational mode anharmonicity and the structural phase transition (SPT) in phenothiazine single crystals has been studied by Brillouin scattering spectroscopy and uniaxial piezomodulated Raman spectroscopy (UPRS). Polarized Brillouin spectra of crystals of phenothiazine were measured for thirty-six independent acoustic phonons at eleven separate temperatures in the range 295-225K. No long wavelength acoustic instabilities were observed in this range. A complete set of elastic constants for four possible crystal structures was determined. Volume and linear compressibilities were also determined. A relationship between the intensity of a Brillouin component and disorder or domain formation within the crystal is also discussed. UPRS has been used to determine the extent of anharmonicity in the crystal vibrations of phenothiazine at room temperature and 90K. Strain induced coupling constants have been determined from the relative intensities of the UPRS response for several modes known to be unstable throughout the SPT. The UPRS spectra, above and below T$\sb{\rm c}$, are consistent with two possible mechanisms for the SPT in phenothiazine: (1) As the temperature is raised an increase in coupling between two unstable lattice modes, both initially Raman active below T$\sb{\rm c}$, occurs. One lattice mode develops large amplitude vibrational motion which at T$\sb{\rm c}$ causes the molecules to either vibrate around an unstable position or settle into a disordered lattice. This lattice mode persists as a Raman active mode and the other unstable mode becomes IR active due to the formation of inversion symmetry following the rule of mutual exclusion. (2) The room temperature phase of crystalline phenothiazine remains as a metastable intergrowth in the low temperature phase. The UPRS spectra are also consistent with a lattice assignment to a Raman band previously assigned as a butterfly wag of the phenothiazine molecule.
Subject Area
Chemistry
Recommended Citation
Sartwell, Joel Frank, "Experimental investigation of mode anharmonicity in the phenothiazine single crystal" (1991). ETD collection for University of Nebraska-Lincoln. AAI9200151.
https://digitalcommons.unl.edu/dissertations/AAI9200151