http://www.tandfonline.com/doi/abs/10.1080/01418638108222162
Calculations of off-centre displacements of divalent substitutional ions in
CaO, SrO and BaO from model potentials
Philosophical Magazine Part B
Volume 43, Issue 4, 1981
DOI:10.1080/01418638108222162
M. J. L. Sangsterab & A. M. Stonehama
pages 597-608
Many thanks!