Charge and orbital order effects in La$_{x}$Sr$_{1-x}$MnO$_{2.6}$

Low temperature annealing in hydrogen have been used to obtain oxygen vacancy ordered manganites Sr$_{4+n}$Mn$^{3+}_{4}$Mn$^{4+}_{n}$O$_{10+3n}$ (n=0, 1, 3) displaying charge and orbital ordering. For the La-substituted n=1 phase four Mn$^{3+}$ cations exhibit elongated pyramidal coordination while the fifth one in octahedral coordination shows decreasing formal valence Mn$^{(4- 5x)+}$. This selective doping produces structural strain resulting in unusual apically compressed coordination leading to complex magnetic interactions and frustration. Similar structures have been previously observed for the (La,Ba)CuO$_{3-d}$ cuprates revealing common vacancy ordering relationships in perovskites for which highly distorted Mn$^{3+}$ (Cu$^{2+})$ and symmetric Mn$^{4+}$ (Cu$^{3+})$ ions are present simultaneously. Work at NIU was supported by the NSF-DMR-0706610 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357.