Abstract
Phase transformation of yellow orthorhombic PbO to red tetragonal form was confirmed by grinding at room temperature. The long time grinding in atmosphere converted the PbO powder into white lead, the basic lead carbonate, Pb3(CO3)2(OH)2 finally. The first reaction product in the nitrogen stream was not white lead, but lead oxy-carbonate, PbO. PbCO3. Thus the effect of moisture was prevented well in this condition, though the carbonate formation was not avoided.
The line profiles of the X-ray powder diffration patterns of the ground product are fairly sharp. The average size of the crystallites determined by the line breadths is about 200Å in diameter, and this value agrees well with that of the electron microphotograph.
The density of the white lead thus obtained is rather high compared with the ordinary one. The sharp line profiles must be due to the crystallization by constant grinding. Consequently it may be assumed that a quasiequilibrium state is established between the decomposition and the formation of white lead crystallites.
Pb3(CO3)2(OH)2_??_Decomposition by grindingH2O+CO2X-ray amorphous PbO
When the Pb3O4 is converted into a mixture of white lead and α-PbO2 by grinding in atmosphere the product turns dark brown.
The photoconductivity measurement of the ground samples was carried out at their early stage of grinding in their visible and near infra-red region. Two new peaks appeared at 680mμ and 950mμ. The peak at 680mμ was largely affected by moisture and in some cases this peak was very high and overlapped the second lower 950mμ peak. The 680mμ peak will be attributed to the absorption of water molecules on the active site of the PbO surface, and the 950mμ peak to some lattice imperfection originated by grinding on the surface region of the PbO grains, respectively.
The synthetic reaction of basic lead carbonate by grinding red PbO is explained on the basis of “active states” that appeared in the PbO grains by grinding.
