Defects can appear accidentally or intentionally in crystalline solids. Question is if a defect participates in phonon motions of a host lattice or it has its own dynamics. If so, what is a coupling between defect motions and collective motions of the crystal lattice? These questions are important in doped semiconductors, and in solids where defects are generated for modification of physical properties.

Phonons spectra are available for rather simple inorganic solids and can be theoretically calculated or measured by neutron scattering technique. The phonon spectra usually deviate significantly from Debye-type phonon spectra presented in most of textbooks of solid-state physics. Vibrations of a simple defect appear in a narrow frequency range are called local mode. Because of a low defect concentration the local modes can be not detected by neutron scattering, but can be detected by classical spectroscopy methods. Local modes can be identified by Raman and IR spectroscopy when appear in an optical phonon region. Local modes appearing in acoustic phonon region are difficult to observation. They can be detected in studies of electron spin relaxation measured by pulsed electron paramagnetic resonance methods. Electron spin system of paramagnetic defects after a pulse excitation relaxes (in time order of micro- to milliseconds) to initial equilibrium transmitting the excitation energy to phonons and/or to local mode of the defect. A method of local mode detection with electron-spin echo will be described and results will be presented for paramagnetic ions doped into inorganic and organic solids and for free radicals generated by ionizing radiation in crystals.

[1] J. Murphy, Spin-lattice relaxation due to local vibrations with temperature independent amplitudes, Phys. Rev. 145, 241-247 (1966)

[2] J. Goslar, S. K. Hoffmann, W. Hilczer, Local vibration mode mechanism of electron spin-lattice relaxation of PO₃^2–-radicals in g-irradiated (glycine)H₃PO₃ crystal, Sol. State Comm. 121, 423-427 (2002).

[3] S. K. Hoffmann, W. Hilczer, T. Radczyk, Electron spin-lattice relaxation in polymers and crystals related to disorder and structure defects. Acta Phys. Pol. A 103, 373-385 (2003).

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