Electron Spin-Lattice Relaxation in Polymers and Crystals Related to Disorder and Structure Defects

Abstract

Temperature dependences (4-300 K) of the electron spin-lattice relaxation have been determined by electron spin echo technique for free radicals in two polymeric systems: phenol-formaldehyde resin and polyhydrazodisulphide. The dependences are described in terms of dynamics specific for amorphous systems involving two-level tunnelling states at low temperatures, exchange-coupled clusters of paramagnetic centres and local oscillators. Some universal temperature behaviour of the spin-lattice relaxation of amorphous systems is suggested, with a strong increase in relaxation rate with temperature at low temperatures and much weaker increase above 50 K with characteristic linear temperature dependence in a broad temperature range and cosech(Δ/kT)-type behaviour. It is also shown that the amorphous-type behaviour appears in low temperatures relaxation studies of single crystals but it is due to a non-uniform distribution of doped paramagnetic ions. Such behaviour we have found in Tutton salt crystals doped with Cu 2 + , as well as for free radical centres produced by ionising irradiation in (NH 4 ) 3 H(SeO 4 ) 2 and Li(N 2 H 5 )SO 4 single crystals, where extended phonons are suppressed around radiation damage centres suggesting a local amorphisation of the crystal structure.