Consequences of lead incorporation in fluorite structured thoria

The outcome of incorporation of lead in fluorite structured thoria in terms of structure and electrical properties have been discussed. Samples with stoichiometries Th1-xPbxO2-delta (x = 0.00 to 0.70) have been synthesized by a solution combustion route and characterized extensively. The maximum solubility limit of lead in fluorite structured thoria was deduced to be 50 mol % from powder X-ray diffraction and Raman spectral measurements. The micro strain of the fluorite lattice increased with the introduction of lead. X-ray photoelectron spectroscopy analysis of Th0.50Pb0.50O2-delta showed the existence of lead in both +2 and + 4 oxidation states. The ratio between the intensities of peaks located at 548 cm(-1) and T-2g mode (I-548/I-T2g) was found to increase with increase in lead content in the samples and quantified utilizing grain size and half width half maxima. The defect concentrations in these samples have been estimated in the range 10(17)-10(20) cm(-3). The direct band gap values shifted from the insulating regime (for thoria) to semiconducting regime (3.4-2 eV) for lead substituted samples. From the frequency dependent conductivity plots of Th1-xPbxO2-delta samples over temperature range (323-773 K), highest conductivity of 1.62 x 10(-6) Scm(-1) was obtained for Th0.50Pb0.50O2-delta at 773 K. From the impedance measurements of lead substituted thoria under flowing oxygen (1 atm), thermal motion of oxygen ion vacancies was found responsible for the observed conductivity.