Hafnium(IV) oxide

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Hafnium(IV) oxide
Hafnium(IV) oxide structure
Hafnium(IV) oxide
Identifiers
CAS number 12055-23-1 YesY
PubChem 292779
ChemSpider 258363 YesY
Jmol-3D images Image 1
Properties
Molecular formula HfO2
Molar mass 210.49 g/mol
Appearance off-white powder
Density 9.68 g/cm3, solid
Melting point 2758 °C
Boiling point 5400 °C
Solubility in water insoluble
Hazards
EU Index Not listed
Flash point Non-flammable
Related compounds
Other cations Titanium(IV) oxide
Zirconium(IV) oxide
Related compounds Hafnium nitride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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.[1] Hafnium dioxide is an intermediate in some processes that give hafnium metal.

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[edit]

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, tetragonal, monoclinic and orthorhombic.[2]

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.[3]

Applications[edit]

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.[4] The advantage for transistors is its high dielectric constant: The dielectric constant of HfO2 is 4-6 times higher than that of SiO2.[5] 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 defficient hafnium oxide) attracts additional interest as a possible candidate for resistive switching memories.[6]

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.[7]

References[edit]