3. Transition ion doping

These transition ions are: Ti ³⁺ (3d ¹ ), Cr ⁴⁺ (3d ² ), Cr ³⁺ and V ²⁺ (3d ³ ), Cr ²⁺ (3d ⁴ ), Co ²⁺ (3d ⁷ ) and Ni ²⁺ (3d ⁸ ) (table 1 ).