zirconium

Zr. Atomic number 40. Electron configuration [Kr] ^4d25s2 . Periodic table group ⁴ transition metal element. Atomic weight 91.224(2). Naturally occurring stable isotopes exist with mass numbers 90 (51.45(40)%), 91 (11.22(5)%), 92 (17.15(8)%), 94 (17.38(28)%), and 96 (2.80(9)%), as well as radioactive isotopes with mass numbers 78-110. In 1789, German MH Klaproth identified zircon as an oxide from the mineral zircon _ZrSiO4 , and in 1824, Swedish JJ Berzelius isolated it as a metal. Zircon comes in a variety of colors ranging from red to black, but most are golden-yellow, and were known in ancient India and Persia. The mineral's name comes from the Arabic word zargûn, which means golden-yellow. The element's name was proposed by Sir Humphry Davy in 1808, based on the name of the mineral. Udagawa Yoan called it girconium in his book "Semi Kaisou" published in 1837. The main minerals include zircon, as well as baddeleyite ( _ZrO2) .
Abundance in the earth's crust is 100 ppm. Recoverable reserves are in the order of South Africa (37%), Australia (27%), Ukraine (10%), India, and the United States (each 9%). Smelting is done by the Kroll process, in which the oxide ore is reacted with carbon and chlorine in a red-hot state to convert it into crude chloride _ZrCl4 , which is then reduced with metallic magnesium in an inert atmosphere. Refining is done by pyrolysis on _{a ZrI4} filament (> 1300 °C, van Arkel-de Boer process). Separation from hafnium, which has almost the same ionic and metallic radii and almost identical chemical behavior, is done at the tetrachloride stage by solvent extraction. Metallic zirconium is silvery white, softer than its cousin titanium, and has two transformations, α and β. At room temperature, it is in the α phase with a hexagonal close-packed lattice, and changes to the β phase with a body-centered cubic lattice at 862 °C. Its melting point is 1852 °C and its boiling point is 4377 °C. Density: 6.506 g cm ^-3 (20 °C). First ionization energy: 6.84 eV. Standard electrode potential: Zr4 ⁺ /Zr-1.55 V. Metallic bond radius: 0.160 nm. ^ZrⅣ (hexacoordination): 0.072 nm. It is not affected by hot concentrated sulfuric acid, hot concentrated hydrochloric acid, or hot concentrated nitric acid, and its acid resistance is superior to that of titanium and stainless steel. It is also resistant to alkali, and can withstand concentrated NaOH aqueous solutions up to the boiling point and molten NaOH up to 1000 °C. It dissolves in hydrofluoric acid and aqua regia. It can be attacked by hydrofluoric acid even in dilute solutions (up to 0.1%). In air at room temperature, the oxide film that gradually forms on the surface of the solid protects the interior, making it highly corrosion-resistant, but the oxidation reaction is exothermic, and powders with a large surface area may ignite. At high temperatures, it reacts with oxygen and halogens (X) to produce Zr ^Ⅳ O ₂ (colorless) and Zr ^Ⅳ X ₄ (colorless, only when X = I is yellow). It can exhibit oxidation numbers from 1 to 4, but the predominant oxidation number is 4, and the only oxidation number known is [Zr ^Ⅲ (H ₂ O) ₆ ] ³⁺ , which is unstable except for 4, especially in aqueous solutions. Zr ^Ⅱ X ₂ and Zr ^Ⅲ X ₃ are blue-black to dark green solids. Zr ⁴⁺ is classified as a hard acid ([see alternative terms] HSAB principle), so it easily forms complexes with ligands that have F ^- , Cl ^- , O, or N as coordinating atoms. It is commonly hexa-, heptad-, or octacoordinated. Rb ₂ [ZrF ₆ ] is six-coordinate and octahedral, Na ₃ [ZrF ₇ ] is seven-coordinate and pentagonal bipyramidal, and [Zr ₄ (OH) ₈ (H ₂ O) ₁₆ ] ⁸⁺ is eight-coordinate and decahedral.
Zirconium is important as a nuclear reactor material because it has the smallest thermal neutron absorption cross section (0.18 barns) of any metallic material. In Japan, 90% of metallic zirconium is used in nuclear reactors as fuel cladding tubes in the form of Zircaloy alloys, the main component of which is Zr, and the remainder is used as corrosion-resistant structural materials for the chemical industry. High-purity metal is used after removing hafnium (105 barns), a homologous element with a large neutron capture cross section. The largest use of ore is for refractories for steel and refractory bricks, with other uses including abrasives, ceramics, and pigments. High-purity _ZrO2 is used in PZT (lead zirconate titanate) piezoelectric elements, ceramic capacitors, optical lenses, etc. In fiscal year 2005, Japan imported approximately 700 t of metal, mainly from Australia and the United States, and 58,000 t of ore, mainly zircon, from Australia and South Africa. Zirconium compounds are designated as dangerous and harmful substances, the names of which must be notified under Article 18 of the Industrial Safety and Health Act. [CAS 7440-67-7]

