Abstract In the CsNi 1− x Cd x F 3 system a domain of 12R-structure is obtained for 0.25 ⩽ x ⩽ 0.50. Neutron diffraction experiments have been performed on the composition CsNi 0.75Cd 0.25F 3, i.e. Cs 4Ni 3CdF 12. An ordering has been detected between Ni 2+ and Cd 2+. The Ni 2+ cations are located in octahedra sharing faces and forming linear trimers of composition Ni 3F 12. No magnetic order has been found down to 4.2 K. Magnetic measurements characterize ferromagnetic couplings within the trimers. Good agreement is obtained between experimental data and χ values calculated using a HDvV hamiltonian and an exchange constant J k = +9 K between adjacent Ni 2+ ions.