Structure, Mossbauer spectroscopy and vibration phonon spectra in valence-bond force-field model approach for distorted perovskites AFeO3 (A = La, Y)

Two distorted perovskite orthoferrites, AFeO(3) (A = La, Y), were synthesized by solid-state reaction method and investigated using the experimental techniques, e.g., X-ray diffraction, Mossbauer spectra, and infrared and Raman spectra at room temperature. The internal hyperfine magnetic field at La-site is slightly larger than for Y-site due to the tilt angle and distortion of the oxygen octahedral, and Neel temperature dependence on the ionic radius of A-site ions. Vibration spectra were analysed employing the normal coordinate analysis method within the framework of a short-range valence-bond force-field model. The present lattice dynamical calculations of optical phonon spectra provide useful insights towards the dependence of phonon frequencies on different bond strengths caused by steric and magnetic effects of A-site cations. Mode mixing occurs due to coupling of different A(g) and B-g modes in the Raman spectra of LaFeO3 and YFeO3. The assignment and comparison of the observed phonon spectra of LaFeO3 and YFeO3 to different symmetry modes differ strikingly from the other isomorphous rare-earth orthoferrites, which may be due to different degree of structural distortion and complexity of magnetic interactions between A and Fe ions through intermediate oxygen ions over the ferrite series.