Composite Spin Chain Structures Built up of Dimeric and Trimeric Polyhedral Units: The Oxides A1+y [(Mn1–x Co x )1–z □ z ]O3 (A = Ca, Sr; x = 3/8)

The investigation of the substitution of calcium for strontium in the composite spin chain structure of the oxide Sr11/8(Mn5/8Co3/8)­O3+δ has been performed combining powder/single crystal X-ray diffraction and 3D electron diffraction. A large family of composite commensurate/incommensurate spin chain structures A1+y [(Mn5/8Co3/8)1–z □ z ]­O3, built up of face sharing MnO6/CoO6 octahedra and a CoO6 trigonal prism and containing cationic vacancies (□) on the prismatic sites, has been synthesized, varying the alkaline earth ion content on the A site according to the nominal composition Sr11–n Ca n Mn5Co3O24. It is shown that the structural nature of the chains and content of cationic vacancies can be directly designed from the Ca/Sr ratio, based on the larger oxidation power of strontium compared to calcium during synthesis in air. In this way, the cationic vacancy content z can be significantly decreased from 1/33 to 0.002 by increasing the Ca content from n = 0 to n = 6.8. Correlatively, the proportion of dimeric units (1 octahedron/1 trigonal prism) with respect to trimeric units (2 octahedra/1 trigonal prism) increases continuously from 0 to 1/3 as the Ca content increases from n = 0 to n = 6.8. It is also shown that the influence of the proportion of these dimeric/trimeric units on the magnetism of these oxides is remarkable, suggesting that, besides the size effect of calcium, it plays a role in the appearance of long-range ordering in this system.