To do this, we created a condition under which it was favorable for the carbon and nitrogen atoms to sit in the proper position. Haller credits the theoretical investigations of Cohen and his then graduate student, Amy Liu, with pointing the way for the efforts of his experimental team. The work of Cohen and Liu showed that if you put carbon and nitrogen atoms into a silicon nitride configuration, you get a stable structure. It was to learn whether materials harder than diamond were even possible that Cohen and Liu began their investigations. To find a good candidate, they first screened a number of hypothetical crystals using a relatively simple empirical model devised by Cohen.
Bulk modulus is the inverse of compressibility.
However, for an ideal crystal, hardness is generally proportional to bulk modulus, which, in turn, is dependent upon the properties of the chemical bonds between atoms in the material. Low compressibilities are achieved with short bonds and low ionicity. Cohen and Amy Liu determined that a carbon-nitrogen compound, despite being partially ionic, would have a bond length short enough to give the material a higher bulk modulus than diamond. A megabar is one million times atmospheric pressure at sea level. In testing this theoretical prediction, Haller and Hansen deposited thin films of carbon and nitrogen on germanium as well as silicon wafers.