Transition Metal Oxides and the Effect of Stoichiometry

Plastic deformation of single crystals of the transition metal oxides Mi. O with the rock-salt structure has been studied for M = Mn [43], Fe [44], Co [45-48], and 

Ni [49,50]. The maximum value of X varies from 0.001 for Ni and 0.013 for Co to 0.15 for Fe and Mn. Fei 0 is unstable below 570 °C, and cannot therefore be deformed plastically at low temperatures. There have been no reports of low-temperature deformation of Mni O, but there is no reason to believe that this should not be possible. On the other hand, plastic deformation on the 110 (110) slip system occurs readily in Coi 0 and Nii 0 at low temperatures [46,48-50) in fact Coi jcO has even been deformed at 4.2 K [48]. Care must be taken in the case of Coi to cool 

It is a different story at high temperatures, where there have been a number of studies of the influence of stoichiometry on the creep rate [43-45,47,49] or the flow stress [43, 47]. Typically, for a given applied stress, the creep rate increases with increasing po, that is, as the crystals become more stoichiometric (i.e., as x decreases). However, the behavior is less straightforward in Mni-j O at temperatures between 1000 and 1400 °C, the flow stress first increases with increasing po and then decreases with further increase in po, by a factor of about 2 in each case. 

The creep behavior is often analyzed in terms of the phenomenological equation 

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