Optical-active phonons in A3Fe2B″O9 (A=Ca, Sr; B″=Te, W) double perovskites

Abstract

double perovskites at 300 K. While SFTO and SFWO crystallize in a tetragonal structure belonging to the I4 / m space with two formulas per unit cell ͑Z=2͒, CFWO crystallizes in a monoclinic structure belonging to the space group P2 1 / n with Z = 2. The observed spectra are very similar to that of the prototype cubic ͑Fm3 ¯m͒ double perovskite, indicating that both the tetragonal and monoclinic structures result from small distortions of the cubic cell. The assignment of the optical phonons follows that given for the prototype Fm3 ¯m double perovskites.

Nanocrystalline Antiferromagnetic High-κ Dielectric Sr2NiMO6 (M = Te, W) with Double Perovskite Structure Type

“…Optical phonons for both compounds are summarized in Table S1 in the Supporting file. Similar to Silva et al [56], we also observe a larger difference in the ν 1 vibration between Te 6+ -(d 10 ) and W 6+ -based (d 0 ) compounds. These authors correlated the increase of wavenumber in W-containing compounds compared to Te-containing compounds with the increase of the force constant (bonding energy) of the W-O bond in the WO 6 octahedron.…”

“…Ayala et al [58] studied tetragonal and monoclinic double perovskites and came to conclusion that these spectra resemble to those of cubic Fm-3m perovskites. The similarity comes from the fact that tetragonal and monoclinic structures arise from the small distortions of the cubic cell [56]. When comparing our SNWO spectrum in Figure 7a with the bulk SNWO reported by Manoun et al [59], there is a progressive shift to lower wavenumbers (lower energies); line shapes are broadened and asymmetric on the lower energy side.…”

“…The Te 6+ cation has a fully-occupied d 10 orbital configuration, which avoids the formation of π-type Te-O bonds. On the other hand, the W 6+ cation has a d 0 orbital configuration, allowing the overlap of the t 2g orbitals The increase of force constant/bonding energy occurs due to the overlapping of t 2g orbitals of the octahedrally-coordinated W 6+ cation [56]. Similar arguments are valid for the absence of vibrations of Ni 2+ O 6 octahedron for SNWO.…”

“…Hence, for SNTO, the frequencies corresponding to vibrations of Ni 2+ O 6 octahedron are not expected and indeed were not observed. Peaks assigned to ν 2 mode are due to asymmetric stretching that appear in the range 470-650 cm −1 according to Silva et al [56]. This mode is represented by two peaks for both compounds, at 497 and 564 cm −1 for SNWO and at 510 and 600 cm −1 for SNTO.…”

“…They are observed at 134 cm −1 for SNWO and at 142 cm −1 for SNTO. Peaks in the range 350-450 cm −1 are assigned to ν 5 mode, which appears due to oxygen bending in octahedra [56,57]. It is observed at 440 cm −1 for SNWO and at 416 cm −1 for SNTO.…”