Magnetic susceptibility studies in the dual hydrous oxide system NiO–Al_O_
Richardson, James Thomas, 1928-
Milligan, W. O.
Master of Arts
Nickelous oxide, NiO, as normally prepared, consists of crystals larger than the colloidal range of particle size, and exhibits a transformation at about 250°C, detectable by back reflection x-ray diffraction, magnetic peaks in neutron diffraction patterns, and the appearance of a Neel point in magnetic susceptibility measurements. Below 250°C, NiO is rhombohedral and antiferromagnetic, whereas above 250°C, it is cubic and paramagnetic. In samples of NiO consisting of extremely finely-divided crystals: or consisting of aggregates of small crystallites existing in a plate-like or rod-like type of morphology, the reduction in the average number of next nearest magnetic neighbors would result in an attenuated magnetic interaction: thereby preventing the occurrence of antiferromagnetism, and such crystals would exhibit a typical paramagnetic behavior. Crystals of Ni0 of extremely small size have been prepared by the heat-treatment of hydrous nickelous hydroxide gel at temperature levels in the range 300-1300°C. A sample prepared at 300°C, exhibits a paramagnetic type of susceptibility-temperature curve. As the heat-treatment temperature level is increased, the crystal size increases, the average number of magnetic neighbors increase, and the magnetic properties change from strong paramagnetism (300°C), through temperature independent paramagnetism (1200°C), to antiferromagnetism (1300°C). The Ni0 samples prepared from the hydroxide below 1300°C are all cubic and paramagnetic. The cubic structure is stabilized at the temperatures below the normal Neel point (250°C) by the presence of adsorbed trace amounts of sodium, or possibly silica. Heating to 1300°C expels the impurity, and the sample becomes rhombohedral and antiferromagnetic. Samples prepared by the heating of hydrated nickelous nitrate crystals do not contain the stabilizing impurities, and were found to consist of rhombohedral crystals at room temperature: and exhibit a normal Neel point at 250°C, in agreement with the results of other investigators.