Influence of iron insertion in phase and Raman spectra of sol–gel-derived Ruddlesden Popper-type Sr2(Ce1-xFex)O4 (x = 0–1.0) powders.

Ruddlesden Popper-type (RP-type) oxides have been attracted due to their immense applications in oxygen separation, antennas, luminescence, catalysts, lithium-ion battery electrodes, sensors, and solid oxide fuel cells. The RP- type Sr2(Ce1-xFex)O4 (x = 0–1.0) series has been synthesised via a novel sol–gel technique and characterised with regards to structure, detailed bond length/angles, and Raman spectra. It involves the mixing of nitrate salt solutions with oxalic acid, digestion for 12 h, dried at 130 °C for 24 h and calcined at 900–950 °C for 5 h in air for the formation. X-ray diffraction cum Rietveld refinement depicts single (orthorhombic), dual (orthorhombic + tetragonal), and single (tetragonal) phase formation with x = 0–0.03, 0.05–0.80, and 0.90–1.0, respectively. It possesses lattice parameters ao = 6.123 Å, bo = 10.359 Å, co = 3.598 Å, Z = 4, space group Pbam for x = 0 and tetragonal parameters at = bt = 3.867 Å, ct = 10.167 Å, Z = 2, space group I4/mmm for x = 1.0. However, the volume of orthorhombic and tetragonal unit cells decreases and increases, respectively, with iron. As iron in Sr2(Ce1-xFex)O4 increases, the structure becomes distorted, and octahedral angles B–O–B and O–B–O deviate drastically. The average bond length decreases from 2.715 to 2.308 Å with 'x', showing the analogous nature of the orthorhombic cell volume. Also, total cell volume decreases by 228.1–181.8 Å3 with iron. The particle size lies in the range of 100–200 nm. Raman spectra consist of bands at 144, 287, 343, 385, and 553 cm−1 for x = 0, and reveal vibrational and stretching modes. It depicts vibrational mode of strontium (Sr2+) ions coordinated by oxygen, stretching mode of Ce–O2 (equatorial), Ce–O1 (terminal), strontium displacement along c-axis, and vibration of Ce–O bonds. Bands at 139, 178, 273, 378, 456, and 581 cm−1, for x = 1 (tetragonal phase), reveal vibration of Sr–O2, Sr–O1 in ab plane, Sr–O2 in the c-plane, bridging in Sr–O2–Sr in ab plane, and vibration of FeO6 octahedra along c-axis, respectively. The intensity at 287 cm−1 band is higher than that at 385 cm−1 indicating applications in luminescence. Further, the band shifted towards a higher wave number with iron. Interestingly, structural and Raman results are supporting each other in the Sr2(Ce1-xFex)O4 (x = 0–1.0) series. [ABSTRACT FROM AUTHOR]