Flash-induced defects in single-crystal 8YSZ characterized by TEM, XRD, and Raman spectroscopy

We present direct evidence, for the first time, for the flash-induced generation of crystal defects in single-crystal cubic zirconia. The defects are characterized by multiple techniques. The crystals were flashed and then cooled down to ambient temperature in Ar atmosphere to preserve the electronic conductivity from the steady state of flash. Transmission electron microscopy revealed colonies of defects. The crystal structure, the non-stoichiometry, and the electronic structure of oxygen ions within these colonies were characterized. They consist of oxygen-depleted compositions. Selected area electron diffraction revealed the structure of these zirconia suboxides to be epitaxially coherent with the parent cubic structure but with a smaller lattice parameter. Electron energy loss spectroscopy spectra showed the peak near 25 mV to shift toward metallic zirconium. The electronic conductivity of the flashed cubic zirconia is attributed to this suboxide phase. The phase also gave rise to new peaks in Raman spectroscopy. A surprising finding was the presence of a distinct, similar to 30 nm thick layer of the suboxide on the surface of the crystal. The surface layer showed a high degree of oxygen deficiency. It was also epitaxial, but with an even smaller lattice parameter than the defect colonies underneath the surface. Mechanical polishing of the crystal to remove this surface layer gave a broad view of the interconnected network of defects across the entire specimen; the length scale of this network was about 0.2 mm. Recent work, where an overlay of a magnetic field caused the flash to migrate from a flashing surface into a free-standing workpiece, suggests the presence of evanescent plasma, which may have a connection to this surface layer.