Interrelation between the Solid-State Synthesis Conditions and Magnetic Properties of the NiCr2O4 Spinel

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 Neel temperatures. We propose that the Neel temperature value serves as a measure of a departure of a composition from the nominal NiCr2O4. The lowest Neel temperature among our series was T-N = 63 K which we consider the closest to the intrinsic quantity of the NiCr2O4 compound.