Europium compounds

Europium compounds are compounds formed by the lanthanide metal europium. In these compounds, europium generally exhibits the +3 oxidation state, such as EuCl₃, Eu₃ and Eu₃. Compounds with europium in the +2 oxidation state are also known. The +2 ion of europium is the most stable divalent ion of lanthanide metals in aqueous solution. Many europium compounds fluoresce under ultraviolet light due to the excitation of electrons to higher energy levels. Lipophilic europium complexes often feature acetylacetonate-like ligands, e.g., Eufod.

Properties of europium compounds

Chalcogenides

Oxides

can be obtained by the reduction of europium oxide with metallic europium at high temperatures. It has a rock-salt structure, is a deep red solid, and is ferromagnetic at 77 K. It has the potential to become a magnetic refrigeration material. Europium sulfide is also ferromagnetic, but europium telluride is antiferromagnetic. The mixed valence oxide Eu₃O₄ of europium can be obtained by reducing europium oxide with a reducing agent in a hydrogen atmosphere, such as:
Europium oxide is the most stable oxide of europium, a light pink solid with a high melting point, which can be obtained by the thermal decomposition of europium nitrate. It reacts with water to give EuOOH. The reaction of soluble europium salts with ammonia or sodium hydroxide can precipitate hydroxide Eu₃, but in the presence of polyhydroxyl compounds, the precipitation is incomplete.
Eu and Eu₂ complexes can be obtained by the reaction of metallic europium in solid argon with water. Eu is rearranged to obtain HEuOH, which is further decomposed into EuO and H₂; Eu₂ is decomposed into Eu₂ and H₂.

Other chalcogenides

Europium sulfide can be obtained by the decomposition of Eu₃ then at 500~600 °C. Europium sulfide can also be obtained by the decomposition of the thiocyanate Eu₃; Its two crystal forms, α-type and γ-type, belong to orthorhombic and cubic crystal systems, respectively. Europium sulfide is prepared by sulfiding the oxide at temperatures sufficiently high to decompose europium oxide:
The selenides, europium selenide and europium selenide, and tellurides, europium telluride and europium telluride, are also known. They can generally be prepared by reacting europium with selenium or tellurium in a vacuum ampoule at a high temperature. Europium selenide can also be obtained by heating europium oxalate with an excess of selenium.
Europium oxysulfide is obtained by reacting europium oxide in a carbon disulfide/argon/low-pressure oxygen stream. It is a solid of the triclinic crystal system, with the space group P''m''1, and its optical band gap is 4.4 eV. Europium oxyselenide and europium oxytelluride can be prepared by reacting europium oxide with selenium or tellurium at 600 °C. The oxyselenide is heated in air and oxidized to oxyselenite. A similar reaction occurs with oxytelluride to give Eu₂TeO₆.

Halides

Europium metal reacts with all the halogens:
This route gives white europium fluoride, yellow europium chloride, gray europium bromide, and colorless europium iodide. Europium also forms the corresponding dihalides: yellow-green europium fluoride, colorless europium chloride , colorless europium bromide, and green europium iodide.
Europium can form all four trihalides. They are strong electrolytes in water, and all but the fluoride are soluble in water. Anhydrous europium trihalides can be prepared by reacting oxides or the halides' hydrates:
Among them, europium iodide can only be obtained by reacting europium oxide and hydroiodic acid.
In addition, europium can also form all four dihalides. They can generally be prepared by reducing the corresponding europium trihalide with hydrogen gas or europium:
Europium iodide can also be obtained by direct treating the reagent with ammonium iodide. Of the dihalides, EuF₂ and EuI₂ are yellow, and EuCl₂ and EuBr₂ are white, although when irradiated by ultraviolet light, EuCl₂ has a bright blue fluorescence.

Pnictides

Europium nitride is a black solid that can be prepared by the reaction of metallic europium in a stream of ammonia in corundum boats in fused quartz tubes at 700 °C:
In this reaction, the europium is oxidized and the hydrogen in the ammonia is reduced. Europium nitride shows Van Vleck paramagnetism and crystallises in the rock salt structure. Thin films of rare earth nitrides, including europium nitride, tend to form oxides in the presence of oxygen. Europium phosphide can be produced from a solution of europium metal in liquid ammonia with phosphine at -78 °C. This produces hydrogen and first Eu₂ is formed, but it then decomposes to EuP and PH₃. It crystallises cubically like NaCl. Pure europium phosphide also shows Van Vleck paramagnetism. Europium diarsenide, Eu₂As₂, is unique which it contains the ion instead of the As^3- ion, unlike other lanthanide arsenides. It crystallizes in the distorted Na₂O₂ structure, similar to nickel arsenide, and is produced from reacting europium and arsenic at 600 °C. Other arsenides, antimonides and bismuthides of europium are also known.

