The thermodynamics of the titanium + oxygen system: an isothermal gravimetric study of the composition range Ti3O5 to TiO2 at 1304 K

Abstract

The compositions of titanium oxide samples at 1304 K have been studied by, as far as possible, equilibrating them with (H2 + H aO) gas buffer mixtures of various compositions. The high precision with which the mass, and therefore the sample’s composition, was determined reveals that complete equilibrium is rarely attained, even after several days in a constant ambient: at most compositions there is gross hysteresis. Two composite hysteresis loops exist in the regions occupied by the (121) and (132) crystallographic shear structure families. Between these loops the oxidation and reduction paths are very nearly coincident: this is the region in which a change in composition can be accommodated by varying only the orientation and not the number of crystallographic shear planes. A third hysteresis loop is believed to exist between the (132) loop and the composition of slightly reduced rutile. A plausible explanation of the behaviour is developed. All the observations appear to be consistent with those from parallel electron microscope studies of reduced ‘rutile’. Nucleation of the product phase appears to be the most important factor affecting the behaviour in the region Ti30 5 to Ti40 7, as would be expected from a consideration of their structures. The differences between the behaviour observed by us and that reported by other workers is believed to be only apparent: it results from (a) the higher precision of our measurements, and (b) the much higher density and number of data points we have acquired. Various thermodynamic data are calculated for the many higher titanium oxides on the basis of a reasonable assumption for deducing approximate equilibrium oxygen potentials from the observed values. Most of these have not previously been available. By incorporating Zador’s (1967) data for T i0 199 to T i0 1999 we deduce for ^Ti30 5+ i 0 2 = TiOa (rutile), AG® (1304 K) = — 78.3 kj mol-1 compared with — 79.4 kj mol-1 in the JANAF tables. The stabilities of the Tiw0 2w_1 phases with respect to mixtures of the (n— 1) and (w+1) neighbours are seen to be extremely small ( —104 J/mol of Ti70 13, i.e. — 5.2 J/mol of atoms), and to decrease with increasing n. 1