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 citra te gel process at temperatures as low as 900 degrees C under flowing o xygen. The compounds were characterized by X-ray powder diffraction, R ietveld profile analysis, thermal gravimetric analysis, iodometric tit rations, Mn K-edge X-ray absorption spectroscopy, temperature-dependen t magnetic susceptibility, and resistivity. Rietveld analysis shows th at there is an elongation in the apical Mn-O distances of the MnO6 oct ahedra, which increases with decreasing dimensionality. Mn K-edge X-ra y absorption spectroscopy and iodometric titrations show no detectable amount of Mn3+ (a Jahn-Teller distorted d(4) ion) in the samples indi cating that the MnO6 deformation may be associated with a splitting in the half-filled t(2g) levels of Mn4+. Magnetic susceptibility measure ments show spontaneous magnetic ordering to antiferromagnetic states a t similar to 125 K for CaMnO3 and Ca4Mn3O10 and at similar to 110 K fo r Ca2MnO4 and Ca3Mn2O7. The effective magnetic moment is greatly suppr essed in all of the RP manganates, even in the three-dimensional CaMnO 3, n = infinity phase, and decreases dramatically with decreasing dime nsionality. Large deviations from Curie-Weiss behavior are observed ab ove the ordering temperature, which increase with decreasing n. These observations are discussed in terms of two-dimensional magnetic fluctu ations and alternative mechanisms. The compounds are all poor electron ic conductors with room-temperature resistivities in the range of 10(2 )-10(4) Omega cm.