Filling the gap: electrochemical access to the missing group IV HfO-based rocksalt compound.

Rocksalt structured group IV transition metal carbides and nitrides (δ-MeX) are well-known materials and are used for various applications. However, the corresponding group IV transition metal monoxides (δ-MeO) are either rarely researched, or, in the case of hafnium, their existence is unknown. In this investigation, electrical loading of yttria-stabilised hafnia was conducted in an inert atmosphere (argon) to achieve electrochemical reduction. Resistive heating of the sample was maintained as the furnace cooled down such that on removing the current, the sample was rapidly quenched, avoiding reoxidation or diffusion-limited phase transformations. XRD, SEM, EDS, and Raman spectroscopy showed that defective rocksalt structure δ-(Y)HfO with a lattice constant of 0.4527 nm was formed and retained at room temperature for the first time. Yttria was rejected from the hafnia-based phase during reduction, leaving a relatively low concentration of yttrium in solid solution. The new compound has a metallic-gold-brown colour and conducts electricity at room temperature (∼0.67 Ω cm). Furthermore, the compound fills the gap in the non-radioactive group IV transition metal carbide, nitride, monoxide rocksalt structure series.