Actinium compounds

Actinium compounds are compounds containing the element actinium (Ac). Due to actinium's intense radioactivity, only a limited number of actinium compounds are known. These include: AcF₃, AcCl₃, AcBr₃, AcOF, AcOCl, AcOBr, Ac₂S₃, Ac₂O₃, AcPO₄ and Ac(NO₃)₃. Except for AcPO₄, they are all similar to the corresponding lanthanum compounds. They all contain actinium in the oxidation state +3. In particular, the lattice constants of the analogous lanthanum and actinium compounds differ by only a few percent.

Here a, b and c are lattice constants, No is space group number and Z is the number of formula units per unit cell. Density was not measured directly but calculated from the lattice parameters.

Actinium(III) oxide is the only oxide that actinium can form, with the chemical formula Ac₂O₃. In this compound, actinium is in the oxidation state +3. It is similar to the corresponding lanthanum compound, lanthanum(III) oxide. It can be obtained by heating the hydroxide at 500 °C or the oxalate at 1100 °C, in vacuum. Its crystal lattice is isotypic with the oxides of most trivalent rare-earth metals.

Actinium trifluoride can be produced either in solution or in solid reaction. The former reaction is carried out at room temperature, by adding hydrofluoric acid to a solution containing actinium ions. In the latter method, actinium metal is treated with hydrogen fluoride vapors at 700 °C in an all-platinum setup. Treating actinium trifluoride with ammonium hydroxide at 900–1000 °C yields oxyfluoride AcOF. Whereas lanthanum oxyfluoride can be easily obtained by burning lanthanum trifluoride in air at 800 °C for an hour, similar treatment of actinium trifluoride yields no AcOF and only results in melting of the initial product.

Actinium trichloride is obtained by reacting actinium hydroxide or oxalate with carbon tetrachloride vapors at temperatures above 960 °C. Similar to oxyfluoride, actinium oxychloride can be prepared by hydrolyzing actinium trichloride with ammonium hydroxide at 1000 °C. However, in contrast to the oxyfluoride, the oxychloride could well be synthesized by igniting a solution of actinium trichloride in hydrochloric acid with ammonia.

Reaction of aluminium bromide and actinium oxide yields actinium tribromide:

and treating it with ammonium hydroxide at 500 °C results in the oxybromide AcOBr.

Actinium hydride was obtained by reduction of actinium trichloride with potassium at 300 °C, and its structure was deduced by analogy with the corresponding LaH₂ hydride. The source of hydrogen in the reaction was uncertain.