lattice energy trends

[whizkid]

taking the anion constant ,and taking the cation from s block, the lattice energy should increase on ascending the group . according to fajan's rules covalent nature of componds takes wil also increase on ascending the group which should result in lower lattice energy. from whre does this contradiction arise?

[Clanad]

It is found that the relative intensity of green emission band to blue emission band decreases with increasing lattice constant of the host lattice. This can be qualitatively accounted for as being caused by a nonradiative energy transfer from the emission band of Ce3 + to the absorption band due to defects associated with anion vacancies in the host lattice. (Source: American Institute of Physics)