Second-neighbours contribution to the repulsive energy of crystalline alkaline-earth oxides

The Born-Mayer model in the alkaline-earth oxides is reconsidered, using new compressibility data. All four oxides cannot share a unique hardness parameter ?. An exact solution is found by assuming that ?(MgO) = ?(CaO) and ?(SrO) = ?(BaO); another solution is proposed, which involves three distinct ? values in the family of salts. Results are tested by calculating the electron affinity of O^2- from each salt.