Internal field in ferroelectric triglycine sulphate crystals as studied by epr of V02 impuritiest

Epr spectra of VO +-doped triglycine sulphate crystals have indicated that the proton configuration in defective hydrogen bonds surrounding the impurity ions yields more than two differently oriented crystalline fields. On annealing the crystals at 80°C or above, these protons are rearranged in a stable configuration, resulting in simpler spectra. The stable VO2+ complexes have C2 symmetry, and the ligand hyperfine splittings due to a proton pair are partially resolved in their spectra. The observed anisotropics are slightly temperature dependent, implying that V02+ions execute a random motion with a small amplitude in the crystalline potential.VO2+ ions possess electric dipoles, which interact with the microscopic internal field at the trapping sites. In the ferroelectric phase, VO2+ spectra from well-annealed crystals exhibit a doublet splitting on each of the sl V hyperfine components. The equilibrium intensity-ratio of the doublet has been found to follow exp (εPs(T)lkT), where Ps(T) is the macroscopic spontaneous polarization at temperature T, ε being a constant related to the electric dipole of VO2+ions. The static internal field is therefore proportional to Ps(T),as postulated in the Lorentz-Weiss mean field theory. The mechanism for vanadyl dipole reversal involves proton relocation through vacant proton sites. The relaxation time depends on temperature and domain movement in the sample crystals varying from fractions of a minute near the transition point to the order of an hour at room temperature.