Effect of magnesium on the XPS and Raman spectra of (Ba0.5Sr0.5)(Al0.2-xMgxFe0.8)O3-ξ (x ≤ 0.2)

ABSTRACT The perovskite-type cubic oxides have found useful applications in catalysis, solid oxide fuel cells, gas sensors, and membrane technology. XPS and Raman studies of a recently developed (Ba0.5Sr0.5)(Al0.2-xMgxFe0.8)O3-ξ (x = 0–0.2) oxygen-permeable system are reported here. The effect of magnesium is shown to alter the relative amounts of Fe3+, Fe4+, and oxygen species via Fe (2p3/2 and 2p1/2) and O 1s signals. Besides, it causes a surge in oxygen vacancies, an increase in Fe3+ ions, a decline in B – O bond strength (B = Al/Mg/Fe), and a rise in anion flow. The O1s signals at ~ 528.5 and ~531.0 eV correspond to surface oxide and adsorbed (O2 2−, O2−, or O−) species, respectively. Raman spectra offer evidence for symmetric A1g stretching, oxygen vacancies, distorted BO6 octahedra, A-O stretching vibration modes (A = Ba, Sr), and bending B-O linkages. A good correlation is advanced between the XPS and Raman results, in conformity with lattice expansion, Mossbauer data, improved oxygen permeability, enhanced electrical conductivity, and high structural stability realized by means of partial replacement of Al3+ with Mg2+ in (Ba0.5Sr0.5)(Al0.2Fe0.8)O3-ξ earlier.


Introduction
The perovskite-type cubic oxide (ABO 3 ; A = La, Ba, Sr; B = Co, Fe, Al, Mg; O-oxygen) based systems have generated immense interest due to their potential applications as gas separation membranes, solid electrolyte materials, and catalysts besides in fuel cells, sensors, etc [1][2][3][4][5][6][7][8][9][10]. Their membranes possess a unique capacity of oxygen separation from the air and so play a vital role in industry and power generation [10][11][12][13]. The flow of oxygen occurs due to the driving force brought by the pressure gradient across the membrane. The main advantage of the process lies in the continuous production of ~ 100% pure oxygen and costeffectiveness. In contrast, cryogenic distillation and pressure swing adsorption methods yield oxygen of only 90-95% purity and involve large-scale plants with high operation cost [10,12,13]. The practical issues in the effective utilization as ceramic membranes are however pertain to their structural stability, mechanical strength, and oxygen permeability at operating temperatures. Intensive efforts have been made in the past to develop multi-component solid-state membranes exhibiting high oxygen permeability with adequate structural stability at elevated temperatures. Some known oxygenpermeable system based on iron and cobalt are La 1−x B x Co 1−y Fe y O 3-ξ (B = Sr and/or Ba, Ca; x = 0-1; y = 0-1), Sr 0.9 Ca 0.1 Co 1−y Fe y O 2.5+ξ (y = 0, 0.1), Gd 0.2 Ba 0. 8 [14][15][16][17]. But the cobalt-based membranes, though exhibit reasonable oxygen permeability, are expensive and show poor stability (thermal and mechanical) and high thermal expansion [1,10]. So, the attention was shifted to the iron-based compounds, viz., SrFeO 3-ξ , Ba 1 [18][19][20][21][22][23][24][25][26][27][28][29]. In this series, the present authors developed a multicomponent (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ (x = 0-0.2) system by partial replacement of Sr 2+ with Ba 2+ (at A-site) and Al 3+ with Mg 2+ (at B-site) in SrAl 0.2 Fe 0.8 O 3-ξ and focused on its phase stability, oxygen permeability, and electrical conductivity [30]. The work described here is in continuation and present some results of X-ray photoelectron spectroscopy and Raman studies of (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ (x = 0-0.2) to understand the effect of magnesium and possible correlation with earlier findings [30].

