Crystal growth and cation order of Ni 3− x Co x B 2 O 6 oxyborates

A series of single crystals of Ni 3− x Co x B 2 O 6 compounds with the kotoite structure and with different concentrations of transition metal ions ( x = 0; 0.19; 0.6; 0.93 and 2) were obtained. The lattice parameters and atomic coordinates were determined using X-ray diffraction. The theoretical calculations using the WIEN2k package predict that nickel ions tend to occupy the 4f crystallographic position, while cobalt ions tend to occupy the 2a crystallographic position. The study of the diffuse scattering spectra and comparison of the Racah parameters for the compounds Ni 3 B 2 O 6 and Co 2 NiB 2 O 6 provides experimental evidence that nickel ions occupy crystallographic position 4f.