“…The optical absorption spectra (Figure a) of AM 2 Ni 6 Te 3 O 18 (A = Pb, Sr; M = Cd, Mn) exhibit broad triplet absorption bands that range from approximately 400 to 1200 nm, which leaves a window of almost no absorption in the green region of the visible spectrum. The octahedral Ni 2+ ions generally exhibit three spin-allowed transitions (Figure S8): (i) 3 A 2g → 3 T 2g , IR region; (ii) 3 A 2g → 3 T 1g (F), visible; and (iii) 3 A 2g → 3 T 1g (P), visible. , The splitting over ν 1 , ν 2 , and ν 3 can be assigned to the lowering of the symmetry. , On the other hand, Co-compounds display two absorption maxima centered around 1250 and 550 nm (Figure b). Octahedral Co 2+ normally exhibits three spin-allowed transitions (Figure S8): (i) 4 T 1g → 4 T 2g (F) (∼1250 nm), (ii) 4 T 1g → 4 A 2g (F) (∼700 nm), and (iii) 4 T 1g → 4 T 1g (P) (∼580). ,, The shoulders of the visible band can be attributed for transitions involving the split of higher energy terms ( 2 G, 2 H, 2 D, and 2 P). , Bands below 400 nm could be due to a ligand–metal charge transfer transition.…”