Experimental
(Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ (x = 0-0.2) powder samples were synthesized via sol-gel route comprising of mixing of the metal nitrates and oxalic acid solutions in ethanol, gel formation and drying at 150°C for 24 h, decomposition at 950°C for 5 h in air, and sieving [30]. The synthesis procedure is given in the supplementary information. A Thermo-electron X-ray diffractometer (model ARL X / TRA) was employed for phase analysis. The molecular vibrational spectra having sensitivity to local structure and chemistry were obtained using a Raman microscope (Renishaw model Invia) at the excitation wavelength of 1064 nm (YAG laser); resolution being 0.3 cm −1 (FWHM). To explore the electronic states of species, the X-ray photoelectron spectra (sample area 100 μm x 100 μm) were recorded with a PHI 5000 VersaProbe (Model Ulvac) using an Al K α (hυ = 1486.6 eV) at the pressure of ~ 6.7 × 10 −8 Pa. The signal was gathered at the take-off angle of 45° to get a contribution from the sub-surface region. The high resolution (step 125 meV) spectra of species were obtained inappropriate energy regimes. Carbon 1s peak at 284.6 eV (with the precision of 0.1-0.2 eV) was used as a reference for deducing the binding energy (BE) accurately. The data accumulation time interval was fixed at 3 -60 min and an Origin 8.5 software was utilized for fitting of each peak assuming the Gaussian/Lorentzian distribution(s).

