Chalkogenides of the transition elements. VI. X-Ray, neutron, and magnetic investigation of the spinels Co3O4, NiCo2O4, Co3S4, and NiCo2S4

Abstract

The crystal structures of the spinels Co3O4, NiCo2O4, Co3S4, and NiCo2S4 were refined from X-ray and neutron powder data. Their lattice parameters at room temperature and the positional parameters of the oxygen and sulfur atoms were: Co3O4, 8.0835 ± 6 Å, 0.2640 ± 8; NiCo2O4, 8.114 ± 14 Å, 0.2583 ± 34; Co3S4, 9.4055 ± 12 Å, 0.2591 ± 5; NiCo2S4, 9.3872 ± 7 Å, 0.2591 ± 3. The thermal stabilities of the two oxides in air and in oxygen at 1 atm were investigated by thermogravimetric analysis and differential thermal analysis, and the magnetization of NiCo2O4 was measured down to 4.2 °K in fields up to 11.70 kOe. From neutron diffraction NiCo2O4. was found to be inverse, while NiCo2S4 was shown to be normal. The results of the magnetization measurements and the neutron-diffraction patterns at 111 and 393 °K were found to be equally consistent with the magnetic structure proposed for NiCo2O4 by Blasse, Co2+[Ni3+Co3+]O4 (Co2+ in a high-spin and Ni3+ andCo3+ in low-spin states), and with Co3+[Ni2+Co3+]O4 (Co3+ in 8(a) in a high-spin and Co3+ in 16(d) in a low-spin state). The sublattice magnetizations were not completely aligned even at 4.2 °K; the net magnetic moment derived from the magnetization measurements was only 1.25 μB, which is lower than the value of 2 μB expected from either model. At 111 °K the moments of the ions in the tetrahedral and octahedral sites were estimated to be 1.9 ± 0.2 and −0.5 ± 0.1 bohr magnetons respectively.