A Dimensionless Relationship Between Bulk Modulus, Molar Volume And Cohesive Energy for Some Ionic Diatomic Solids

Rabia Ahmad² and M. Z. Rahman Khan¹

¹Department of Chemistry, Jamia Milia Islamia, New Delhi - 110 025 (India) ²Department of Physics, Aligarh Muslim University, Aligarh - 202 002 (India)

KoVo/Eo is an important dimensionless quantity, formed from the bulk modulus Ko the molar volume Vo and the cohesive energy Eo. It may be called dimensionless elastic energy. Its significance in high pressure physics is well known. The two classes of compounds being considered here are, on the one hand, the alkali halides and the halides of Cu, Ag and Tl for which data are available, and on the other, the chalcogenides. Average KoVo/Eo is distinctly different for the two classes of compounds. Thus KoVo/Eo turns out to be a parameter that distinguishes between these two classes of chemical compounds. The quantity KoVo/Eo is also calculated using a relationship between KoVo/Eo and the pressure derivative of the bulk modulus Ko, obtained using the Born-Mayer formula. Nearly the same average value is reproduced by the theoretical relationship for the two classes of compounds. The agreement is a pointer to the usefulness of the Born-Mayer formula, at least, for semi-quantitative purposes. The quantity KoVo/Eo may be used to provide an alternative form of the Born-Mayer equation.

Cohesive energy; molar volume; bulk modulus; dimensionless elastic energy; diatomic solids