Relation between the Co-O bond lengths and the spin state of Co in layered Cobaltates: a high-pressure study

Abstract

The pressure-response of the Co-O bond lengths and the spin state of Co ions in a hybrid 3d-5d solid-state oxide Sr2Co0.5Ir0.5O4 with a layered K2NiF4-type structure was studied by using hard X-ray absorption and emission spectroscopies. The Co-K and the Ir-L 3 X-ray absorption spectra demonstrate that the Ir5+ and the Co3+ valence states at ambient conditions are not affected by pressure. The Co Kβ emission spectra, on the other hand, revealed a gradual spin state transition of Co3+ ions from a high-spin (S = 2) state at ambient pressure to a complete low-spin state (S = 0) at 40 GPa without crossing the intermediate spin state (S = 1). This can be well understood from our calculated phase diagram in which we consider the energies of the low spin, intermediate spin and high spin states of Co3+ ions as a function of the anisotropic distortion of the octahedral local coordination in the layered oxide. We infer that a short in-plane Co-O bond length (<1.90 Å) as well as a very large ratio of Co-Oapex/Co-Oin-plane is needed to stabilize the IS Co3+, a situation which is rarely met in reality.

Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3

“…At 0.5 GPa, the spectrum represents a main peak located at ∼7650 eV corresponding to the K β 1,3 line, and a pronounced satellite peak at ∼7637 eV corresponding to the K β′ line. The intensity ratio of the low-energy K β′ line to the main emission K β 1,3 line is proportional to the number of the unpaired electrons in the incomplete 3d shell , and can be used to determine the spin state of Co ion as a function of temperature , and/or pressure. ,,− With increasing pressure, the intensity of the low-energy K β′ line decreases and almost disappears above ∼15 GPa (Figure ), suggesting a decreasing spin moment of Co 2+ ion. The inset of Figure contains a plot of the integrated absolute difference (IAD) as a function of pressure. ,,− To get the IAD, all of the XES spectra were normalized with respect to the area under the curves and then shifted to have centers of mass at the same position.…”

Experimental and Theoretical Soft X‐Ray Absorption Study on Co3+ Ion Spin States in Sr2−xCaxCoO3F

“…Therefore, spin states of Co ions and consequently the physical properties of cobaltates can be altered by external stimuli, such as temperature and pressure, or by internal pressure through chemical substitution. For example, pressure‐induced spin‐state transitions have been observed in BiCoO 3 , LaCoO 3 , SrCo 0.5 Ru 0.5 O 3‐δ , Pr 0.5 Ca 0.5 CoO 3 , (Pr 0.7 Sm 0.3 ) 0.7 Ca 0.3 CoO 3 , and Sr 2 Co 0.5 Ir 0.5 O 4 , while temperature‐induced spin‐state transitions have been found in LaCoO 3 and (Pr 0.7 Sm 0.3 ) 0.7 Ca 0.3 CoO 3 , accompanying insulator‐to‐metal and nonmagnetic‐to‐magnetic transitions . Furthermore, spin‐state crossover induced by chemical pressure from an equal valence substitute has been observed in RCoO 3 (R = rare earth) and Sr 3− x Ca x YCo 4 O 10.5 …”

Quantum criticality in a layered iridate

“…Another candidate is the Sr 2 Ir 1−x Co x O 4 system. Based on first principle calculations for Sr 2 Ir 0.5 -Co 0.5 O 4 , it was inferred that Co 3+ /Ir 5+ charge partitioning is energetically stable 42 , which was confirmed by subsequent experiments on this material [43][44][45] . On the other hand, more dilute Co substitutions have not been investigated in much detail yet, although it is already known that even moderate Co substitution levels (x ≤ 0.1) are not sufficient to reduce T N appreciably 46 .…”