Results and discussion
(Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ (0 ≤ x ≤ 0.2) powder samples exhibit a perovskite-type cubic structure with lattice parameter 3.953-3.978 Å ± 0.002 Å ( Fig  S1 supplementary information). XPS data contain information about the oxidation state(s) of species as well as local charge transfer related effects in the material [31]. The electronic/bonding states and local chemical environment around oxygen are examined through XPS studies to understand the anion vacancy formation with magnesium insertion (x ≤ 0.2) at Al 3+ sites in (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 ) O 3-ξ . The carbon C 1s peak at 284.6 eV is used as a standard for calibration of XPS data. X-ray photoelectron (XP) spectra of (Ba 0.5 Sr 0.5 ) 2) having contributions from Ba 3d, Sr 3d, Al 2p, Mg 1s, Fe 2p, and oxygen (O 1s) levels shown in Figure 1 appear nearly similar for all the powder samples. For instance, the Ba 3d doublet is invariably observed in the energy range of 775-800 eV with FWHM of ~ 1.5-1.7 eV (Figure 2(a)). But the peaks shift slightly toward higher energy and show progressive broadening with magnesium insertion, i.e. from ~794.79 and 779.54 eV for x = 0 to ~ 794.92 and 779.67 eV for x = 0.20, indicative thereby increasing distortion. The presence of two peak maxima at ~ 779 and ~795 eV also gives the signature of the perovskite structure [31,32]. Figure 2(b) depicts the XPS scans of Ba 3d 5/2 and Ba 3d 3/2 peaks at respective binding energies (marked) in different compositions. The peak is deconvoluted into two with centers around 779 eV and 777 eV in the case of Ba 3d 5/2 . The exact position, width, and percentage area of each fitting peak for both the cases are summarized in Table 1. Notice that the (%) peak area decreases at ~779.5 eV but increases at ~ 777.5 eV with a rise in magnesium content. These correspond to barium lying on the surface and in subsurface regions, respectively. The figures suggest depletion of barium at the surface with increased magnesium content. Further, the presence of both the peaks points toward relaxed states for Baspecies [31][32][33][34][35]. The Sr 3d signal appears as a doublet due to spin-orbit coupling of 3d 5/2 and 3d 3/2 levels; their binding energies (BE) being ~ 133 and ~ 135 eV, respectively ( Figure 3). These peaks shift marginally toward lower energy with magnesium insertion, i.e. from 133.42 and 134.92 eV for x = 0 to 133.04 and 134.79 eV for x = 0.20. On examining closely, a hump is observed at the lower energy side (~ 131.67 eV) in the spectrum of composition x = 0.20. The above binding energy values match well with the Sr 2+ ion data observed before in the perovskite-type cubic oxides [33,34,36]. The Sr 3d and Ba 3d XPS data support XRD findings, which conform to occupancy of barium and strontium at A-sites in ABO 3-δ cubic structure [30]. Figure 4 depicts the XP spectra of (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ samples for aluminum (Al 2p) and magnesium (Mg 1s).  [37][38][39][40]. The relative amounts of Fe 3+ and Fe 4+ ions are deduced from respective areas of the fitting curves and summarized in Table 2. Clearly, there is decrease of Fe 4+ ions and increase of Fe 3+ species with rise in magnesium content (x), both occupying B-sites in ABO 3-δ structure. It essentially means that magnesium substitution induces some Fe 4+ → Fe 3+ conversion with added anion vacancies and consumption of electron released by oxygen desorption as molecule. These findings are consistent and in agreement with the Mössabuer data, revealing upsurge of Fe 3+ ion concentration in (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ (0 ≤ x ≤ 0.2) [30].
The high-resolution XP spectrum of O 1s level presented in Figure 6 is fitted with two Gaussian peaks at ~ 528. 5    Structural symmetry of oxides can be evaluated by Raman spectroscopy because of its sensitivity to local chemistry and coordination [46]. Figure 7(a) displays a few typical Raman spectra of (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ (x = 0-0.20) powder samples in the wavenumber range of 300-900 cm −1 . Interestingly, the main peak having Raman shift (> 600 cm −1 ) moves toward higher wavenumber with increasing magnesium content (x); notice that the Mg 2+ ion is lighter and has less positive charge than Al 3+ . Also, a shoulder in the Raman band observed in the sample of composition x = 0.20 indicates splitting of the signal into two peaks centered around 540 and 680 cm −1 (Figure 7(b)). A band with Raman shift in the range of 600-700 cm −1 corresponds to a totally symmetric A 1g stretching mode of BO 6 selfdistorted octahedra (or in-phase stretching breathing mode of oxygen in close vicinity of B-ion). The band near 680 cm −1 provides evidence for oxygen vacancies too [46,47]. These characteristics possibly arise due to local distortion of the crystal symmetry [32,48]. Generally, the anti-stretching (E g ) mode is two-fold (E g 1 and E g 2 ) whereas that of bending F 2g i is three-fold (i = 1,2,3) degenerate and represent octahedra tilting as well as Ba 2+ /Sr 2+ translation. Both E g and F 2g modes exhibit Raman shift in the range of 500-600 cm −1 . Thus, the split peak at 540 cm −1 arises due to octahedral tilting and/or Sr/Ba-ion translation induced by the creation of extra anion vacancies in the vicinity. The orthorhombic distortion of BO 6 octahedra displays a total of 24 Raman active modes involving rotation, anti-stretching, bending, and stretching effects [49][50][51]. The oxygen vibrates along M-O-M'    These characteristics are consistent and well supported by XPS data and Raman spectra discussed above, evident in Mossbauer studies reported before [30], and responsible for improving the oxygen permeability (J O2 ) of the membrane (thickness ~ 1 mm) to ~ 4.05-4.51 ml/cm 2 .min at 1000 °C (measurement details given briefly in the supplementary information). The process involves (i) a reaction O 2 (Air) + 4e − → 2O 2at preferential sites on the front surface, (ii) migration of O 2ions via anion vacancies and due to pressure difference across the membrane, and (iii) liberation of oxygen as a molecule with release of associated electrons at the exit surface. The electrons then travel back to the front surface as such or by hopping via Fe 3+ -O -Fe 4+ framework for continuation/recycle of the process. The enhancement in (J O2 ) with Mg 2+ content results due to enlarged unit cell, increased  oxygen vacancies, and improved electrical conductivity. Further, the (Ba 0.5 Sr 0.5 )(Fe 0.8 Mg 0.20 )O 3-ξ membrane retains its original cubic phase and sustains high oxygen permeability at 900 °C for 120 h continuously. The stability (structural as well as mechanical) of membranes at elevated temperatures can therefore be attributed to magnesium in (Ba 0.5 Sr 0.5 )(Al 0.2-x Mg x Fe 0.8 )O 3-ξ .

Conclusions
Magnesium in (Ba 0.5 Sr 0.5 ) (Al 0.2-x Mg x Fe0 .8 ) O 3-ξ (x = 0-0.2) brings distortion in oxygen octahedra, creates additional oxygen vacancies, weakens the B-O (B = Al, Mg, Fe) bonds in perovskite ABO 3-ξ cubic structure, and induces some Fe 4+ → Fe 3+ conversion. These features accompanied by unit cell enlargement provide phase stability and significant improvement in the oxygen permeation in the system.

Disclosure statement
No potential conflict of interest was reported by the authors.