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

The La_2-2xCa₁₊₂ₓMn₂O₇ $(0.6<~x<~1.0)$ 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~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 t2g levels of Mn⁴⁺. X-ray absorption spectroscopy measurements confirm that the Mn-valence variation in these series tracks formal valence expectations. In the region $0.6<~x<~0.8$ the magnetic susceptibility manifests a peak or shoulder at {~}280 K due to charge and orbital ordering, and antiferromagnetic order develops at lower temperatures (TN~150--200K) with quasi-two-dimensional antiferromagnetic fluctuation effects being evidenced above TN. The magnetic properties change significantly at $0.825<~x<~1.0:$ at higher temperatures two-dimensional magnetic coupling is observed and at {~}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.