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

• Ab-initio calculations indicate strong spin–phonon coupling in SrFeO2 in contrast to that in case of CaFeO2.
• Neutron inelastic scattering experiments on SrFeO2 have been analyzed using the ab-initio calculations.
• Distortion in SrFeO2 above 300 K is deduced to be similar to that in CaFeO2 at ambient conditions.
• An unstable Bu mode at the zone center couples with a stable phonon mode at the M-point (1/2 1/2 0) and leads to the cell doubling and the distorted structure.
• Magnetic exchange interactions reduce significantly with increasing distortion.
• At high pressure the distortion in CaFeO2 diminishes and the undistorted planer structure becomes dynamically stable.
• The computed phonon spectra in SrFeO3 are used to understand the role of the difference in geometry of oxygen atoms around the Fe atom with respect to planer SrFeO2.

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 Bu 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 Bu 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.