The two-dimensional double layered antiferromagnetRb3Mn2Cl7—a neutron diffraction study

The structure of ${\mathrm{Rb}}_3$ ${\mathrm{Mn}}_2$ ${\mathrm{Cl}}_7$ is similar to that of ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$. Whereas in ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$ there are single Mn${\mathrm{Cl}}_2$ layers which are shifted one from the other by the (1/2,1/2,1/2) translation, in ${\mathrm{Rb}}_3$ ${\mathrm{Mn}}_2$ ${\mathrm{Cl}}_7$, instead of single layers there are pairs of layers. In the temperature range $64.5<T<\mathrm{}100\mathrm{}$ °K the crystal behaves like a two-dimensional antiferromagnet. Its magnetic moments are oriented along the $c$ axis. Every moment is antiparallel to its five ${\mathrm{Mn}}^{+2\mathrm{}}$ nearest neighbors. In this temperature range there are no correlations between the various pairs of layers. Therefore, instead of points in the reciprocal space one expects to obtain rods of reflections along $c^{*}$, as in the case of the single-layer type compounds. In the double-layer case, however, one expects the intensity of the rods to be modulated with a wave vector $\frac{2\pi }{t}$ where $t$ is the intrapair distance between layers. Several scans made along and across the ($0,1,l$) rods confirmed these expectations. Below $T_N\sim 64.5$°K the compound orders in three dimensions and as in ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$ we find here (the same kind of) magnetic polytypism.