The synthesis of the NiCr₂O₄ compound with the spinel structure via the high-temperature solid-state reaction leads to different deviations of the cationic composition from the nominal depending on the atmosphere in the furnace chamber. The samples prepared from the same starting NiO and Cr₂O₃ compounds but in different atmospheres differ in phase composition and orbital and spin ordering temperatures. We find that a common route of synthesis in the air and a possible presence of the Ni₂O₃ in initial NiO lead to the incorporation of the Ni³⁺ ions into the octahedral sites regularly occupied by the Cr³⁺ ions. This results in a decrease in the orbital ordering and an increase in the Nèel temperatures. We propose that the Nèel temperature value serves as a measure of a departure of a composition from the nominal NiCr₂O₄. The lowest Nèel temperature among our series was T_N = 63 K which we consider the closest to the intrinsic quantity of the NiCr₂O₄ compound.

We present the new synthesis root of spinel-structure FeCr₂O₄ and its single crystal growth by the optical floating zone method, ensuring its single phase and near-ideal composition. The advantage of the proposed synthesis method is the creation of the reducing atmosphere in the oven needed for preserving the Fe²⁺ oxidation state via decomposition of the iron (II) oxalate FeC₂O₄ used as one of the initial components. The occurrence of the Fe³⁺ ions in the obtained polycrystalline samples as well as grown single crystals was carefully monitored by means of Mössbauer spectroscopy. Magnetic susceptibility and heat capacity temperature dependences reveal a sequence of the structural (138 K) and magnetic (at 65 K and 38 K) phase transition characteristics for the FeCr₂O₄ compound.

Atomic ions and free electrons embedded in condensed helium play important role in modern research on quantum fluids and solids. Here we present experimental and theoretical study of Ba⁺ cations immersed in superfluid He. We observe laser-induced fluorescence of Ba⁺ injected into liquid He from the plasma of radio frequency discharge in He-Ba gas mixture. The structure of trapping site is studied theoretically in the frame of the atomic bubble model making use of ab initio Ba⁺ - He pair potentials available in the literature. Calculated spectra of the ion are compared with the results of the experiment.