On the poisson ratio of thin films of Ce_0.8Gd_0.2O_1.9 II: Strain-dependence

We have adapted the X-ray diffraction technique of measuring crystal lattice parameters as a function of sample orientation in order to investigate the Poisson ratio (ν) of thin films of Ce_0.8Gd_0.2O_1.9 with different degrees of in-plane compressive strain. Since Gd-doped ceria is mechanically inelastic, the unstrained lattice constant is not well-defined. Therefore in order to determine the Poisson ratio, more than one strain state of the same sample must be characterized. We have accomplished this with a home-built, diffractometer-compatible sample-bender. Using this device, we find that the Poisson ratio of as-deposited thin films of Gd-doped ceria is a decreasing function of the in-plane compressive strain. At total strain ≈ −0.9 %, ν is > 0.45, indicating that the material tends to preserve volume. Increase of the in-plane strain to ≈ −2 % produces a monotonic decrease in the Poisson ratio to ~0.2. Such low values are indicative of reduced volume. We explain this behavior by noting earlier results from our group suggesting that, at temperatures below 200 °C and at low strain values, Ce_0.8Gd_0.2O_1.9 responds to non-isotropic mechanical stress by reorienting the locally distorted Ce_Ce-V_O –O₇ units. At higher strain values this mechanism is apparently no longer effective. Measurement of the Poisson ratio of inelastic thin films by this technique is straightforward and not diffractometer-dependent. It therefore represents a practical method for ceramic thin film characterization.