For cations with radius ratios close to the critical value of .414, 2 rules are developed to predict a preference for tetrahedral or octahedral coordination: (1) If for MaXbOcthe ratio γ = a b is greater than a certain critical value (generally γc≅ 1.0), X prefers tetrahedral coordination; (2) the greater the MO bond strength, the greater the tendency for octahedral coordination of X. These rules follow from a consideration of Pauling's electrostatic valence rule and reflect the probability of having strong MO bonds to compete with XO bonds. The coordination of Te6+, V5+, As5+, Ge4+, Ti4+, Fe3+, Ga3+, Al3+, B3+, Be2+, and Zn2+in many oxides are consistent with these rules. Exceptions occur when the cations are found in highly stable structures such as perovskite, spinel, quartz, garnet, scheelite, and K2SO4. When applied to high-pressure transitions these rules allow one to predict that small γ values and strong MO bonds will lower the pressure at which an increase in the coordination of X should occur. © 1975.
Shannon, R. D., Chenavas, J., & Joubert, J. C. (1975). Bond strength considerations applied to cation coordination in normal and high-pressure oxides. Journal of Solid State Chemistry, 12(1–2), 16–30. https://doi.org/10.1016/0022-4596(75)90174-7