Ca(2.5)Sr(0.5)GaMn2O8: diamagnetic Ga in control of the structural and electronic properties of a bilayered manganate

The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.     

Magnetism and structural chemistry of the n=2 Ruddlesden-Popper phase La3LiMnO7

Polycrystalline samples of the n=2 Ruddlesden-Popper phase La₃LiMnO₇ have been prepared and characterized. X-ray and neutron diffraction suggest that the structure is tetragonal with a disordered distribution of Li and Mn cations over the octahedral sites, but ⁶Li MAS NMR shows that the Li and Mn are 1:1 ordered locally. Electron microscopy shows that the stacking of the cation-ordered, perovskite-like bilayers along the crystallographic z-axis is disordered on the distance scale sampled by X-ray and neutron diffraction. Magnetometry data and neutron diffraction data collected at 2 K together suggest that the Mn cations within each structural domain order antiferromagnetically at 14 K, but that the disorder along z prevents the establishment of long-range magnetic order.     

Crystal Structure and Magnetic Properties of SrCaMnGaO5+δ

The room-temperature crystal structure of the brownmillerite SrCaMnGaO_5+δ (δ=0.035) has been refined from neutron powder diffraction data; space group Ima2, a=15.7817(6), b=5.4925(2), c=5.3196(2)> Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μ_B per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6 T, increasing to 90% in 13 T.