Related compounds Molar mass 210.49 g/mol Melting point 2,758 °C Appearance off-white powder | Density 9.68 g/cm³ Formula HfO2 Boiling point 5,400 °C | |
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Hafnium(IV) oxide is the inorganic compound with the formula HfO2. Also known as hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of 5.3~5.7 eV. Hafnium dioxide is an intermediate in some processes that give hafnium metal.
Contents
Hafnium(IV) oxide is quite inert. It reacts with strong acids such as concentrated sulfuric acid and with strong bases. It dissolves slowly in hydrofluoric acid to give fluorohafnate anions. At elevated temperatures, it reacts with chlorine in the presence of graphite or carbon tetrachloride to give hafnium tetrachloride.
Structure
Hafnia adopts the same structure as zirconia (ZrO2). Unlike TiO2, which features six-coordinate Ti in all phases, zirconia and hafnia consists of seven-coordinate metal centres. A variety of crystalline phases have been experimentally observed, including cubic (Fm-3m), 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 recently observed in thin films of hafnia.
Thin films of hafnium oxides, used in modern semiconductor devices, are often deposited with an amorphous structure (commonly by atomic layer deposition). Possible benefits of the amorphous structure have led researchers to alloy hafnium oxide with silicon (forming hafnium silicates) or aluminium, which were found to increase the crystallization temperature of hafnium oxide.
Applications
Hafnia is used in optical coatings, and as a high-κ dielectric in DRAM capacitors and in advanced metal-oxide-semiconductor devices. Hafnium-based oxides were introduced by Intel in 2007 as a replacement for silicon oxide as a gate insulator in field-effect transistors. The advantage for transistors is its high dielectric constant: the dielectric constant of HfO2 is 4–6 times higher than that of SiO2. The dielectric constant and other properties depend on the deposition method, composition and microstructure of the material.
In recent years, hafnium oxide (as well as doped and oxygen-deficient hafnium oxide) attracts additional interest as a possible candidate for resistive-switching memories.
Because of its very high melting point, hafnia is also used as a refractory material in the insulation of such devices as thermocouples, where it can operate at temperatures up to 2500 °C.
Multilayered films of hafnium dioxide, silica, and other materials have been developed for use in passive cooling of buildings. The films reflect sunlight and radiate heat at wavelengths that pass through Earth's atmosphere, and can have temperatures several degrees cooler than surrounding materials under the same conditions.