Piezomodulated-Raman-spectroscopy study of the phase transition in phenothiazine

Single crystals of phenothiazine undergo a displacive order-disorder phase transition near 250 K. The microscopic mechanism of this transition is not clear owing to the complexity of a disordered lattice and the lack of direct mode-anharmonicity data. Unstable optical modes have been observed in phenothiazine and are regarded as coupled, by symmetry, to an acoustic mode that becomes overdamped through the phase transition. The latter is considered to have the primary role in the phase transition. This premise is in doubt, however, based on the optical-mode anharmonicity directly observed by uniaxial-piezomodulation-Raman-spectroscopy results presented here. Anharmonic responses are observed in the low-temperature spectra at frequencies that correspond to room-temperature Raman values and thus support persistent metastable, room-temperature, lattice structures that are intergrown within the low-temperature phase. This result is indicative of a first-order phase change. A 22-${\mathrm{cm}}^{\mathrm{-}1}$ mode at room temperature displays a signficantly anharmonic response that may correspond to the primary order parameter or to coupling with an overdamped infrared active mode.