hafnium hydride is a refractory, face-centered cubic or tetragonal crystal that is formed by reacting hafnium with hydrogen gas. It is typically produced as an intermediate in the process of making hafnium powder from mass hafnium metal.

The structure of most transition metal hydrides is CaF2 (Fm-3m space group) where the metal atoms form a face-centered cubic lattice while the hydrogen atoms occupy the tetrahedron centers. The group IVB dihydrides TiH2, ZrH2 and HfH2 display a much more basic face-centered tetragonal (fct) cell structure of the I4/mmm space group and exhibit fascinating superconducting properties6,7,8.

Stable phases of HfH2 under pressure

A comprehensive investigation of the pressure-induced phase transformations and properties of a unique, high-Tc superconductor, HfH2, under high pressure has been performed using structural searches, first-principle calculations and X-ray photoelectron spectroscopy. The results indicate that a metastable stochiometric HfH9 is formed by the stabilization of a unique, H12 tube-like structure in a HfH3 framework under 200 GPa.

Hydrogen-rich hydrides are important materials in search of exotic properties such as high-Tc superconductivity3,4 and the existence of diverse hydrogen chemistries at high pressure is key to these efforts5,6. This study provides a new insight into the chemistry of hydrogen-rich hydrides, which will lead to better understanding of their structural and electronic properties under high pressure.

The bonding nature of HfH2 is studied by electron localization function (ELF), difference charge density and Bader charge analyses, which reveal that the ionic bond is formed between Hf and H atoms and that HfH2 is classified as an ionic crystal with the charges transferring from Hf to H atoms. These findings are complemented by the structural studies of I4/mmm, Cmma and P21/m at 100, 200 and 250 GPa, which show that the HfH2 can be characterized as a stable crystalline phase under these conditions.

tags: Calcium nitride powder