The ${\mathrm{La}}_{2-2x}{\mathrm{Ca}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ $(0.6\mathrm{}<~x<~1.0)$ 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 ${\mathrm{Mn}}^{4+}$-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 $n=2\mathrm{}$ Ruddlesden-Popper structure comprised of perovskite bilayers separated by rocksalt layers. The lattice parameter c reaches a minimum at $x\sim 0.7$ 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 ${\mathrm{Mn}}^{4+}.$ 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 ${(T}_N\sim 150–200\mathrm{K})$ with quasi-two-dimensional antiferromagnetic fluctuation effects being evidenced above $T_N.$ 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 $x=0.7\mathrm{}$ material.

• Received 24 January 2000

DOI:https://doi.org/10.1103/PhysRevB.62.6485