Probing Oxide-Ion Mobility in the Mixed Ionic-Electronic Conductor La2NiO4+δ by Solid-State (17)O MAS NMR Spectroscopy

Abstract

While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic-electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution (17)O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic (17)O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.

Figure 1

Room-temperature ¹⁷O MAS NMR spectrum of La₂NiO_4+δ with proposed assignments. (a) Crystal structure of the high-temperature tetragonal (space group I4/mmm) phase of La₂NiO_4.17 as reported by Skinner et al. Partially occupied sites (O_ax, O_i) are depicted as partially filled spheres. (b) Individual subspectra collected at different offset frequencies (colored) summed to give the broadband spin–echo mapping spectrum (black). Proposed assignments depict the local geometry about each oxygen environment (equatorial O_eq, axial O_ax, and interstitial O_i). A rotor-synchronized Hahn echo pulse sequence (π/6−τ–π/3−τ–acquire) was used for each subspectrum. Spectra were collected at 7.05 T at a MAS rate of 12.5 kHz, with 120 000 scans per subspectrum and a recycle delay of 0.5 s. Asterisks denote spinning sidebands. (c) Inset showing the “diamagnetic region” of the summed spin–echo mapping spectrum in (b). Features at 532 and 170 ppm are assigned to interstitial oxygen (O_i) in La₂NiO_4+δ, and a LaAlO₃ impurity phase, respectively. Asterisks denote spinning sidebands.

Figure 2

Local structural distortion induced by nearby interstitial defect (O_i), from part of the DFT-optimized La₁₆Ni₈O₃₃ supercell. Axial sites (in orange) closest to the interstitial undergo the largest displacement toward the Ni center, with concomitant tilting of the NiO₆ octahedra. The four types of axial oxygen sites, ordered by increasing Ni–O_ax bond length, are depicted in orange (O_ax,1), green (O_ax,2), cyan (O_ax,3) and purple (O_ax,4). Nickel atoms are depicted in gray and nonaxial (equatorial) oxygen atoms in red. (For clarity only part of the structure is shown, omitting La.)

Figure 3

Comparison of Hahn echo and MATPASS NMR spectra of La₂NiO_4+δ, with quadrupolar filtering. Spectra were acquired at 4.7 T with a MAS rate of 40 kHz. Splitting of O_ax is partially resolved in the Hahn echo and fully resolved in the MATPASS data. Both experiments show pulse length dependence (π/6 vs π/2) consistent with a single highly quadrupolar environment (O_eq). Asterisks denote spinning sidebands where apparent.

Figure 4

Variable-temperature NMR spectra of La₂NiO_4+δ, focusing on the interstitial oxygen site. (a, top to bottom) ¹⁷O MAS NMR spectra of La₂NiO_4+δ acquired at the indicated temperatures, and at 35 °C after cooling from high temperature (red). The difference between normal room temperature and the lowest sample temperature (35 °C) is due to frictional heating by MAS. Spectra were acquired at 16.4 T under a MAS rate of 12.5 kHz. Spectra shown are normalized to the number of scans. (b) Detail of (a) with spectra scaled to highlight broadening and shift of interstitial site. Asterisks denote spinning sidebands (for clarity shown only at 35 °C).

Figure 5

Broadband variable-temperature NMR spectra of La₂NiO_4+δ. Spectra were acquired at 7.05 T at a MAS rate of 12.5 kHz. Spectra were normalized to number of scans (between ∼300 000 and ∼6 700 000 per spectrum) and then scaled as shown to obtain similar intensity for the O_ax feature (∼3500 ppm) present in all spectra. ^#Indicates the feature at 2400 ppm assigned to the La₃Ni₂O₇/La₄Ni₃O₁₀ impurity phase (see SI, section 5). Asterisks denote visible spinning sidebands, for clarity only indicated for O_i at 35 °C and for O_eq at 140 °C. (A close-up view of the spectrum at 140 °C depicting the weakly resolved peaks in the diamagnetic region is shown in Figure S9.)