Polarized neutron diffraction study of the magnetization density distribution in Rb ₂ CrCl ₄ : a two-dimensional ionic ferromagnet

<jats:p> The magnetization density distribution in the ionic ferromagnet Rb <jats:sub>2</jats:sub> CrCl <jats:sub>4</jats:sub> has been determined by polarized neutron diffraction from a single crystal at 4.5 K. Magnetic structure factors <jats:italic>F</jats:italic> <jats:sub>M</jats:sub> were derived from the flipping ratios <jats:italic>R</jats:italic> of 660 main structure reflections (189 independent) and 57 super-structure reflections with sin <jats:italic>θ</jats:italic> /λ ≼ 0.8 Å <jats:sup>-1</jats:sup> (λ = 0.90 Å). The main structure reflections were corrected for extinction using a secondary extinction parameter. The <jats:italic>F</jats:italic> <jats:sub>M</jats:sub> 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 <jats:sup>2+</jats:sup> moment occupying each 3d orbital is 0.26(2)( <jats:italic>yz</jats:italic> ), 0.25(2)( <jats:italic>xz</jats:italic> ), 0.25 0.25(2)( <jats:italic>z</jats:italic> <jats:sup>2</jats:sup> ) and — 0.01 (2)( <jats:italic>x</jats:italic> <jats:sup>2</jats:sup> — <jats:italic>y</jats:italic> <jats:sup>2</jats:sup> ), where <jats:italic>z</jats:italic> is the principal axis of Jahn-Teller elongation at each Cr site. The optimum refined distribution of the magnetic moment (μ <jats:sub>B</jats:sub> ) 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). </jats:p>