SELECTIVITY AND COMPOSITIONAL DEPENDENCE OF RESPONSE OF GAS-SENSITIVERESISTORS .4. PROPERTIES OF SOME RUTILE SOLID-SOLUTION COMPOUNDS

Solid solutions based on the rutile structure offer the possibility of independently varying the cation composition and dopant density in th e lattice and hence exploring the effects that these have upon the ele ctrical response to reactive gases. Thus, solid solutions of CrNbO4 (p -type response) with Nb-doped TiO2 and FeNbO4 (n-type response) have b een investigated. The resistivities and activation energies increased in the order FeNbO4 < CrNbO4 < Ti0.975Nb0.025O2, which is explained in terms of the emptying and decreasing energy of the e(g) and t(2g) sta tes within the bandgap. The n-p transitions can similarly be discussed in terms of the localised occupancy of these levels and the energy of the reactive oxygen surface state with respect to them. Sensitivity t o CO decreased in the order Ti0.975Nb0.025O2 > CrNbO4, FeNbO4 and this was postulated to be due either to Nb5+ surface segregation or to the narrowing of the conduction bandwidth leading to valence trapping of electrons. An enhanced gas response at T < 300 degrees C was found for the compounds after they had been heated to ca. 700 degrees C in air. This was attributed to the desorption of surface hydroxy, carbonyl an d lattice oxygen species to produce a vacancy around which the surface reconstructed. It is proposed that ionosorption of oxygen at this sit e produced a species able to react with CO and produce a resistance ch ange at low temperatures.