Synthesis of stoichiometric lead molybdate PbMoO 4 : An x-ray diffraction, Fourier transform infrared spectroscopy, and
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Synthesis of stoichiometric lead molybdate PbMoO4 : An x-ray diffraction, Fourier transform infrared spectroscopy, and differential thermal analysis study H. C. Zeng Department of Chemical Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511 (Received 13 April 1995; accepted 10 October 1995)
The PbOyMoO3 system with 47% : 53%, 53% : 47%, and 50% : 50% molar ratios at various processing temperatures has been studied with x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and differential thermal analysis (DTA) methods. It is found that in addition to the crystallization of primary PbMoO4 phase, subphases such as Pb2 MoO5 and PbMo2 O7 are also formed. The remaining PbO and MoO3 are detected at certain stages of the thermal process due to localized powder inhomogeneity. Physical processes, such as sublimation, eutectic melting, solid to liquid, and liquid to vapor transformations are also investigated. In particular, evaporations of excessive PbO or MoO3 in the nonstoichiometric PbOyMoO3 can be correlated to thermal processing parameters. The current study has led to the following three processing guidelines to obtain stoichiometric PbMoO4 : (i) for high temperature application, such as the Czochralski melt growth, it is suggested an excessive MoO3 (a few mol %) must be included and a slow heating rate should be employed; (ii) for low temperature synthesis, the stoichiometric PbO –MoO3 can be used, but with a fast heating rate; and (iii) PbO-rich PbOyMoO3 system is not recommended in PbMoO4 synthesis.
I. INTRODUCTION
Lead molybdate (PbMoO4 ) is a congruently melting compound whose materials have received increasing attention due to their important applications as acoustooptic modulators, deflectors, and ionic conductors.1–3 In recent years, the single crystal of PbMoO4 has demonstrated a great potential to be used as an efficient low-temperature scintillator for nuclear instrumental application.4 In relation to the importance of technological application, quality evaluation of PbMoO4 singlecrystal material has also been performed extensively over the last twenty years. In particular, crystal imperfections and inhomogeneity have been studied by petrography and electroprobe microanalysis,5 chemical etching method,6–8 thermal etching and annealing technique,1,8 Twyman–Green interference method,1 Schlieren method, and x-ray topography.9 The crystal imperfections and inhomogeneity so far are identified mainly as coloration, foreign inclusion, such as crucible material Pt,1 crystalline particles due to the second phase formation,5 minute scattering center arising from structural defects,1 and crystal dislocations.10,11 To improve optical performance of the crystal, it is important to examine key process parameters in the field of crystal growth. In this connection, thermal history of high temperature melt, growth ambient effects, and dendritic growth of PbMoO4 have been studied and reported J. Mater
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