Magnetic Structure of RbMn<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cl</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

The compound RbMn${\mathrm{Cl}}_{3}$ is paramagnetic at room temperature and belongs to the space group $\frac{{D}_{6h}^{4}\ensuremath{-}P{6}_{3}}{\mathrm{mmc}}$ with six molecules per unit cell. Neutron diffraction of RbMn${\mathrm{Cl}}_{3}$-powder sample was studied as a function of the sample temperature. A transition to antiferromagnetism was found at ${T}_{N}\ensuremath{\sim}94$\ifmmode^\circ\else\textdegree\fi{}K. The magnetic structure determined from the diffraction pattern at 4.2\ifmmode^\circ\else\textdegree\fi{}K consists of ferromagnetic planes stacked in the sequence $A(+)$, $B(\ensuremath{-})$, $B(+)$, $A(\ensuremath{-})$, $C(+)$, $C(\ensuremath{-})$, along the $c$ axis. Spin direction is perpendicular to the $c$ axis, defining an infinite number of structures which belong to a two-dimensional irreducible representation ${\ensuremath{\Gamma}}_{6}^{+}$ of ${D}_{6h}^{4}$. Two distinctly different structures of orthorhombic symmetry, $C{m}^{\ensuremath{'}}{c}^{\ensuremath{'}}m$ and $\mathrm{Cmcm}$, span the subspace of this representation and are consistent with the diffraction data. Dipolar energy calculations show that the direction perpendicular to the $\stackrel{^}{z}$ direction is of lower dipolar energy than that parallel to $\stackrel{^}{z}$. The existence of a spin component perpendicular to $\stackrel{^}{z}$ is also confirmed with the aid of an external magnetic field. A magnetic moment of 4.1 ${\ensuremath{\mu}}_{B}$ is calculated for the ${\mathrm{Mn}}^{2+}$ ion from the diffraction pattern at 4.2\ifmmode^\circ\else\textdegree\fi{}K.

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