hafnium

Hf. Atomic number 72. Group ^{4 transition} metal element with electron configuration [Xe] ^4f145d26s2 . Atomic weight 178.49(2). Natural isotopic abundances are ^174Hf 0.16(1)%, ^176Hf 5.26(7)%, ^177Hf 18.60(9)%, ^178Hf 27.28(7)%, ^179Hf 13.62(2)%, ^{and 180Hf} 35.08(16)%. Radioactive nuclides with mass numbers from 153 to 188 have been produced. G. Hevesy and D. Coster discovered Hf in zirconium mineral (zircon) in 1923 by X-ray spectroscopy. The element's name comes from "Hafnia," the Latin name for Copenhagen, Denmark, where the experiment that discovered it took place. The element's symbol is also derived from its name.
Abundance in the earth's crust: 3 ppm. Hafnium has no endogenous mineral, but is always present in the related element zirconium, zircon and baddeleyite. Zircon is the main mineral of hafnium, accounting for 1-4% of zirconium. The main resource countries are the same as zirconium resource countries, with Australia and South Africa accounting for 80% of the world's total reserves. Japan imports the entire amount of the metal. Separation from its congener zirconium is difficult, and Hevesy et al. succeeded by repeated recrystallization of the double salt with potassium fluoride or ammonium fluoride. Currently, it is separated by ion exchange chromatography and solvent extraction of the thiocyanato complex salt. Industrially, methods include solvent extraction and distillation from a KCl-AlCl ₃ molten salt solution of the tetrachloride. Historically, van Arkel and de Boer obtained the metal by reducing tetraiodide vapor over a tungsten filament, but it is now obtained by reduction of the tetrachloride with magnesium (Kroll process). The pure metal has a hexagonal crystal structure, a silvery-white luster, and is soft. Density is 13.310 g cm ^-3 (6.506 g cm ^-3 ) (both at 20 °C). Melting point is 2230 °C (1852 °C), boiling point is 5197 °C (4377 °C). First ionization energy is 654.1 kJ mol ^-1 (659.9 kJ mol ^-1 ). Atomic radius is 0.156 nm (0.159 nm). Ionic radius is 0.085 nm (six-coordinated) (0.086 nm). ( ) indicates zirconium. It is difficult to separate from zirconium because the radii are almost the same due to lanthanide contraction, and their chemical properties are very similar. It reacts with oxygen, nitrogen, and sulfur at high temperatures. It absorbs hydrogen at temperatures above 700 °C to form the compound Hf H _1.86 . When heated, it reacts with halogens to form tetrahalides. It usually has an oxidation number of 2 to 4. Its maximum oxidation number is 6. It is not affected by cold mineral acids or aqueous alkaline solutions. It does not react with water at room temperature.
Its use includes control rods in nuclear reactors due to its extremely large thermal neutron absorption cross section (105 barn, 1 barn = 10 ^-28 m ² compared to 0.185 barn for Zr). It is also used in super-strong heat-resistant alloys for the blades of jet engines and power-generating gas turbines, as a material for high-temperature ceramics, and as an oxygen and nitrogen getter. Hafnium carbide is used in plasma electrodes, high-voltage discharge tube electrode materials, cutting tools, etc. It is most widely used for superalloys. "Hafnium and its compounds" are "harmful substances the names, etc. of which should be notified" as defined in Article 57-2, Paragraph 1 of the Industrial Safety and Health Act. [CAS 7440-58-6]

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

Hf．原子番号72の元素．電子配置[Xe]4f ¹⁴5d²6s²の周期表4族遷移金属元素．原子量178.49(2)．天然同位体存在比は ¹⁷⁴Hf 0.16(1)％，¹⁷⁶Hf 5.26(7)％，¹⁷⁷Hf 18.60(9)％，¹⁷⁸Hf 27.28(7)％，¹⁷⁹Hf 13.62(2)％，¹⁸⁰Hf 35.08(16)％．質量数153～188までの放射性核種がつくられている．1923年，G. Hevesy(ヘベシー)とD. CosterがX線分光分析法によってジルコニウム鉱物(ジルコン)中から発見した．元素名は発見の実験が行われたデンマークのコペンハーゲンのラテン語名“Hafnia”から．元素記号も名称から．
地殻中の存在度3 ppm．固有の鉱物はなく，親縁元素のジルコニウムの鉱物，ジルコン，バッデレイ石中につねに存在する．ジルコンがハフニウムの主要鉱物でジルコニウムの1～4％ 存在する．主要資源国は，ジルコニウムの資源国と同じで，オーストラリア，南アフリカで世界の全埋蔵量の80％ を占める．日本は全量，金属を輸入している．同族のジルコニウムとの分離は困難で，Hevesyらは，フッ化カリウムまたはフッ化アンモニウムとの複塩の再結晶の繰り返しによって成功した．現在はイオン交換クロマトグラフィー，チオシアナト錯塩の溶媒抽出で分離する．工業的には，溶媒抽出法，四塩化物のKCl-AlCl₃溶融塩溶液からの蒸留などの方法がある．金属は，歴史的にはvan Arkelとde Boerによって四ヨウ化物の蒸気をタングステンフィラメント上で還元して得られたが，現在は四塩化物のマグネシウムによる還元で得られる(Kroll法)．純粋な金属は六方晶系構造で，銀白色光沢を示し，軟らかい．密度13.310 g cm^－3(6.506 g cm^－3)(ともに20 ℃)．融点2230 ℃(1852 ℃)，沸点5197 ℃(4377 ℃)．第一イオン化エネルギー654.1 kJ mol^－1(659.9 kJ mol^－1)．原子半径0.156 nm(0.159 nm)．イオン半径0.085 nm(六配位)(0.086 nm)．(　)内はジルコニウム．ジルコニウムとの分離が困難な理由は，ランタノイド収縮により半径がほぼ等しく，化学的性質もよく似ているためである．高温で酸素，窒素，硫黄と反応する．700 ℃ 以上で水素を吸蔵してHf H_1.86 の化合物をつくる．加熱するとハロゲンと反応して四ハロゲン化物をつくる．通常は酸化数2～4．最高酸化数6．冷鉱酸，アルカリ水溶液には侵されない．常温で水と反応しない．
用途は，熱中性子吸収断面積がいちじるしく大きい(Zrの0.185 barn に対して105 barn，1 barn ＝ 10^－28 m²)ので，原子炉の制御棒に用いられる．ジェットエンジンや発電用ガスタービンのブレードに用いられる超強力耐熱合金や，高温用セラミックの材料，酸素，窒素のゲッターなどとしても用いられる．炭化ハフニウムはプラズマ電極，高圧放電管電極材料，切削用工具などに使われる．超合金用の需要がもっとも多い．「ハフニウム及びその化合物」は，労働安全衛生法第57条2第1項に定める「名称等を通知すべき有害物」である．[CAS 7440-58-6]

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