Organoeuropium compounds

Organoeuropium compounds are a class of organic metal compounds containing Eu-C bonds. The cyclopentadienyl complexes of europium were studied in the early stage. They can be prepared by the reaction of sodium cyclopentadienide and anhydrous europium halide in tetrahydrofuran, such as:
Europium bis is an orange-red solid that can be melted at 165 °C. The complex of cyclononatetraene and europium can be prepared by a similar method, and its toluene solution emits blue-green fluorescence at 516 nm, compared with other organic europium sandwich complexes with a clear blue shift.
In addition to the preparation of organo-europium compounds by metathesis reaction, metal europium can also be directly involved in the reaction, such as the reaction of europium and pentamethylcyclopentadiene to generate light orange bis europium; and the reaction between cyclooctatetraene and europium gives the pale green cyclooctatetraene europium.

Other compounds

is the sulfate of divalent europium, which can be obtained by electrolysis of europium sulfate solution with mercury as the cathode, or by reducing europium chloride with zinc amalgam, and then reacting with sulfuric acid. It reacts with sodium carbonate or ammonium oxalate to obtain europium carbonate and europium oxalate, respectively:
Europium sulfate can be directly obtained by reacting europium oxide and dilute sulfuric acid, and crystallized, and dehydration of hydrate can obtain the anhydrous form. Europium sulfate is soluble in water, and its octahydrate has a solubility of 2.56 g at 20 °C. Europium sulfite and its basic salt are known, and heating the sulfite in a carbon monoxide atmosphere will dehydrate to obtain the anhydrous form, and after Eu₂O₂SO₄, finally obtain the oxysulfide Eu₂O₂S.
Europium hydroxide can be obtained by reacting metallic europium with sodium hydroxide. Europium hydroxide belongs to the orthorhombic crystal system. It decomposes to form europium hydroxide, a pale pink solid that reacts with acids and produces europium salts. It can be prepared by reacting europium with water, or by reacting europium nitrate with hexamethylenetetramine at 95 °C or with ammonium hydroxide.
Europium nitrate can be obtained by reacting europium oxide and nitric acid and crystallizing. The crystal is dried with 45~55% sulfuric acid to obtain hexahydrate. Its anhydrous form can be obtained by the reaction of europium oxide and dinitrogen tetroxide, while heating the hydrate can only obtain the basic salt EuONO₃. Europium phosphate can be obtained by reacting europium chloride and diammonium hydrogen phosphate, and its white monohydrate precipitates from solution. It loses water at 600~800 °C, and changes from a hexagonal phase with water to an anhydrous monoclinic phase. Europium oxide reacts with arsenic pentoxide to obtain europium arsenate, which is a colorless crystal with a xenotime structure.
Europium carbonate is one of the carbonates of europium, which can be obtained by reacting a dilute solution of sodium bicarbonate saturated with carbon dioxide with a soluble europium salt. It is heated and decomposed to generate europium oxide and carbon dioxide. Its basic salts and double salts are known. Europium acetate is a pale pink solid that can crystallize from an aqueous solution as the tetrahydrate, which is dried with sulfuric acid to give the trihydrate. The reaction of europium nitrate and oxalic acid gave europium oxalate decahydrate, which was converted to pentahydrate at 100 °C. Using potassium oxalate as raw material can only get double salt KEu₂·2H₂O. The coordination polymer _n can be obtained by reacting europium oxalate and oxalic acid with oxalic acid at 200 °C. Europium oxalate is heated to 320 °C in a carbon dioxide atmosphere to obtain europium oxalate:

Applications

Compounds of Eu³⁺ can emit red light under excitation. For example, europium oxide can be used in picture tube televisions and europium-doped yttrium oxysulfide can be used as phosphors. In addition, europium compounds can also be used in the manufacture of anti-counterfeiting materials.
Based on the properties of europium oxide, thin layers of europium oxide deposited on silicon are being studied for use as spin filters. Spin filter materials only allow electrons of a certain spin to pass, blocking electrons of the opposite spin. The synthesis of europium oxide, as well as its europium sulfide, because of their potential as laser window materials, insulating ferromagnets, ferromagnetic semiconductors, and magnetoresistant, optomagnetic, and luminescent materials. Europium sulfide was used in an experiment providing evidence of Majorana fermions relevant to quantum computing and the production of qubits.
Eu₃ serves as a Lewis acid catalyst in organic synthesis including stereoselective Diels-Alder and aldol addition reactions. For example, Eu₃ catalyzes the cyclocondensations of substituted dienes with aromatic and aliphatic aldehydes to yield dihydropyrans, with high selectivity for the endo product.