Structural chemistry and electronic properties of the n=3 Ruddlesden-Popper phases Ca4Mn2FeO9.75 and Sr4Mn2FeO9.80

The room-temperature crystal structures of the n = 3 Ruddlesden-Popper phas es Ca-4-Mn2FeO9.75 and Sr4Mn2FeO9.80 have been refined from neutron and X-r ay powder diffraction data. Both adopt space group I4/mmm with (a,c)= (Ca, 3.73683(1), 27.0860(1) Angstrom), (Sr, 3.83393(1), 27.8148(1) Angstrom). In both compounds the cation site at the center of the perovskite blocks is p referentially occupied by Fe (Ca, Mn:Fe = 0.424:0.576(4)), and the anion va cancies are found around this site. The occupied anion sites show static di sorder in Ca4Mn2FeO9.75 but not in Sr4Mn2FeO9.80. Both compounds are electr ical insulators which order antiferromagnetically at T-N = 75 K (Ca) or 90 K (Sr). Susceptibility and M(H) data suggest that not all the Mn and Fe cat ions take part in the long-range magnetic ordering, and there is evidence o f a spin glass transition in both compounds at similar to 11 K. The magneti c structure of Ca4Mn2FeO9.75 at 5 K has been determined by neutron diffract ion. No ordered moment was detected on the Mn/Fe site at the center of the perovskite blocks; 0.74(1) mu(B) per transition metal cation was measured a t the sites on the block edges. Possible causes of magnetic frustration in this crystal structure are considered. Ca4Mn2FeO9.75 has a magnetotresistan ce of -4% at 137 K in a 14 T field.