A low-temperature X-ray study of the rhombohedral deformation of nickel oxide containing magnesium

X-ray studies of polycrystalline Ni_1−xMg_xO solid solutions with 0 ≤ x ≤ 0.13 have been carried out. The effects of temperature and of Mg²⁺ concentration upon the parameter of the pseudocubic lattice, a_c, and the rhombohedral angle α_rh, have been investigated. At temperatures below the Néel temperature it has been found that as the Mg²⁺ concentration is increased up to x = 0.01, a_c and α_rh increase. With x > 0.01, the changes of a_c follow Wegard's law, and α_rh decreases.