Spin-state transition, magnetism and local crystal structure in Eu _1-xCa_xCoO_3-δ

The doping series Eu_1-xCa_xCoO_3-δ provides a rather peculiar way to study the spin-state transitions in cobalt-based complex oxides since partial substitution of Eu³⁺ ions by Ca²⁺ ions does not increase the mean valence of cobalt but is accompanied by appearance of oxygen vacancies in the ratio δ ~ x/2. In the parent compound EuCoO₃, the low spin (LS)-intermediate spin (IS) or high spin (HS) transition takes place at temperatures so high that the chemical decomposition prevents its direct observation. The substitution of Eu ³⁺ by Ca²⁺ shifts the spin state transition to lower temperatures and the associated energy gap Δ in octahedrally-coordinated Co³⁺ ions changes from 1940K in EuCoO₃ to 1540K in Eu _0.9Ca_0.1CoO_2.95 and 1050K in Eu _0.8Ca_0.2CoO_2.9. Besides, each O^2- vacancy reduces the local coordination of two neighboring Co³⁺ ions from octahedral to pyramidal thereby locally creating magnetically active sites which couple to dimers. These dimers form another gapped magnetic system with a very different energy scale, D_dim ~ 3 K, on the background of the intrinsically non-magnetic lattice of octahedrally-coordinated low-spin Co ³⁺ ions.