The coercive field measured from ferroelectric hysteresis loop for pure and Mg:SLT (0.5 mol% doped) crystals were found to be independent of ramp rate of voltage, while it depends strongly on ramp rate for Mg:SLT (1 mol% doped) crystals. The measured coercive field decreased monotonously depending on the Mg concentration and low Ec value of 6 kV/cm is obtained for Mg(1.0 mol%)SLT crystal. Internal field of about 1 kV/cm present in pure SLT completely gets vanished in Mg(1.0 mol%)SLT. Spontaneous polarization is insensitive to the Mg addition. The origin of the internal field and large changes in switching fields appear largely to be dependent on the [Li]/[Li+Ta] ratio in crystals. On repeated cycling, the coercive field is found to have a marginal variation in pure and Mg doped SLT crystals.