Frustrated magnetic structures: II. Antiferromagnetic structure of the ordered modified pyrochlore NH4FeIIFeIIIF6 at 4.2 K

The nuclear and magnetic structures of the ordered modified pyrochlore NH₄Fe²⁺Fe³⁺F₆ were solved at 4.2 K. NH⁺₄ tetrahedra are almost regular ($\text{〈}\text{N}\text{?}\text{H}\text{〉 =}\text{0.924}\text{A}\text{?}$) and the fluorinated skeleton at 4.2 K does not exhibit large deviations from the room-temperature positions (R_N = 0.042). Below T_N = 19 ± 1 K the magnetic and nuclear cells are identical. The strictly antiferromagnetic arrangement of Fe²⁺ spins (μ = 3.12(9)μB) along the [0 1 0] direction contrasts with the canted antiferromagnetic sublattice of Fe³⁺ ions (μ = 4.13 [8] μB) which is quasi-orthogonal (α = 76°) to the Fe²⁺ sublattice (R_mag = 0.078). The main component of Fe³⁺ moments lies along a. The antiferromagnetism of NH₄Fe³⁺Fe³⁺F₆ is compared to the spin-glass like behaviour which occurs when M²⁺ and M³⁺ ions are randomly distributed on the cationic sites.