Wüstite (FexO), where x ranged from 0.95 to 0.99, was studied under static high pressures. It was found that Fe0.99O was much less compressible than Fe0.98O. Such observations constituted the only direct evidence for a significant effect of defect concentration upon elastic properties, and supported the results of crystal-field modelling for ideal stoichiometric wüstite. The bulk moduli of FexO exhibited a discontinuous variation at compositions that were within the predicted range at which defect clusters formed. It was suggested that this behavior might be caused by an order-disorder transition of the defect structure.

Effect of Defects on the Elastic Properties of Wüstite. J.Zhang: Physical Review Letters, 2000, 84[3], 507-10