Structure, magnetism, and properties of Ruddlesden-Popper calcium manganates prepared from citrate gels

Ruddlesden-Popper (RP) phases with the formula (CaO)(CaMnO3)(n), where n = 1, 2, 3, and infinity, have been prepared using the Pechini citrate gel process at temperatures as low as 900 degrees C under flowing oxygen. The compounds were characterized by X-ray powder diffraction, Rietveld profile analysis, thermal gravimetric analysis, iodometric titrations, Mn K-edge X-ray absorption spectroscopy, temperature-dependent magnetic susceptibility, and resistivity. Rietveld analysis shows that there is an elongation in the apical Mn-O distances of the MnO6 octahedra, which increases with decreasing dimensionality. Mn K-edge X-ray absorption spectroscopy and iodometric titrations show no detectable amount of Mn3+ (a Jahn-Teller distorted d(4) ion) in the samples indicating that the MnO6 deformation may be associated with a splitting in the half-filled t(2g) levels of Mn4+. Magnetic susceptibility measurements show spontaneous magnetic ordering to antiferromagnetic states at similar to 125 K for CaMnO3 and Ca4Mn3O10 and at similar to 110 K for Ca2MnO4 and Ca3Mn2O7. The effective magnetic moment is greatly suppressed in all of the RP manganates, even in the three-dimensional CaMnO3, n = infinity phase, and decreases dramatically with decreasing dimensionality. Large deviations from Curie-Weiss behavior are observed above the ordering temperature, which increase with decreasing n. These observations are discussed in terms of two-dimensional magnetic fluctuations and alternative mechanisms. The compounds are all poor electronic conductors with room-temperature resistivities in the range of 10(2)-10(4) Omega cm.