Composite Spin Chain Structures Built up of Dimeric and Trimeric Polyhedral Units: The Oxides A_1+y[(Mn_1–xCo_x)_1–z□_z]O₃ (A = Ca, Sr; x = 3/8)

Abstract

The investigation of the substitution of calcium for strontium in the composite spin chain structure of the oxide Sr 11/8 (Mn 5/8 Co 3/8 )O 3+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 A 1+y [(Mn 5/8 Co 3/8 ) 1−z □ z ]O 3 , built up of face sharing MnO 6 /CoO 6 octahedra and a CoO 6 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 Sr 11−n Ca n Mn 5 Co 3 O 24 . 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.