Isobaric hysteresis loop

FIGURE 8 Isobaric hysteresis loop of the TbO system for different heating and cooling processes (after Eyring, 1979). 

Fig. 7. Isobaric hysteresis loop for (a) the reaction f Tb70i2- Tb20j+ O fe) (dashed loop is the intrinsic hysteresis envelope), and (b) the reaction n TbiiO20 Tb7Oi2 +n O2 (after Lowe and Eyring 1975).

The hysteresis loop of the oxygen content of the PrO and TbO is a obvious peculiar characteristic of the lanthanide higher oxides, but the hysteresis of the oxygen content of the CeO is difficult to observe directly in the isobaric curves because... 

A complete isobar at 159 mm. of Hg shows the normal breaks at CmOi.8 and CmOi.7 in the reduction half of the cycle but exhibits an extreme hysteresis loop in the oxidation part of the cycle which did not close until a composition near Cm02 was reached. This behavior, to a lesser degree, is exhibited by the rare earth oxide systems discussed above. 

Very extensive temperature-pressure-composition observations have been made on the rare earth higher oxides as is clear from the discussion of the previous sections. Certain observations should be made explicit. There are no instances of classical behavior, i.e., none of the single phases are line phases but have at least some range of composition and none of the two-phase isobaric transformations occur at a fixed temperature but rather over a range of temperatures. There is also clearly intrinsic hysteresis in all two-phase regions. Furthermore the hysteresis loops are not symmetric, but rather are skewed such as to give a retarded and almost linear approach when a phase of lower symmetry is being formed. This may occur either in oxidation or reduction is reproducible and intrinsic - it has been termed pseudophase formation (Hyde et al., 1966). 

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