SPIN DISTRIBUTION, CANTING AND COVALENCE IN (ND4)2CR(SO4)2.6D2O BY POLARIZED NEUTRON-DIFFRACTION

Polarized neutron diffraction experiments on deuterated ammonium chromous Tutton salt at 1.6 K and 4.6 T, in two orientations of the crystal in the magnetic field, are reported. 216 (b/ /H) and 212 (a/ /H) data were refined using a model of the magnetization density involving only the spin distribution on the [Cr(OD2)6]2+ ion, extinction and six canting parameters, three for each data set, to give a goodness-of-fit of 1.4. The canting parameters describe the projection of the total ion spin moment onto the a, b and c* directions. The spin density corresponds closely to a 'spin hole' of 1.03(4) spins in the 3dx2-y2 orbital of this Jahn-Teller distorted Cr2+ ion. with a small but significant contribution from covalence from the coordinated water molecules, and spin polarization of both those molecules and the Cr(II) ion. The two water molecules attached by the long Cr-O(8) bonds behave differently from the four shorter bonded ligands, reflecting their lesser covalence in, respectively, spin occupied and unoccupied Cr(II) 3d obitals. The canting parameters, which correspond to angles of 24-degrees and 29-degrees for the two datasets, and further data on their field and temperature dependencies, are fitted well by a tetragonal crystal field model in which only variation in the spin-orbit coupling constant is important. This quantity has a value 17(5)% greater than for the free Cr2+ ion. Experiment and theory both confirm that orbital magnetization effects are important only in determining the canting of the magnetic moments and are otherwise small compared with the spin moment in this compound.