Using a combination of convergent beam electron diffraction and X-ray diffraction techniques, a team of materials researchers at Arizona State University have achieved startlingly clear images of electron orbitals responsible for bonding in Cu2O, also known as cuprite, a ceramic semiconductor with a rare structure.
The images map the charge density of non-ionic bonds in Cu2O and thus provide the first experimental verification of the controversial hypothesis that both ionic and covalent bonding occurs in the material. In addition, the images show that the covalent bonding exists not just between oxygen and copper atoms, but also between pairs of copper atoms.
The research findings, authored by J.M. Zuo, M. Kim, and J.C.H. Spence of the ASU Department of Physics and Astronomy, and M. O'Keeffe of the ASU Department of Chemistry, appear in the September 2 issue of the journal Nature and one of the associated images appears on the issue's cover.
The first accurate experimental images (not computer simulations) ever achieved showing the electron formations responsible for atomic bonding in Cu2O, the charge-density maps show electron clouds in a distinct dumbbell shape, with a torus and two three-petaled rings surrounding the middle. This complex formation (resembling an elaborate baby teether) is predicted by theory for a "s-dz2 orbital hybridization," which leaves a "hole" on the copper ions. The maps also show fainter, less defined distributions of electrons between the copper atoms in the crystal matrix, indicating a metal-to-metal bond.
According to the theory of quantum mechanics, the extremely fast moving electrons (described as both particles and waves) that surround atomic nuclei cannot actually be seen in a specific location, but instead can only be known by the areas in which they are likely to occur - orbital "clouds." An atom's electrons can form bonds - the glue that ties atoms together - including covalent bonds, where electrons are closely shared between two atoms, and ionic bonds, where atoms literally lose some of their electrons to other atoms. A radical extension to this simple picture of chemical bonding theory was proposed some years ago, and it has finally been shown to be correct.
"Much of current theory predicts ionic but not covalent or metal-to-metal bonding in oxides like cuprite," said Zuo, the paper's lead author. "Understanding bonding in copper oxides is the key to solving the biggest unsolved problem in solid state theory - the nature of high temperature superconductivity in copper oxides. Here we see direct experimental pictures of bonding that explain the structure of cuprite."
Metal oxides like copper oxides are known to materials scientists as "complex materials" because they have many interesting electrical and magnetic properties and have a wide variety of technological applications, including uses as magnetic media for computer disks, for a new type of dynamic memory for computers, and for making miniature electric motors and magnetic sensors for mine-detection.
Though physicists have long argued that the interesting properties of metal oxides indicate the presence of bonding that was not ionic in the materials, finding direct evidence of metal-to-metal bonding in cuprite was still somewhat of a scientific surprise.
"In particular, the evidence of covalent bonding between metals is likely to make them re-write the chemistry textbooks," said Spence. "Chemistry has always assumed that these are only possible between copper and oxygen in this material. These chemical bonds have not been seen before, because they differ so slightly from the charge distributions in un-bonded atoms."
Both X-ray and electron diffraction were used in the mapping. Electrons were used for small scattering angles to avoid the "extinction effect" that otherwise distorts X-ray measurements, and X-rays at high angles where they are more accurate - a combination that gave the team sufficient accuracy for the fine details of the images. The clear definition of the covalent Cu-O bonds was obtained by using a technique that first moved all ions (Cu+ and O--) to the background of the map and then subtracted the background from the image.
Though the team's maps are computer generated, they are not simulations, but actual images produced directly from the electron diffraction results, much as traditional photographs are the direct result of focused beams of light of known wavelengths leaving a record on film.
"This is really exciting," said Zuo. "It's the first time that we've ever seen an orbital at this level of accuracy. It's direct, experimental proof of the quantum model."
The team's research was performed at ASU's high resolution electron microscopy center and was funded by grants from the National Science Foundation.
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