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

Abstract

The polytypism and physical properties of BaTi_1−yCo_yO_3−δ for y = 0.8 and 0.9 prepared with different δ values are reported. Samples quenched from 1293 K are strongly reduced, 3C-type perovskites (space group Pmm) with δ ∼ 0.4 and an average oxidation state of ∼Co³⁺ whereas slow cooled (4 °C min⁻¹) samples are 5H-type perovskites (space group Pm1 and stacking sequence (ccchh)) with δ = 0.2 and an average oxidation state of ∼Co^3.5+. An unusual feature of the 5H polymorph is the presence of ∼15 to 20% Ti(IV) on the tetrahedral (M3) sites. The 3C-polytypes are antiferromagnetic semiconductors with Néel 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.