Unusual visible absorption in high PbO lead borate glass

A series of glasses in the system xPbO center dot(100 - x)B2O3 (x = 45, 55, 65 mol %) were prepared using analytical grade PbO consisting of a mixture of orthorhombic and tetragonal phases (ca. 1:1, w/w) and H3BO3 melted in silica beaker in the temperature range of 575-900 degrees C for 30 min under normal atmosphere. Only the 55PbO center dot 45B(2)O(3) glass prepared at 575 (U), 650 (W) and 700 degrees C (L4) showed several optical absorption bands in the visible region. However, all the bands disappeared either when a 1.7 mol % equivalent PbO from Pb3O4 was added in the batch or the melting temperature was increased to more than 800 degrees C. L2 glass showed a broad absorption band at ca. 485 nm which was thought to be due to combined effect of nanometallic lead (Pb-0) and Pb+. The formation of hexagonal nano-Pb-0 particles of 7-13 nm size in 55PbO center dot 45B(2)O(3) glass melted at 575-700 degrees C was confirmed by TEM and Pb+ was identified by a broad EPR. signal with 9-value at ca. 2.00. Optical absorption at ca. 485 nm and 565 nm (broad shoulder) was observed in L3 glass whereas a new peak at ca. 525 nm appeared in L4 which was identified due to Pb3+ by EPR signal with g-value ca. 2.176 and A-tensor, ca. 6340x10(-4) cm(-1). A mechanism for the formation of Pb-0, Pb+ and Pb3+ has been proposed