Editing Hafnium (section)

==Chemical compounds==
{{Main|Hafnium compounds}}
Due to the [[lanthanide contraction]], the [[ionic radius]] of hafnium(IV) (0.78&nbsp;ångström) is almost the same  as that of  [[zirconium]](IV) (0.79&nbsp;[[angstrom]]s).<ref name="lanl72" /> Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties.<ref name="lanl72" /> 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 chemistry|inorganic compounds]] in the oxidation state of +4. [[Halogen]]s react with it to form hafnium tetrahalides.<ref name="lanl72" /> At higher temperatures, hafnium reacts with [[oxygen]], [[nitrogen]], [[carbon]], [[boron]], [[sulfur]], and [[silicon]].<ref name="lanl72" /> Some hafnium compounds in lower oxidation states are known.<ref>{{Greenwood&Earnshaw2nd|pages=971–975}}</ref>

[[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.<ref name="Holl" /> These tetrachlorides are precursors to various [[Organozirconium chemistry|organohafnium compounds]] such as hafnocene dichloride and tetrabenzylhafnium.

The white [[hafnium oxide]] (HfO<sub>2</sub>), with a melting point of {{convert|2,812|C|K F}} and a boiling point of roughly {{convert|5,100|C|K F|sigfig=2}}, is very similar to [[zirconia]], but slightly more basic.<ref name="Holl" /> [[Hafnium carbide]] is the most [[refractory]] [[binary compound]] known, with a melting point over {{convert|3,890|C|K F|0}}, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of {{convert|3,310|C|K F|0}}.<ref name="lanl72">{{cite web|url = http://periodic.lanl.gov/72.shtml |title = Los Alamos National Laboratory – Hafnium|access-date=2008-09-10}}</ref> This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures. The mixed carbide [[tantalum hafnium carbide]] ({{chem|Ta|4|HfC|5}}) possesses the highest melting point of any currently known compound, {{cvt|4263|K|C F}}.<ref>{{cite journal|title=Researches on Systems with Carbides at High Melting Point and Contributions to the Problem of Carbon Fusion|journal=Z. Tech. Phys.|author1=Agte, C. |author2=Alterthum, H. |name-list-style=amp | volume= 11|year= 1930|pages=182–191}}</ref> Recent supercomputer simulations suggest a hafnium alloy with a melting point of {{convert|4,400|K|C F|0}}.<ref name="hong2015">{{cite journal|last1=Hong|first1=Qi-Jun|last2=van de Walle|first2=Axel|title=Prediction of the material with highest known melting point from ab initio molecular dynamics calculations|journal=Phys. Rev. B|date=2015|volume=92|issue=2|page=020104|doi=10.1103/PhysRevB.92.020104|bibcode=2015PhRvB..92b0104H|doi-access=free}}</ref>