Origin of the Lone Pair of α-PbO from Density Functional Theory Calculations

We present ab initio calculations using density functional theory (DFT) within the local density approximation (LDA) on PbO in the α-phase and the idealized CsCl structure. The electron densities clearly show that CsCl-structured PbO has a symmetric lead site while α-PbO shows asymmetry, forming what could be described as a sterically active lone pair. However, analysis of the partial density of states indicates that mixing of the Pb 6s with the oxygen 2p gives rise to filled bonding and antibonding states, and therefore, the 6s 2 electrons are not available to form a lone pair. The stabilization of the α-PbO structure over the CsCl structure is shown to be the result of mixing of Pb 6p states into the filled antibonding O 2p and Pb 6s states, contrary to the classical view of hybridization of the lead 6s and 6p to form a lone pair.