Ionic thermocurrent spectrum is characterized by the maximum depolarization current (IM), the temperature TM at which IM appears and the linear heating rate. Relaxation time τM at TM is found to be fixed for a particular crystalline structure when data corresponding to a specific type of dopant recorded at the same heating are considered. τM has been found to decrease with the increase in the heating rate. Also, τM for a particular crystalline structure changes when a different type of dopant is introduced in the lattice.