Synthesis and characterization of nanostructured lead molybdate and calcium tungstate

Dr. M. Abdul Khadar, former Director & Professor, Centre for Nanoscience & Nanotehnology,Department of Physics, University of Kerala, Kariavattom campus, Thiruvananthapuram, Kerala, India

Materials belonging to the tungstate and molybdate family have a long tradition of practical applications and have been the subject of intense research over the past century. In the present work nanoparticles of lead molybdate (PbMoO4) and calcium tungstate (CaWO4) having different grain sizes were prepared using chemical routes and their physical properties were analysed using different experimental techniques. Lead molybdate (PbMoO4) is a technologically important material, extensively used in acousto-optic devices such as acousto-optic light deflectors and modulators. PbMoO4 has high acousto-optic figure of merit, low optical loss in the region 420 to 3900 nm and good mechanical impedance for acoustic matching. Lead molybdate crystals have been used as scintillators for the neutrinoless double β decay experiment. Calcium tungstate finds application as a phosphor, in scintillation counter and laser host material owing to its superior luminescence properties. Lead molybdate and calcium tungstate crystallize in tetragonal system with scheelite structure and belong to the point group symmetry C4h and space group symmetry I41/a.

Chapter 1 of the thesis gives a general introduction to the field of nanostructured materials. A broad review of different synthesis methods, characterization techniques and applications of nanocrystalline materials is illustrated.

The details of preparation of nanostructured PbMoO4 and CaWO4 of different grain sizes using chemical techniques are described in chapter 2. It also explains the analysis of the crystal structure and determination of average grain sizes using XRD, TEM and high resolution TEM. The determination of the crystallite sizes of the different samples using Hall-Williamson analysis of XRD data is also given in this chapter.

The optical properties of nanostructured PbMoO4 and CaWO4 are explained in chapter 3. The size dependent evolution of UV-Visible absorption spectral features of nanocrystalline PbMoO4 and the Urbach behaviour of the absorption edge are analysed in detail. The analysis and interpretation of features in the absorption and photoluminescence spectra of nanocrystalline CaWO4 and the grain size dependent variations of these are explained in this chapter.

Chapter 4 presents the vibrational properties of nanostructured PbMoO4 and CaWO4. FTIR spectra of nanocrystalline PbMoO4 and CaWO4 in the far IR and mid-IR regions are analysed. Also, the analysis and interpretation of FT Raman and micro-Raman spectra of nanocrystalline samples of PbMoO4 and CaWO4 are given in this chapter.

Studies of load and grain size dependence of microhardness of pelletized samples of nanostructured PbMoO4 and CaWO4 is presented in chapter 5. Variation of microhardness with load, grain size, compaction pressure and time of annealing are studied in detail. The load dependence of microhardness is analysed based on theoretical models. Enhancement of microhardness with reduction in grain size is analysed in the light of Hall-Petch relation.

Studies of load and grain size dependence of microhardness of pelletized samples of nanostructured PbMoO4 and CaWO4 is presented in chapter 5. Variation of microhardness with load, grain size, compaction pressure and time of annealing are studied in detail. The load dependence of microhardness is analysed based on theoretical models. Enhancement of microhardness with reduction in grain size is analysed in the light of Hall-Petch relation.

The variation dc conductivity with temperature of compacted nanostructured samples of PbMoO4 and CaWO4 are presented in chapter 6. The variations of dc conductivity with grain size, chamber pressure and time of annealing of the pellets are studied.

Chapter 7 is a summary of the conclusions extracted from the experimental analyses conducted on the samples of nanostructured PbMoO4 and CaWO4. It also illustrates the future scope and need for research on these two technologically important materials in the nanosized regime.