The porosity at 900 °C may be attributed to the degradation and oxidation of the Ni substrate. The XRD patterns in Fig. 5 indicates that the LaNiO3 layer was mostly eliminated above 850 °C [58] while the Ni was heavily oxidized. The oxidation or cracking of Ni may have degraded the quality of the PZT film. Thickening the HfO2 did not eliminate oxidation, suggesting that Ni foil may not be the best solution for such devices.