Polarized neutron diffraction study of the magnetization density distribution in Rb2CrCl4: a two-dimensional ionic ferromagnet

The magnetization density distribution in the ionic ferromagnet Rb$\_2$CrCl$\_4$ has been determined by polarized neutron diffraction from a single crystal at 4.5 K. Magnetic structure factors F$\_M$ were derived from the flipping ratios R of 660 main structure reflections (189 independent) and 57 super-structure reflections with $\sin \theta/\lambda \leqslant 0.8 \overset\circ{\mathrm A}^{-1}$ ($\lambda$ = 0.90 $\overset\circ{\mathrm A})$. The main structure reflections were corrected for extinction using a secondary extinction parameter. The F$\_M$ were modelled by representing the magnetization density by a superposition of densities from orbitals centred at each atom: Cr(3d, 4s) and Cl(3s, 3p). The results verify the presence of orbital ordering arising from the cooperative Jahn-Teller distortion in the basal plane, previously postulated as responsible for the ferromagnetic exchange interaction. The proportion of the Cr$^{2+}$ moment occupying each 3d orbital is 0.26(2)(yz), 0.25(2)(xz), 0.25(xy), 0.25(2)(z$^2$) and -0.01(2)(x$^2$-y$^2$), where z is the principal axis of Jahn-Teller elongation at each Cr site. The optimum refined distribution of the magnetic moment ($\mu_B$) was Cr(3d) 3.16(8), Cr(4s) 0.6(1), equatorial Cl(3s) 0.05(2), Cl(3p) 0.06(3) and axial Cl(3s) 0.03(3), Cl(3p) 0.08(5).