ИСТИНА

Ground state of trivalent cobalt (Co3+) characterizes by three different spin states: low-spin (LS, t2g6eg0, S = 0), high-spin (HS, t2g4eg2 S = 2), and so-called intermediate-spin state (IS, t2g5eg1, S = 1). The particular spin state of cobalt ions depends upon different factors like their coordination number (CN), strength of crystal field (ligand type), temperature and etc. For octahedral coordination (CN = 6) in three-dimensional framework structures the LS ground state was observed for LaCoO3 [1] at low temperatures. For the Sr2CoO3Cl compound [2] CN = 5 that lower than in LaCoO3 and leads to stabilizing only HS state of Co3+ cations. For the octahedrally coordinated Co3+ cations in layered structures (LaSrCoO4 [3]) coexistence of LS and HS states occurs. It should be noted that no reliable data of existence of IS Co3+ in the recent literature are not presented. In the present work we demonstrate the spin state of Co3+ in Sr2Co1.2Ga0.8O5, which is table in wide temperature range. A sample of Sr2Co1.2Ga0.8O5 was prepared by citrate-based route. We used low-temperature neutron powder diffraction (NPD) and soft X-ray absorption spectroscopy to identify Co3+ spin state and high-temperature powder X-ray diffraction (HT PXRD) to investigate temperature behavior of this compound. Rietveld refinement was carried out from room- and low-temperature NPD data using structure model with disordered orientation of tetrahedral chains (space group Icmm). Refined unit-cell parameters are a = 5.6148(6) Å, b = 15.702(2) Å, c = 5.4543(6) Å; Rwp = 3.39%, Rp = 4.43%, χ2 = 7.75%. Refinement of magnetic structure showed the G-type antiferromagnetic arrangements of spins with c-axis oriented magnetic moments (Icm’m’ magnetic space group). Occupancies of Co/Ga positions were refined from room-temperature data and refined composition slightly differs from the nominal and corresponds to formula Sr2Co1.3Ga0.7O5. Structural refinement shows large interatomic distances between and highly distorted CoO6 octahedron with 1.9591(4) Å and 2.257(6) Å for equatorial and axial Co–O bonds, respectively, that can lead to stabilizing of high-spin state of Co3+ cations. Considering the static disorder of atoms in the tetrahedral layer and the presence of non-magnetic Ga3+ cations, during the final refinement the magnetic moment of cobalt on the M2 tetrahedral site was set at 4μB, as expected for HS-Co3+, because at the tetrahedral site only HS-Co3+ can be assumed [4]. The refined magnetic moment of cobalt on the octahedral M1 site is 3.36(7)μB per site. HT PXRD data shows monotonically increasing of unit-cell parameters with thermal expansion coefficients (TEC) 9.2, 21.3 and 11.6 ppm K−1 for individual unit-cell parameters a, b and c, respectively. Same trend was also observed for unit-cell volume and linear TEC was calculated from it (V1/3) and was 13.1 ppm K-1. This value much lower in comparison with other Co-containing perovskite-like oxides (for LaCoO3 TEC ~ 24.6 ppm K-1, it should be noted that high values of TEC for Co-contained oxides are presence due to temperature initiated spin-state transition from LS to HS states) that indirectly confirms that Co cations are represented in the single spin state in the Sr2Co1.2Ga0.8O5. XAS spectra show that for the Sr2Co1.2Ga0.8O5 spectrum, overall spectral features are very similar to those of HS-Sr2CoO3Cl, thus strongly hinting towards the HS-Co3+ state that also confirm presence of only high-spin cobalt in the Sr2Co1.2Ga0.8O5.