SOLID-SOLUTION FORMATION, ELECTRICAL-PROPERTIES AND ZERO-RESISTANCE BEHAVIOR IN THE SPINEL SYSTEM MG2TIO4-MGTI2O4

A range of solid solutions in the system Mg2-yTi1+yO4 with average Ti valence x, x = (4 + 2y)/(1 + y), have been prepared for 3.2 < x < 4.0. The lattice parameter varied smoothly with composition, increasing from a value of 8.443 angstrom for x = 4.0 to 8.484 angstrom for x = 3.2. Room-temperature resistance was found to be a minimum for compositions between x = 3.25 and 3.3. On cooling from room temperature to 100 K, resistivity rose by several orders of magnitude for compositions with x > 3.3. For compositions with x < 3.25, the temperature dependence of resistivity was much less, with a rise of only one order of magnitude between 300 and 20 K. For compositions between x = 3.25 and 3.3, transitions from semiconducting to zero-resistance behaviour were observed at ca. 50 K. This zero-resistance behaviour was lost on standing, except when the sample was stored in vacuum.