Zirconium dioxide coating on particles of titanium dioxide pigments, as a refractory material, in insulation, abrasives and enamels. Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has the ability to allow oxygen ions to move freely through the crystal structure at high temperatures. This high ionic conductivity (and a low electronic conductivity) makes it one of the most useful electroceramics. Zirconium dioxide is also used as the solid electrolyte in electrochromic devices. Zirconia is a precursor to the electroceramic lead zirconate titanate ("PZT"), which is a high-K dielectric, which is found in myriad components. The very low
Group 4 element to which Coster and Hevesy were guided by Bohr's prediction that hafnium would be a transition metal rather than a rare earth element. While titanium and zirconium, as relatively abundant elements, were discovered in the late 18th century, it took until 1923 for hafnium to be identified. This was only partly due to hafnium's relative scarcity. The chemical similarity between zirconium and hafnium made a separation difficult and, without knowing what to look for, hafnium was left undiscovered, although all samples of zirconium, and all of its compounds, used by chemists for over two centuries contained significant amounts of hafnium.