Local atomic structure of the layered compound SrFeO$_{2}$

SrFeO$_{2}$ exhibits several unexpected structural and physical properties. Its antiferromagnetic transition temperature T$_{N}$=473K is unusually high for a two-dimensional layered structure. First-principle calculations on SrFeO$_{2}$ showed that the Fe 3d down-spin elections occupy the nondegenerate d$_{z2}$ level rather than the degenerate (d$_{xz}$, d$_{yz})$ levels. This is in good agreement with the absence of a Jahn-Teller instability and the existence of the three dimensional antiferromagnetic ordering because the out-of plane direct Fe-Fe exchange is comparable in strength to the in-plane Fe-O-Fe superexchange. Therefore, it is expected that there is no structure instability in SrFeO$_{2}$. Using the pair distribution function (PDF) analysis to characterize the local structure of SrFeO$_{2}$, we observed that the local symmetry is lower than the average P4/mmm crystal symmetry. In particular, the FeO$_{2}$ planes are buckled, with two unique buckling angles along the a-axis. The buckling angle of Fe-O-Fe is reduced from 180$^{o }$with increasing temperature, accompanied by a reduction of the Fe magnetic moment. Thus the local structure instability correlates with the magnetism where the distortions suppress orbital overlap.