Structure, Magnetism, and Properties of Ruddlesden−Popper Calcium Manganates Prepared from Citrate Gels

Abstract

Ruddlesden−Popper (RP) phases with the formula (CaO)(CaMnO3)n, where n = 1, 2, 3, and ∞, have been prepared using the Pechini citrate gel process at temperatures as low as 900 °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 d4 ion) in the samples indicating that the MnO6 deformation may be associated with a splitting in the half-filled t2g levels of Mn4+. Magnetic susceptibility measurements show spontaneous magnetic ordering to antiferromagnetic states at ∼125 K for CaMnO3 and Ca4Mn3O10 and at ∼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 = ∞ 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 102−104 Ω cm.