Study of Defect Chemistry in the System La_2–xSr_xNiO_4+δ by ¹⁷O Solid-State NMR Spectroscopy and Ni K-Edge XANES

Abstract

The properties of mixed ionic–electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution 17O (I = 5/2) solid-state NMR spectroscopy of La2–x Sr x NiO4+δ, an MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of 17O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (x). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted x = 0 material (Halat et al., J. Am. Chem. Soc. 2016, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) materials. In particular, the intensity of the 17O resonance arising from mobile interstitial defects decreases, and then disappears, at x = 0.5, consistent with reports of lower bulk ionic conductivity in Sr-substituted phases. Second, local distortions among the split axial oxygen sites diminish, even on modest incorporation of Sr (x < 0.1), which is also accompanied by faster spin–lattice (T 1) relaxation of the interstitial 17O resonances, indicating increased mobility of the associated sites. Finally, the hyperfine shift of the equatorial oxygen resonance decreases due to conversion of Ni2+ (d8) to Ni3+ (d7) by charge compensation, a mechanism associated with improved electronic conductivity in the Sr-substituted phases. Valence and coordination changes of the Ni cations are further supported by Ni K-edge X-ray absorption near-edge structure (XANES) measurements, which show a decrease in the Jahn–Teller distortion of the Ni3+ sites and a Ni coordination change consistent with the formation of oxygen vacancies. Ultimately, these insights into local atomic and electronic structure that rely on 17O solid-state NMR spectroscopy should prove relevant for a broad range of aliovalently substituted functional paramagnetic oxides.

Electro-Oxidation Reaction of Methanol over La2–xSrxNiO4+δ Ruddlesden–Popper Oxides

“…The substitution of trivalent La 3+ with bivalent Sr 2+ in the A site brings about charge neutrality and, therefore, Ni 2+ oxidizes to Ni 3+ in the B site. The Ni 3+ sites in an octahedral environment possesses asymmetrical occupancy of electrons in the e g orbital, with a t 2g 6 e g 1 configuration, which could be responsible for tetragonal elongation. , The elongation along the “ z ” axis signifies a lesser overlap of d z 2 and axial O 2p, whereas compression along the equatorial plane indicates a higher overlap of d x 2 –y 2 and O 2p with an increase in Sr 2+ up to x = 0.6. The significant shrinking along the equatorial plane has also reduced the unit-cell volume upon Sr 2+ doping …”

Classical and Emerging Characterization Techniques for Investigation of Ion Transport Mechanisms in Crystalline Fast Ionic Conductors

“…Quadrupolar interaction, on one hand, is very sensitive to local symmetry, as different coordination environments can yield very different quadrupolar coupling constants ( C Q ), related to structure deviation from cubic symmetry, and asymmetry factors (η), related to deviation from axial symmetry. These parameters were investigated in several works on lithium conductors, revealing various distortion existing in polyhedra. − In the oxide-ion conductor Bi 1– x Sr x VO 4–0.5 x , different coordination environments formed with various extents of Sr substitution in the BiVO 4 structure led to distinct resonances in NMR spectra because of quadrupolar interactions (Figure ). NMR spectra of the VO x group in various coordination environments, i.e., the V 2 O 7 group with two neighboring Sr atoms and five or six neighboring Bi atoms, VO 4 tetrahedra neighboring with two Sr atoms and six Bi atoms, with one Sr atom and seven Bi atoms, and surrounded by eight Bi atoms, were calculated.…”

A 17O paramagnetic NMR study of Sm2O3, Eu2O3, and Sm/Eu-substituted CeO2

“…2,3 17 O NMR in such systems is hampered not only by the direct bonding interactions between paramagnetic ion and oxygen sites, but also by both the unfavourable natural abundance (0.037%) and quadrupolar character (I = 5/2) of the 17 O nuclide. 4 Nonetheless, the sensitivity of paramagnetic 17 O NMR spectra to distances from, and the electronic and magnetic properties of, the paramagnetic centre has enabled insights in recent years into materials as diverse as metal-organic frameworks, 5 battery materials, 6 mixed ionic-electronic conductors, 7,8 and phases of geological and radiochemical relevance. 9,10 In these studies, computational results from periodic DFT calculations have also played a critical role, aiding in spectral assignment.…”