Structure-property relations in anion deficient 5H-and 3C-polytype Ba(Ti,Co)O3-δ perovskites

The polytypism and physical properties of BaTi(1-y)CoyO(3-delta) for y = 0.8 and 0.9 prepared with different delta values are reported. Samples quenched from 1293 K are strongly reduced, 3C-type perovskites (space group Pm (3) over barm) with delta similar to 0.4 and an average oxidation state of similar to Co3+ whereas slow cooled (4 degrees C min(-1)) samples are 5H-type perovskites (space group P (3) over bar m1 and stacking sequence (ccchh)) with delta = 0.2 and an average oxidation state of similar to Co3.5+. An unusual feature of the 5H polymorph is the presence of similar to 15 to 20% Ti(IV) on the tetrahedral (M3) sites. The 3C-polytypes are antiferromagnetic semiconductors with Neel temperatures <40 K and the 5H-polytypes are ferromagnetic semiconductors with Curie temperatures >285 K. The reasons for the changes in polymorphism and physical properties with cation and anion content are rationalised on the basis of the different chemical nature and concentration of the Co(III), Co(IV) and Ti(IV) ions on the B-sites of these perovskites. The results are discussed in context to behaviour observed in related mixed-valent B-site perovskites to obtain a better understanding of the structure-composition-property relationships in these materials.