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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. Powder neutron inelastic scattering experiments on SrFeO2 have also been performed at temperatures from 5 K to 353 K in the antiferromagnetic phase and analyzed using the ab-initio calculations. These suggest distortion of the ideal infinite planer structure above 300 K. 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 known 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 centre 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.