MICROSTRUCTURAL INVESTIGATIONS OF REDUCED MAGNESIUM TITANATE SPINELS WHICH HAVE SHOWN ANOMALOUS RESISTANCE BEHAVIOR

Samples from the reduced magnesium titanate spinel system ((Mg1+yTi2-y O4)-O-x+) have been examined using selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) imaging. Unusual microstructural features are observed in samples in the region of the system (i.e., where x, the average Ti valency, lies between 3.26<x<3. 33) where transitions to zero electrical resistance on cooling below 5 0 K are reported. These features do not occur in samples from outside this compositional range. Lamellar features with wavelengths up to 750 nm are observed, and associated electron diffraction patterns demonst rate streaking of diffraction spots around the direct beam. The presen ce of streaks, rather than divergent spots, indicates fanning of the s tructure across the lamellar boundaries and HREM images further indica te that interlamellar boundaries are coherent. Linear streaking parall el to < 111 > observed in some crystals may represent the same feature viewed perpendicular to a lamellar boundary. These observations are c onsistent with exsolution of a single high temperature phase to two co -existing spinels of slightly different compositions. The observation of coherent lamellar boundaries and streaking of electron diffraction spots are consistent with spinodal decomposition as the mechanism of e xsolution. Fine-scale lamellar structures parallel to {111} are also o bserved, which are texturally distinct from those described above. Str eaking of electron diffraction spots parallel to < 110 > may be associ ated with these features. These lamellae probably represent intimate s pinel-spinelloid intergrowths, rather than ordering of cation vacancie s or another process. These microstructural features appear to relate to the critical resistivity transition, as they have only been observe d in samples close to the compositions which have displayed such elect rical behaviour. Similar studies on samples away from the region of in terest fail to show these textures. A relationship between the solvus and the zero electrical resistance behaviour is inferred. In the simpl est case, the zero electrical resistance material may represent a meta stable homogeneous high temperature spinel from above the solvus; alte rnatively, the electrical behaviour may derive from the strained inter lamellar boundaries. Fine-scale linear features parallel to {111}, oft en associated with the interlamellar boundaries, are interpreted as fi ne spinelloid domains; the electrical behaviour may also relate in som e way to these features. It should be noted that the presence of no si ngle microstructural feature correlates directly with the occurrence o f this behaviour, nor does the loss of any feature correlate with degr adation; however, it is also clear that the microstructure is continua lly developing as the sample ages at room temperature. Different degra dation rates in air and vacuum are explained in terms of the continuin g exsolution being slowed by the development of spontaneous strain at interlamellar boundaries. Chemical attack by air releases interlamella r strain and allows exsolution to progress at full speed.