Structure and reactivity with oxygen of Pr₂NiO_4+δ: an in situ synchrotron X-ray powder diffraction study

Abstract

The promising SOFC cathode material Pr2NiO(4.22) has been studied in situ under a pure oxygen atmosphere from 25 to 950 °C by high resolution synchrotron X-ray powder diffraction. At room temperature (RT) δ = 0.22(1), the average crystal structure turns out to be monoclinic. The subtle monoclinic distortion (γ = 90.066(1)° at RT), retained up to 460 °C, is interpreted in terms of specific tilt schemes of the NiO6 octahedra. It is also shown that Pr2NiO(4.22) is incommensurately structurally modulated already at room temperature, in the same manner as the homologous cobaltate La2CoO(4.14). The phase transition to the High Temperature Tetragonal (HTT) phase was completed at 480 °C without any evidence for the Low Temperature Orthorhombic (LTO) phase allowing clarifying the phase diagram of this K2NiF4-type ternary oxide. Moreover, it turns out that above 800 °C, the HTT phase transforms reversibly into two coexisting isomorphous tetragonal phases. The incommensurate modulation subsists up to 950 °C, although modified concomitantly with the two abovementioned phase transformations. In addition, the role of kinetics on the decomposition process is highlighted through thermo-gravimetric analyses.

Structural transformations of the La2−xPrxNiO4+δ system probed by high-resolution synchrotron and neutron powder diffraction

“…In general, the refined details for Pr-rich F2/m structures in this study match well with results recently reported for Pr2NiO4+δ by Broux et al. 46 As discussed in that work, it is not realistic that the large refined anisotropic atomic displacement values for apical O(1) and equatorial O(2/3) oxygen sites imply significant thermal motion of these ions in the structure -which should be relatively small at 300 K -but more likely represent disordered or incommensurate ion displacements, such as octahedra tilting and/or steric repulsions from the interstitial oxygen site. Several previous reports on LNO and PNO have proposed structural models with split apical oxygen sites, 31,39,41 however it was recently shown by pair distribution function (PDF) analysis of neutron and X-ray powder that ambient temperature disorder of apical oxygen atoms in La2NiO4.18 can be well modeled by anisotropic atomic displacement parameters.…”

“…The data for the Pr2NiO4+δ end member with δ ≈ 0.25 are well fit by a monoclinic F2/m space group with lattice parameters of a ≈ 5.39 Å, b ≈ 5.46 Å, c ≈ 12.45 Å, and γ ≈ 90.07° (see Table 1). This unit cell corresponds extremely well with details of the low temperature monoclinic (LTM) structure for Pr2NiO4+δ recently published in a separate investigation by Broux et al, 46 and provides confirming experimental evidence for an ambient temperature monoclinic structural distortion of Pr2NiO4+δ first proposed decades ago. 20,34,35 We note that close inspection by monochromatic labbased Cu Kα1 powder X-ray measurements of PNO also suggests a non-orthorhombic distortion, although less distinctly visible than in the HR-SPD datasets.…”

“…This observation of additional non-indexed peaks is consistent with previous literature on Pr2NiO4+δ referenced earlier. 10,[38][39][40][41]46 Also, as shown in Figure 5 and discussed further below, local TEM electron diffraction analysis similarly revealed additional non-indexed reflections. This confirms that these peaks originate from the main LPNO phase.…”

“…9,11,[42][43][44][45] Recently, Hernandez et al used high angular resolution synchrotron powder diffraction to establish that, in fact, oxidized Pr2NiO4+δ compositions (δ ≈ 0.22) do exhibit a clear but subtle monoclinic distortion at ambient temperature. 46 This so-called low temperature monoclinic cell (LTM) was indexed with a F2/m space group (non-conventional setting of space group n o 12, C2/m), and is derived from the parent orthorhombic Fmmm cell, but with one non-90° angle of γ ≈ 90.06°. In situ studies showed that the monoclinic distortion persists until the transition to a HTT form above 700 K. As mentioned above for previous Pr2NiO4+δ works, Hernandez et al also observe weak non-indexed reflections in diffraction patterns, which they attribute to complex incommensurate modulation, for which the average structure can be described by the proposed LTM cell.…”

Activity and Stability of (Pr1-xNdx)2NiO4as Cathodes for Solid Oxide Fuel Cells: Part V. In Situ Studies of Phase Evolution

“…14, 15 The phase transition from orthorhombic to HTT involves a small change in lattice parameters. Both a and b are within 0.06 Å in the orthorhombic structure 10 and equal in the HTT phase.…”