Hafnium compounds

Hafnium compounds are compounds containing the element hafnium (Hf). Due to the lanthanide contraction, the ionic radius of hafnium(IV) (0.78 ångström) is almost the same as that of zirconium(IV) (0.79 angstroms). Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties. Hafnium and zirconium tend to occur together in nature and the similarity of their ionic radii makes their chemical separation rather difficult. Hafnium tends to form inorganic compounds in the oxidation state of +4. Halogens react with it to form hafnium tetrahalides. At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Some compounds of hafnium in lower oxidation states are known.

Hafnium(IV) fluoride (HfF₄) is a white crystalline powder. It has a monoclinic crystal structure, with space group C2/c (No.15), and lattice constants a = 1.17 nm, b = 0.986 nm and c = 0.764 nm. Hafnium(IV) chloride (HfCl₄) is also a white crystalline powder, with a monoclinic structure. It can be prepared in many ways:

Hafnium(IV) bromide (HfBr₄) is a colourless, diamagnetic moisture sensitive solid that sublimes in vacuum. It adopts a structure very similar to that of zirconium tetrabromide, featuring tetrahedral Hf centers, in contrast to the polymeric nature of hafnium tetrachloride. Hafnium(IV) iodide (HfI₄) is a red-orange, moisture sensitive, sublimable solid that is produced by heating a mixture of hafnium with excess iodine. It is an intermediate in the crystal bar process for producing hafnium metal. In this compound, the hafnium centers adopt octahedral coordination geometry. Like most binary metal halides, the compound is a polymeric. It is one-dimensional polymer consisting of chains of edge-shared bioctahedral Hf₂I₈ subunits, similar to the motif adopted by HfCl₄. The nonbridging iodide ligands have shorter bonds to Hf than the bridging iodide ligands.

Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium metal. They are volatile solids with polymeric structures. These tetrachlorides are precursors to various organohafnium compounds such as hafnocene dichloride and tetrabenzylhafnium.

Hafnium does form lower halides such as hafnium(III) iodide. Hafnium trihalides are strongly reducing compounds and as such do not have any aqueous chemistry.

The white hafnium(IV) oxide (HfO₂), also known as hafnium dioxide or hafnia, with a melting point of 2,812 °C and a boiling point of roughly 5,100 °C, is very similar to zirconia, but slightly more basic. It is an electrical insulator with a band gap of 5.3~5.7 eV.

Hafnium(IV) oxide typically adopts the same structure as zirconia (ZrO₂). Unlike TiO₂, which features six-coordinate Ti in all phases, zirconia and hafnia consist of seven-coordinate metal centres. A variety of other crystalline phases have been experimentally observed, including cubic fluorite (Fm3m), tetragonal (P4₂/nmc), monoclinic (P2₁/c) and orthorhombic (Pbca and Pnma). It is also known that hafnia may adopt two other orthorhombic metastable phases (space group Pca2₁ and Pmn2₁) over a wide range of pressures and temperatures, presumably being the sources of the ferroelectricity observed in thin films of hafnia.

Thin films of hafnium oxides deposited by atomic layer deposition are usually crystalline. Because semiconductor devices benefit from having amorphous films present, researchers have alloyed hafnium oxide with aluminum or silicon (forming hafnium silicates), which have a higher crystallization temperature than hafnium oxide.