EPR study of Ni distribution in LaNi_y Co_1–yO₃ solid solutions (0 ≤ y ≤ 0.25)

Abstract

1 Introduction At room temperature, LaCoO 3 possess a rhombohedrally distorted perovskite-type structure, which undergoes a transformation into a cubic perovskite at high temperature [1,2]. LaCoO 3 has been considered as an excellent example of the thermally induced spin transition of Co 3+ . It has been shown that, above 35 K, there is a transition from low-spin (S = 0) to high-spin Co 3+ (S = 2); above 650 K a transition to Co 3+ ions with intermediate spin (S = 1) is also established [3,4].When Ni substitutes for Co, the rhombohedrally distorted perovskite structure is preserved but the magnetic properties show a crossover from antiferromagnetic to ferromagnetic interactions together with promotion of the high-spin state of the cobalt ions [5,6]. The magnetic ground state has been interpreted as ferromagnetic clusters embedded in a nonferromagnetic matrix [7,8]. While the nonferromagnetic matrix is determined by superexchange antiferromagnetic interactions between isovalent ions (Co 3+ -Co 3+ and Ni 3+ -Ni 3+ , respectively), the origin of the ferromagnetic interactions remains unclear. There are two models: (i) superexchange interaction between Ni and Co ions in the same valence state, or (ii) electron transfer between Ni 3+ and Co 3+ leading to the appearance of Ni 2+ and Co 4+ that are coupled by double-exchange interactions with Co 3+ [5,6]. This picture bears a resemblance to the magnetic properties of Sr-doped LaCoO 3 , where Co 4+ ions

Effect of the synthesis route on the microstructure and the reducibility of LaCoO3

“…At 900 • C, all studied LaCoO 3 samples studied exhibited the identical lattice parameters, which coincide with those for LaCoO 3 reported in the literature [47,48]. The organic components of the precursors affect the oxi- [34,38,39,49]. Thus, for LaCoO 3 prepared from freeze-dried citrates with a La:Co:CA ratio of 1:1:2, defects including ferromagnetic coupled Co 3+ and Co 4+ ions were detected by EPR [34].…”

“…Fig. 5 gives the EPR spectra of LaCoO 3 oxides annealed at 700 • C. It is worth mentioning that defectless LaCoO 3 does not give the EPR response in the X-band experiments (9.2 GHz) [34,38,39,49]. This picture is observed for P-LaCoO 3 .…”