Traditionally, metal hydrides were considered to be hydrogen storage materials. However, they are also used in hydrogen sensors and switchable mirrors. They have a complex phase diagram. These transitions determine the performance of the material in real world applications.

Hafnium hydride is a tetravalent transition metal. It is one of the most abundant and lightest elements in the periodic table. It is a promising optical hydrogen-sensing material. It features a sensing range that reaches over six orders of magnitude in hydrogen pressure. It is used in atomic energy and aerospace industries. It is commonly available in submicron and nano-powder forms. It is also used as a neutron control material for fast reactors.

In the solid state, the metal atoms form a bcc lattice. In a face-centered cubic (fcc) lattice, the H atoms take four different 2e sites. The fcc - fct transition occurs at PH2 = 10+1 Pa. This is followed by a significant reduction in volume. The optical transmission is unaffected.

Hafnium hydride has the ability to react with air to form a protective film. This protection makes it resistive to concentrated alkalis and acids. It can be easily crushed to very fine particles.

In the laboratory, hafnium is often studied in zircon. This mineral is the predominant host of hafnium. It has extremely low Lu/Hf ratios. The isotopic composition of hafnium has been relatively stable for the past 20 million years. It is believed that this stability was triggered by increased isolation of the central Pacific deep water.

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