Effect of Defects on the Elastic Properties of Wüstite

Wüstite $\left({\mathrm{Fe}}_x\mathrm{O}\right)$ has been studied under static high pressure in the composition range $x=0.95\mathrm{}–0.99$. ${\mathrm{Fe}}_{0.99}\mathrm{O}$ is substantially less compressible than ${\mathrm{Fe}}_x\mathrm{O}$ with $x<\mathrm{}0.98\mathrm{}$. Such observations provide the only direct evidence of a significant effect of defect concentration on the elastic properties and support the results of crystal-field modeling for ideal stoichiometric wüstite. The bulk moduli of ${\mathrm{Fe}}_x\mathrm{O}$ show a discontinuous variation at compositions that are within the predicted range for the defect clusters to form, a behavior that may be caused by an order-disorder transition of the defect structure.