Direct observation of d-orbital holes and Cu-Cu bonding in Cu2O

A striking feature of metal oxide chemistry is the unusual electronic and c hemical behaviour of Cu(I) and Ag(I): a case in point is that detailed unde rstanding of Cu-O bonding is essential to the theory of high-temperature co pper oxide superconductors. Both cations are usually coordinated in a linea r fashion to two oxygens, particularly for Cu(I). In many compounds, the Cu (I) and Ag(I) cations also adopt close-packed (and related) configurations with short metal-metal distances that are strongly suggestive of the occurr ence of metal-metal bonding(1,2) despite their formal nd(10) configuration. Such observations have been explained(3,4) by invoking the participation i n bonding of electronic orbitals of higher principal quantum number-that is , (n + 1)s and (n + 1)p-accompanied by the creation of d-orbital holes on t he metal ion. To test this hypothesis, we have used a recently developed me thod of quantitative convergent-beam electron diffraction(5) combined with X-ray diffraction to map the charge-density distribution in the simple oxid e Cu2O, the results of which we then compare with electronic-structure calc ulations. We are able to image directly the d holes on the copper atoms, an d also demonstrate the existence of Cu-Cu bonding in this compound.