Oxygen vacancies effect on ionic conductivity and relaxation phenomenon in undoped and Mn doped PZN-4.5PT single crystals

AC-impedance spectroscopic studies in the temperature range 550 - 700 degrees C are carried out on undoped and Mn doped PZN- PT single crystals grown by the flux method. The variation of dielectric permittivity with temperature at different frequencies shows normal ferroelectric and relaxor-like dependence for the doped and undoped crystals, respectively. Temperature-dependent spectroscopic modulus plots reveal a much broader peak for PZN- 4.5PT + 1% Mn compared with that for PZN- 4.5PT, which is different from the dielectric behaviour of the doped one. Complex modulus imaginary part (Z '') versus real part (Z') plots fit well with one semicircle thus indicating only bulk contribution. The relaxation observed in the spectroscopic plots was assigned to mobile relaxor species such as oxygen vacancies and ions. No such relaxation could be observed for PZN- 4.5PT + 1% Mn in the dielectric measurements. For both undoped and Mn doped crystals, the conduction behaviour was modelled by the universal dynamic response equation and by the NTC ( negative temperature coefficient) materials resistance-temperature behaviour. A large difference in behaviour was found between the two single crystals such as the thermistor coefficients and the activation energy values, which could explain the increase in the thermal stability observed in the Mn doped PZN-PT single crystals by many studies.