On the poisson ratio and XRD determination of strain in thin films of Ce_0.8Gd_0.2O_1.9

XRD measurement of the room temperature in-plane and out-of-plane d-spacings of the (422) diffraction peak of 11 thin film samples of fluorite Ce_0.8Gd_0.2O_1.9 demonstrates that the zz and xx (=yy) components of the strain tensor for this material are not related via a constant, i.e. the Poisson ratio, as is the case for elastic materials. Rather, these strains are independent. We attribute this behavior to the inelastic character of Ce_0.8Gd_0.2O_1.9 deriving from the chemical strain effect, i.e. the lability of point defect-containing complexes under stress. Chemical strain is dependent on the thermal and mechanical history of the film, and above 200 C, is no longer observed, being transformed into elastic strain and stress. This transformation may compromise the mechanical stability of Ce_0.8Gd_0.2O_1.9 containing devices, which must operate over a broad temperature range. Measurements analogous to those described here can assist in predicting the magnitude of such an effect.