Phonons and stability of infinite-layer iron oxides SrFeO₂ and CaFeO₂

We present detailed ab-initio lattice dynamical analysis of the Fe-O infinite-layer compounds CaFeO₂ and SrFeO₂ in various magnetic configurations. These indicate strong spin-phonon coupling in SrFeO₂ in contrast to that in case of CaFeO₂. From our ab-initio calculations in SrFeO₂ as a function of volume, we suggest that the distortion in SrFeO₂ above 300 K is similar to that in CaFeO₂ at ambient conditions. The distortion of the planer structure of CaFeO₂ involves doubling of the planer unit cell that may be usually expected to be due to a soft phonon mode at the M-point (1/2 1/2 0). However, our ab-initio calculations show quite unusually that all the M-point (1/2 1/2 0) phonons are stable, but two stable M₃⁺ and M₂^- modes anharmonically couple with an unstable B_u mode at the zone center and lead to the cell doubling and the distorted structure. Magnetic exchange interactions in both the compounds have been computed on the basis of the ideal planar structure (P4/mmm space group) and with increasing amplitude of the B_u phonon mode. These reveal that the magnetic exchange interactions reduce significantly with increasing distortion. We have extended the ab-initio phonon calculation to high pressures, which reveal that, above 20 GPa of pressure, the undistorted planer CaFeO₂ becomes dynamically stable. We also report computed phonon spectra in SrFeO₃ that has a cubic structure, which is useful to understand the role of the difference in geometry of oxygen atoms around the Fe atom with respect to planer SrFeO₂. Finally, powder neutron inelastic scattering experiments on SrFeO₂ have also been performed at temperatures from 5 K to 353 K in the antiferromagnetic phase. The 5-K data are compared to the ab-initio calculations.