The doping series Eu1-xCaxCoO3-d provides a rather peculiar way to study the spin-state transition in cobalt-based complex oxides since partial substitution of Eu3+ ions by Ca2+ ions does not increase the mean valence state of cobalt but is accompanied by appearance of oxygen vacancies in the ratio d \sim x/2. In the parent compound EuCoO3, the low spin (LS)-high spin (HS) transition takes place at temperatures so high that the chemical decomposition prevents its direct observation. The substitution of Eu3+ for Ca2+ in this system shifts the LS-HS transition to lower temperatures. The energy gap associated with this transition in octahedrally-coordinated Co3+ ions changes from 1940 K in EuCoO3 to 1540 K in Eu0.9Ca0.1CoO2.95 and 1050 K in Eu0.8Ca0.2CoO2.9. Besides, each O2- vacancy reduces the local coordination of two neighboring Co3+ ions from octahedral to pyramidal thereby locally creating magnetically active sites which couple into dimers. These dimers at low temperatures form another gapped magnetic system with very different energy scale, D~3 K, on the background of intrinsically non-magnetic lattice of octahedrally-coordinated low-spin Co3+ ions.
@article{arxiv.1210.1289,
title = {Spin-state transition, magnetism and local crystal structure in Eu_{1-x}Ca_xCoO_{3-d}},
author = {A. N. Vasiliev and T. M. Vasilchikova and O. S. Volkova and A. A. Kamenev and A. R. Kaul and T. G. Kuzmova and D. M. Tsymbarenko and K. A. Lomachenko and A. V. Soldatov and S. V. Streltsov and J. -Y. Lin and C. N. Kao and J. M. Chen and M. Abdel-Hafiez and A. U. B. Wolter and R. Klingeler},
journal= {arXiv preprint arXiv:1210.1289},
year = {2013}
}