Rhombic PbO

The solid residues of the combustion of Pb(C2H5)4 in air or in argon-oxygen mixtures in the presence of hydrocarbons are rhombic PbO, PbC03, and small amounts of Pb304 and Pb(N02)2 when the combustion is performed in air [41]. Tetragonal and rhombic lead oxide are products of the oxidation at temperatures between 250 and 560 °C [23], e.g., at410°C [44]. 

Pb-0 bonds. This close similarity of structures presumably allows small impurities to stabilize the yellow rhombic form at ordinary temperatures. The two crystal forms can be obtained simultaneously by heating lead carbonate at 400° in a melt of potassium and sodium nitrates, when over a period of one day red prisms of tetragonal PbO and yellow plates of rhombic PbO are formed along with orange needles of Pb304. 

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