Quantitative convergent-beam electron diffraction and quantum crystallography—the metallic bond in aluminium

The technique of quantitative convergent-beam electron diffraction (QCBED) is reviewed as a method for making very accurate and precise measurements of low-order bonding-sensitive structure factors. As such, it is a technique with the potential to make significant contributions to the field of quantum crystallography. To demonstrate this, the application of QCBED in determining the nature of the metallic bond in aluminium is examined in detail. The importance of precision in structure factor measurement when it comes to determining bonding electron distributions, especially in metals, becomes obvious from this examination. Uncertainties as low as ±0.1% are routinely attainable by QCBED and are shown to be important in reliably locating bonding charge. In the case of aluminium, a nearly free electron gas metal where the bonding electron distribution is subtler than in most other materials, the bonds are entirely tetrahedrally centred. This is a conclusion that could not have been reached experimentally without the levels of precision offered by QCBED.