Diffraction de neutrons dans CoCl2, 6D2O determination de ses structures cristallographique et magnetique

We have performed neutron diffraction experiments on a CoCl₂, 6D₂O monocrystal. We show that a crystallographic transition from the C 2/m monoclinic structure to the P $\text{1}$ triclinic structure occurs at T = Tmt = 73.5K., together with a twinning of the monocrystal. We describe the two possible twins (four types of domains) related to the loss of the initial monoclinic symmetry, say of the elements 2 and m : either m becomes twin plane (Albite twinning) or 2 becomes twin axis (Pericline twinning). Each domain has an antiferromagnetic structure P 2a T, with an easy magnetization axis tilted from the ac plane by an angle φ = (30 ± 4)°, its component on this plane making an angle φ = (3.5 ± 2)° with the c axis.