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Lead zirconium titanate (PZT) piezo ceramic system was discovered by Jaffe.et.al. The coexistence of rhombohedral and tetragonal phases at morphotropic phase boundary (MPB) composition, has been well established by various researchers across the globe to have superior dielectric and piezoelectric properties. Modification of various properties of PZT nano ceramics by addition of dopants have been researched extensively for further improvement of dielectric and piezoelectric properties, so as to make them suitable for various applications such as actuators, transducers and ceramic capacitors. One significant merit of PZT ceramic over other synthetic engineering composites is that, by doping foreign ions to substitute a part of the host atom; can modify their piezoelectric and dielectric properties significantly, depending on the site occupied by the ion in the ABO3 type perovskite structure. The dopants are classified as isovalent, acceptor or donor. For isovalent doping, the substituting ion has the same valency and the same ionic size as the replaced ion. For example Sn+4 substitute (Zr,Ti)4+ or Ca+2 substitute Pb+2 , which has been shown, to enhance the dielectric properties. The isovalent substitution generally causes a little influence on the dielectric properties due to fluxing effect of the doping ions during the period of sintering and hence makes characterization technique easier. For acceptor doping, the substituting ion have lower valency and ionic size as compared to the replaced ion. For example Fe+3, Mg+2 can substitute (Zr,Ti)+4 or Na+ can substitute Pb+2. The existence of intrinsic lead vacancies introduce space charge and internal field inside the PZT grains. It restricts the domain motion, thereby increasing the cohesive field but reduces the dielectric constant significantly. These are known as hard PZTs, because higher electric field is - required to pole these ceramics. The donor ions on the other hand reduce the concentration of intrinsic oxygen vacancies, created due to PbO evaporation during sintering. These substituting ions have higher valency and ionic size as compared to replaced ions. The donor ions introduce lead vacancies to maintain the charge neutrality, for example La+3 can substitute Pb+2 or Nb+5 can substitute (Zr,Ti)+4. The increase of lead vacancies can generate electrons by ionization. Most of the holes from the Pb vacancies are compensated by electrons from donor level to make the resistivity high. They can be easily poled and hence are called soft PZTs due to lower electric field values required to pole these ceramics successfully. They are known to have higher dielectric properties. The main objective of the present work is to investigate the effect of MgO doping with variable weight percentage on structural and dielectric properties of PZT having a fixed Zr/Ti ratio of 70/30, prepared by sol-gel technique, which has been reported to be superior over solid state reaction technique used earlier, with a view to homogeneity, purity and reactivity of the prepared sample. Thereby determining the best doping percentage of Mg with PZT (70/30) with respect to a linear variation of dielectric constant with temperature over a wide range, so as to make the doped material suitable to be used as a thermal sensor, when incorporated as dielectric in a variable capacitor.