1. Ionic crystal is mainly formed by ionicity union.

2. The ionicity of O 2- increases with Mg and Al doping.

3. The classical bond ionicity is corrected from the consideration of both the electronegativity difference and bond distance.

4. A method to calculate new valence electronegativity and the ionicity of chemical bond of the transition metal elements is also presented here.

5. These mechanisms are the internal displacement of the ionic charge, the internal displacement of the electronic charge and the charge in ionicity due to strain.