Properties of the electron-doped layered manganates La2-2xCa1+2xMn2O7 (0.6≤x≤1.0)

The La2-2xCa1+2xMn2O7 (0.6 less than or equal to x less than or equal to 1.0) series has been successfully synthesized using a citrate gel technique in order to study their structure and properties as a function of x in the Mn4+-rich region of the phase diagram. Rietveld refinement of powder x-ray diffraction data, combined with electron microscopy, shows that the phases are of high purity and adopt the n=2 Ruddlesden-Popper structure comprised of perovskite bilayers separated by rocksalt layers. The lattice parameter c reaches a minimum at x similar to 0.7 and the increase with x is attributed to elongation of the apical Mn-O bonds due to preferential A-site occupancy and/or splitting of the t(2g): levels of Mn4+. X-ray absorption spectroscopy measurements confirm that the Mn-valence variation in these series tracks formal valence expectations. In the region 0.6 less than or equal to x less than or equal to 0.8 the magnetic susceptibility manifests a peak or shoulder at similar to 280 K due to charge and orbital ordering, and antiferromagnetic order develops at lower temperatures (T(N)similar to 150-200 K) with quasi-two-dimensional antiferromagnetic fluctuation effects being evidenced above T-N. The magnetic properties change significantly at 0.825 less than or equal to x less than or equal to 1.0: at higher temperatures two-dimensional magnetic coupling is observed and at similar to 115 K the system spontaneously orders antiferromagnetically, but with a ferromagnetic moment. The transport results indicate insulating behavior at all compositions, but with an enhanced localization upon charge/orbital ordering in the x=0.7 material.