Correlation between standard enthalpy of formation, structural parameters and ionicity for alkali halides

The standard enthalpy of formation (ΔHo) has been considered to be an interesting and useful parameter for the correlation of various properties of alkali halides. The interrelation between ΔHo and structural parameters for the halides of Li, Na, K and Rb has been thoroughly analyzed. When cationic component element is kept constant in a homologous series of alkali halides, the negative value of ΔHo has been observed to decrease linearly with increase of interionic distance (d) and accordingly following empirical equation ΔHo = α + βd (where α and β are empirical constants) has been established. However, for common anionic series of alkali halides an opposite nonlinear trend has been observed with the exception of common fluorides. The correlation study on the standard enthalpy of formation has been extended in term of radius ratio and also discussed in the light of ionization energy of the metal, electron affinity of the halogen, size of the ions, ionic character of bond and lattice energy of the compound.