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

The magnetization density distribution in the ionic ferromagnet Rb2CrCl4 has been determined by polarized neutron diffraction from a single crystal at 4.5 K. Magnetic structure factors FM were derived from the flipping ratios R of 660 main structure reflections (189 independent) and 57 super-structure reflections with sin θ/λ ≼ 0.8 Å-1 (λ = 0.90 Å). The main structure reflections were corrected for extinction using a secondary extinction parameter. The FM 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 Cr2+ moment occupying each 3d orbital is 0.26(2)(yz), 0.25(2)(xz), 0.25 0.25(2)(z2) and — 0.01 (2)(x2—y2), where z is the principal axis of Jahn-Teller elongation at each Cr site. The optimum refined distribution of the magnetic moment (μB) 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).