HIGH-TEMPERATURE FORM OF PB2WO5 AND TRANSFORMATION PHENOMENA TO ITS LOW FORM

The phase transformation of Pb2WO5 was studied by means of high temperature x-ray diffractometry, differential scanning calorimetry ( DSC ) and visual observation of the crystal growth in the range between room temperature and to its melting point. Two forms were found to be stable in the range studied. The Pb2WO5, which had been reported by DeVries and Fleischer was confirmed to be low temperature form. The newly synthesized high temperature form is isomorphous with Pb2MoO5 and its cell parameters are; a=14.206 (2) A ̊, b=5.799 (1) A ̊, c=7.346 (1) A ̊ and β=113°54 (0.9)′. The transition temperature was determined by DSC measurements as 330±10°C. The transition from high to low form is remarkably affected by the crystal size of its high form. Large crystals easily transform to its low temperature form. However, small crystalline sample (≲100 μm), the rate of transformation is quite low and the high form is easily retained to the room temperature. One of the previously reported x-ray diffraction pattern of Pb2WO5 was revealed to be the mixture of high and low form of this compound. © 1979.