Hardness correlation with cohesive energies

Other authors have studied other correlations. Two are Povarennykh (1964), and Goble and Scott (1985). The latter emphasized compressibility (inverse bulk modulus) as did Beckmann (1971). The bulk modulus is not a reliable measure for the same reason as the cohesive energy. It is volume dependent rather than shear dependent. Still another attempt to correlate hardness and compressibility was that of Yang et al. (1987). This was followed by a proposal by Liu and Cohen (1990) that hardness and bulk moduli are related. This proposal was refuted by Teter (1998) who showed that hardness values correlate better with shear moduli than with bulk moduli. 

Our immediate concern is whether Eg, or possibly E, serves as a suitable measure of chemical hardness just as (/ — A) does for molecules. Examination of the data in Table 5.5 shows that it does. There is a good correlation btween Eg and the cohesive energy, as long as related solids are compared. That is, the 4-4 compounds show Eg falling just as AE coh does. The alkali halides also are correlated with each other, but not with the 4-4 cases. In the 2-6 examples, we can compare the CN6 compounds with each other, but not with the CN4 cases, which have their own relationship. The 3-5 solids form their own family for CN4, but there are no Eg data for the ionic 3-5 cases, which probably belong to a different family. 

---