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

Zr．原子番号40の元素．電子配置[Kr]4d²5s²の周期表4族遷移金属元素．原子量91.224(2)．天然には質量数90(51.45(40)％)，91(11.22(5)％)，92(17.15(8)％)，94(17.38(28)％)，96(2.80(9)％)の安定同位体が存在し，ほかに78～110の放射性同位体がある．1789年ドイツのM.H. Klaprothが鉱物ジルコンZrSiO₄から酸化物として確認し，1824年スエーデンのJ.J. Berzelius(ベルセリウス)が金属として単離した．ジルコンは赤から黒に至る各色を帯びるが，多くは黄金色の鉱物で古代インド，ペルシャでも知られており，鉱物名は黄金色を意味するアラビア語zargûnに由来する．元素名はSir Humphry Davy(デイビー)が1808年に鉱物名から提案した．宇田川榕菴は天保8年(1837年)に出版した「舎密開宗」で，琪爾古扭母(ギルコニウム)としている．主要鉱物はジルコンのほか，バッデリ石ZrO₂などがある．
地殻中の存在度100 ppm．可採埋蔵量は南アフリカ(37％)，オーストラリア(27％)，ウクライナ(10％)，インド，アメリカ(ともに9％)の順である．製錬は，酸化鉱に赤熱状態で炭素と塩素を作用させ粗塩化物ZrCl₄にかえ，不活性気体中で金属マグネシウムで還元するクロール(Kroll)法，精製はZrI₄のフィラメント上の熱分解( ＞ 1300 ℃，van Arkel-de Boer法)で行われる．イオン半径，金属半径がほぼ等しく化学的挙動がほとんど同一のハフニウムとの分離は四塩化物の段階で溶媒抽出法によって行われる．金属ジルコニウムは銀白色で，同族のチタンより軟らかく，α，βの2変態がある．室温では六法最密格子のα相で，862 ℃ で体心立方格子のβ相にかわる．融点1852 ℃，沸点4377 ℃．密度6.506 g cm^－3(20 ℃)．第一イオン化エネルギー6.84 eV．標準電極電位 Zr^4＋/Zr－1.55 V．金属結合半径0.160 nm．Zr^Ⅳ(六配位)0.072 nm．熱濃硫酸，熱濃塩酸，熱濃硝酸にも侵されず，耐酸性はチタン，ステンレス鋼よりすぐれている．アルカリにも強く，濃NaOH水溶液には沸点まで，溶融NaOHにも1000 ℃ まで耐える．フッ化水素酸，王水に溶ける．フッ化水素酸には希薄溶液(～0.1％)でも侵される．室温の空気中では，固体は表面に徐々にできる酸化皮膜が内部を保護するので，耐食性にすぐれているが，酸化反応は発熱反応で，表面積の広い粉末は発火することがある．高温では，酸素，ハロゲン(X)と反応してZr^Ⅳ O₂(無色)，Zr^Ⅳ X₄(無色，X ＝ Iのみ黄色)を生じる．酸化数1～4を示すが，主要酸化数は4で，とくに水溶液中では，4以外は不安定な [Zr^Ⅲ(H₂O)₆]^3＋ のみが知られている．Zr^Ⅱ X₂，Zr^Ⅲ X₃は青黒色～暗緑色の固体である．Zr^4＋ は硬酸([別用語参照]HSAB原理)に分類されるので，F^－，Cl^－ やO，Nを配位原子とする配位子と錯体をつくりやすい．六，七，八配位が多い．Rb₂[ZrF₆]は六配位・八面体，Na₃[ZrF₇]は七配位・五方両すい，[Zr₄(OH)₈(H₂O)₁₆]^8＋ は八配位・十面体．
ジルコニウムは，熱中性子吸収断面積が金属材料中最小(0.18バーン)なので，原子炉材料として重要である．わが国では金属ジルコニウムの90％ が，Zrを主成分とするジルカロイ合金の形で燃料被覆管などとして原子炉で使用され，残りが化学工業用耐食性構造材料となる．中性子捕獲断面積の大きい同族元素ハフニウム(105バーン)を除去した高純度金属を使用する．鉱石の最大の用途は，鉄鋼用耐火物，耐火れんが向けで，ほかは研磨剤，窯業，顔料などである．高純度ZrO₂はPZT(ジルコン酸チタン酸鉛)圧電素子，セラミックコンデンサー，光学レンズなどに用いられる．2005年度にわが国は約700 t を金属としておもにオーストラリア，アメリカから，鉱石をおもにジルコンで58000 t オーストラリア，南アフリカから輸入した．ジルコニウム化合物は労働安全衛生法政令第十八条の名称等を通知すべき危険物及び有害物指定．[CAS 7440-67-7]

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