Thermal excitation of the Co³⁺triplet spin-state in LaCoO₃determined by polarized neutron diffraction

Abstract

The magnetic moment induced by an external magnetic field of 5.8 T on the Co 3+ ions in LaCoO 3 has been measured by polarized neutron diffraction. Measurements were made at 100 K, near to the spin-state transition, at 1.5 K and at 285 K. The data give evidence for a transition from the ground singlet state (S = 0) to the excited triplet intermediate spin state (S = 1). The energy of 1000 H ≈ ∆ K estimated for the high-spin (S =2) state, is too high to contribute significantly even at T = 285 K. There is evidence for a small negative covalent spin density associated with the oxygen ligands amounting to about one fifth of the aligned Co 3+ moment at all three temperatures.

Local distortion in LaCoO3and PrCoO3: extended x-ray absorption fine structure, x-ray diffraction and x-ray absorption near edge structure studies

“…It may be mentioned that such disagreement in experiment and calculation is quite known in literature and Rehr and Albers [30] have commented on it as one of the challenging problems in the field. The intensity of the main-edge peak D decreases for the orthorhombic structure in conformity with the observed decrease in the intensity of peak D for PrCoO 3 . One can see overall small decrease in intensity in the region of peak G whereas in the region of peak I intensity increases.…”

“…One can see overall small decrease in intensity in the region of peak G whereas in the region of peak I intensity increases. Moreover, orthorhombic distortion also shifts the positions of peaks G and I and dips F and H as observed experimentally for PrCoO 3 . It is also clear from the calculated spectra that the features E and H are more prominent in the orthorhombic structure as seen experimentally.…”

“…The XRD bond lengths obtained from powder data are constrained by the assumed model of the crystal structure-rhombohedral for LaCoO 3 and orthorhombic for PrCoO 3 -whereas EXAFS analysis permitted existence of distortion in CoO 6 octahedron. The validity of EXAFS results is reinforced by its application to the interpretation of XANES in these compounds as discussed in next section.…”

“…These energies are comparable within a few meV and so thermal excitations can readily affect the occupancies of different e g orbitals thereby affecting the spin state of the Co 3+ ion. As temperature increases these compounds show two main magnetic transitions-one at low temperatures around 100 K for LaCoO 3 and 200 K for PrCoO 3 and the other at high temperatures around 500 K for LaCoO 3 and 550 K for PrCoO 3 . Earlier the high temperature transition was generally attributed to a high spin state in these compounds [6] and attributed the low temperature transition to LS to IS transition.…”

“…In the IS state e g orbital possesses one electron making Co studies have shown the existence of two Co-O bond lengths at 100 K [13]. For PrCoO 3 , the x-ray structure is orthorhombic and one expects that the Co-O bond lengths need not be the same. There is no neutron data on this compound to the best of our knowledge.…”