Europium

Eu. Atomic number 63. Electron configuration [Xe] ^4f76s2 , lanthanide element in group ³ of the periodic table. One of the rare earth elements, it is located between the lanthanides, so it can be included in both the yttrium group and the cerium group. ⁶² Sm to ⁶⁶ Dy are also called medium rare earths. Atomic weight 151.964(1). Two stable isotopes with mass numbers 151 (47.81(6)%) and 153 (52.19(6)%) and radioactive isotopes with mass numbers 130 to 167 are known. In 1896, EA. Demarçay separated it from Sm (discovered in the spectrum in 1889 by W. Crookes), and named it europium after the European continent.
Abundance in the earth's crust is 1.1 ppm. One of the rare earth elements with low abundance. From trace amounts of monazite and bastnaesite, the rare earth elements are separated by solvent extraction, and the metal is obtained by molten salt electrolysis. Silvery white, body-centered cubic lattice structure. Melting point 822 °C, boiling point 1597 °C. Density 5.243 g cm ^-3 (25 °C). First ionization energy 5.670 eV. Standard electrode potential C/Eu -1.99 V, Eu ³⁺ /Eu ²⁺ -0.35 V. It dissolves in hot water and acid to produce hydrogen. Oxidation states 2 and 3. Ionic radii Eu ²⁺ 0.131 nm, Eu ³⁺ 0.109 nm. Eu ³⁺ is pale pink, Eu ²⁺ is colorless. It ignites in air at temperatures above 150 °C, becoming Eu ₂ O ₃ . Eu ^II , with its 4f shell half-full (4f ⁷ ), is the most stable oxidation state (II) of the rare earth elements. The ionic radius of Eu ²⁺ is very close to that of Sr ²⁺ (0.132 nm), so Eu ^II compounds exhibit properties similar to Sr compounds. Europium(II) sulfate, like Sr sulfate, is poorly soluble in water. Isomorphous substitution of Sr ²⁺ by Eu ²⁺ is also common. Eu ^III compounds, like other rare earth elements, are insoluble in water as carbonates, oxalates, and fluorides.
Its uses include: Eu ³⁺ is an activator for red phosphors in cathode ray tubes, such as _{YVO 4} Eu ³⁺ , Y ₂ O ₂ S:Eu ³⁺ , Y ₂ O ₃ :Eu ³⁺ , etc. Eu ²⁺ is an activator for blue phosphors, such as YSr ₅ Cl(PO ₄ ) ₃ :Eu ²⁺ . Because of its large neutron capture cross section, it is also used in nuclear reactor control rods. [CAS 7440-53-1]

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

Eu．原子番号63の元素．電子配置[Xe]4f ⁷6s²の周期表3族ランタノイド元素．希土類元素の一つで，ランタノイドの中間に位置するので，イットリウム族，セリウム族の両者にに含まれることがある．⁶²Sm～⁶⁶Dyを中希土類ともいう．原子量151.964(1)．質量数151(47.81(6)％)，153(52.19(6)％)の2種の安定同位体と，質量数130～167の放射性同位体が知られている．1896年，E-A. DemarçayがSmから分離し(スペクトル中の発見は，1889年，W. Crookes)，ヨーロッパ大陸にちなんでeuropiumと命名した．
地殻中の存在度1.1 ppm．存在度の低い希土類元素の一つ．微量に含まれるモナズ石，バストネス石などを原料に，溶媒抽出法で希土類相互分離後，溶融塩電解により金属が得られる．銀白色，体心立方格子構造．融点822 ℃，沸点1597 ℃．密度5.243 g cm^－3(25 ℃)．第一イオン化エネルギー5.670 eV．標準電極電位C/Eu－1.99 V，Eu^3＋/Eu^2＋－0.35 V．熱水，酸に溶けて水素を発生する．酸化数2，3．イオン半径 Eu^2＋0.131 nm，Eu^3＋0.109 nm．Eu^3＋ は淡紅色，Eu^2＋ は無色．150 ℃ 以上では空気中で発火してEu₂O₃となる．電子配置が4f殻半分充満(4f ⁷)の Eu^Ⅱは希土類元素の酸化状態(Ⅱ)中でもっとも安定である．Eu^2＋ のイオン半径は Sr^2＋(0.132 nm)と非常に近いので，Eu^Ⅱ化合物はSr化合物と似た性質を示す．硫酸ユウロピウム(Ⅱ)は，Srの硫酸塩と同様に水に難溶．Eu^2＋ による Sr^2＋ の同形置換もよく見られる．Eu^Ⅲ化合物はほかの希土類元素と同様に，炭酸塩，シュウ酸塩，フッ化物が水に不溶．
用途は，Eu^3＋ はブラウン管用赤色蛍光体の賦活材．YVO₄Eu^3＋，Y₂O₂S：Eu^3＋，Y₂O₃：Eu^3＋ など．Eu^2＋ は青色蛍光体の賦活材．YSr₅Cl(PO₄)₃：Eu^2＋ など．中性子捕獲断面積が大きいところから原子炉制御棒にも利用される．[CAS 7440-53-1]

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