Structural and electronic properties of reduced magnesium titanates
Measurement of electrical resistivity in the magnesium titanate spinel system Mg2-xTiy+1+xO4, give rise to three types of electrical resistivity behaviour, in the composition range y=+3.25 (x=0.6) to y=+3.333 (x=0.5): (I) From room temperature to 100K a rapid non-linear increase in resistivity occurs with decreasing temperature. (II) Below 100K the resistivity decreases linearly with temperature. (III) For some samples below 50K a transition to zero resistance was observed. Type III behaviour was the most interesting, since there is, as yet, no conclusive evidence for the occurrence of superconductivity in the magnesium titanate spinel system. The zero resistance behaviour was very sensitive to composition and sample history, making reproducibility difficult. Powder x-ray diffraction patterns showed the spinel phase to contain a small amount of a second phase, with an x-ray diffraction pattern similar to MgTiO3, which has the ilmenite structure. Care in sample preparation increased phase purity but, did not lead to better reproducibility of the zero resistance behaviour. In addition, the zero resistance only lasted a few hours to a few days. The presence of low resistance, in some samples, and the apparent zero resistance is due to the overlap of the 3d energy levels of the titanium ions, which reside on the octahedral sites. Doping of the magnesium titanate spinels with M3+ cations, in an effort to increase the stability of the zero resistance behaviour, proved to be unsuccessful. Substitution of M3+ ions onto the octahedral sites appears to interfere with the overlap of the 3d energy levels of the titanium ions causing an increase in electrical resistivity. The deterioration of the zero resistance, with time, appears to be catalysed by air and moisture. Keeping the samples in dry, vacuum conditions allowed critical current behaviour to be measured in one sample. Magnetic susceptibility measurements showed no diamagnetic signal, which is necessary, along with the zero resistance and critical current measurements, to prove the existence of superconductivity.