Local structures of Sr2FeMnO5+y (y=0, 0.5) and Sr2Fe1.5Cr0.5O5 from reverse Monte Carlo modeling of pair distribution function data and implications for magnetic order

Abstract

The local structures of the oxygen deficient perovskites Sr2FeMnO5, Sr2FeMnO5.5, and Sr2Fe1.5Cr0.5O5 have been analyzed using neutron pair distribution function data. The results show that locally all three structures are more complex than implied by their average cubic structures and that the distributions of oxygen vacancies are not completely random on a local level. For both Sr2FeMnO5+y compounds it is found that there is no short range ordering of the Fe and Mn cations. For Sr2Fe1.5Cr0.5O5 there is evidence to suggest that the Fe/Cr distribution is not completely random and is locally ordered such that there are fewer FeFe nearest neighbor pairs than in a random distribution. Reverse Monte Carlo modeling of the pair distribution function data has provided the FeO, MnO, and CrO bond length distributions and information on the coordination numbers of the Fe, Mn, and Cr cations. In Sr2FeMnO5 it is found that the Fe3+ cations are most often in 4-fold coordination but there is also a large amount of Fe3+ in 5-fold coordination and a small amount in 6-fold coordination. The Mn3+ is split between 5-fold and 6-fold coordination. The MnO bond length distributions indicate that the Mn3+O6 octahedra and Mn3+O5 square pyramids are locally Jahn–Teller distorted. In Sr2FeMnO5.5 the Fe3+ is almost entirely 5 coordinate while the Mn4+ is almost entirely 6 coordinate. The Cr3+ in Sr2Fe1.5Cr0.5O5 is almost entirely 6-fold coordinated, giving the Fe3+ an average coordination number of 4.67. In Sr2FeMnO5 and Sr2Fe1.5Cr0.5O5 the Fe3+ and Sr2+ cations undergo local displacements which are driven by the oxygen vacancies, while the Mn3+ and Cr3+ cations remain near their positions in the average structures. In Sr2FeMnO5.5 these cations are not significantly displaced. The local coordination geometries are used to explain previously observed but yet poorly understood magnetic properties of these materials.