Features of Solidification of the 2Bi 2 O 3 ·3GeO 2 Melt under Different Cooling Conditions

The influence of heat treatment of the melt (composition 40 mol % Bi 2 O 3 and 60 mol % GeO 2 ), temperature of the beginning of its cooling and cooling rate on the phase composition, macro- and microstructure of the solid phases formed during crystallization were studied. The role of platinum in the formation of a metastable state of the melt and, as a consequence, the formation of metastable phases during crystallization is also considered. Using X-ray phase analysis, optical and scanning electron microscopy, it was found that the main factors determining the formation of the phase composition of crystallization products on the composition of 40 : 60 mol % (Bi 2 O 3 : GeO 2 ) are the preliminary heat treatment of the melt, which provides the most complete implementation of the metastable equilibrium in the melt (thermodynamic factor), and its interaction with solid platinum oxides on the walls and bottom of the crucible (kinetic factor). It is shown that at slow cooling rates (with a furnace) the melt crystallizes with the formation of a mixture of phases consisting of metastable Bi 2 GeO 5 and eutectoid (Bi 2 GeO 5 + α-GeO 2 ). In this case, the decomposition of this metastable mixture is possible with a partial or complete transition to a stable state with the formation of Bi 4 Ge 3 O 12 . It was found that high-speed cooling of the melt is accompanied by the formation of either glass (quenching with a crucible in water) or a mixture of metastable Bi 2 GeO 5 and an amorphous component.