Seeing the Unseen: The Structural Influence of the Lone Pair Electrons in PbWO 4 .

Pair distribution function (PDF) analysis of the scheelite-type material PbWO 4 reveals previously unidentified short-range structural distortions in the PbO 8 polyhedra and WO 4 tetrahedra not observed in the similarly structured CaWO 4 . These local distortions are a result of the structural influence of the Pb 2+ 6s 2 lone pair electrons. These are not evident from the Rietveld analysis of synchrotron X-ray or neutron powder diffraction data, nor do they strongly influence the X-ray PDF (XPDF). This illustrates the importance of neutron PDF (NPDF) in the study of such materials. First-principles density function theory (DFT) calculations show that the Pb 2+ 6s 2 electrons are hybridized with the O 2- 2p electrons near the Fermi level. The presence of local-scale distortions has previously been neglected in studies of structure-functionality relationships in PbWO 4 and other scheelite-structured photocatalytic materials, including BiVO 4 , and this observation opens new avenues for their optimization.