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.[1] In particular, the lattice constants of the analogous lanthanum and actinium compounds differ by only a few percent.[2]
| Formula | color | symmetry | space group | No | Pearson symbol | a (pm) | b (pm) | c (pm) | Z | density, g/cm3 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Ac | silvery | fcc[3] | Fmm | 225 | cF4 | 531.1 | 531.1 | 531.1 | 4 | 10.07 | |
| AcH2 | unknown | cubic | Fmm | 225 | cF12 | 567 | 567 | 567 | 4 | 8.35 | |
| Ac2O3 | white[4] | trigonal[5] | Pm1 | 164 | hP5 | 408 | 408 | 630 | 1 | 9.18 | |
| Ac2S3 | black | cubic[6] | I3d | 220 | cI28 | 778.56 | 778.56 | 778.56 | 4 | 6.71 | |
| AcF3 | white[7] | hexagonal | Pc1 | 165 | hP24 | 741 | 741 | 755 | 6 | 7.88 | |
| AcCl3 | white | hexagonal[8] | P63/m | 165 | hP8 | 764 | 764 | 456 | 2 | 4.8 | |
| AcBr3 | white | hexagonal | P63/m | 165 | hP8 | 764 | 764 | 456 | 2 | 5.85 | |
| AcOF | white | cubic | Fmm | 593.1 | 8.28 | ||||||
| AcOCl | white | tetragonal | 424 | 424 | 707 | 7.23 | |||||
| AcOBr | white | tetragonal | 427 | 427 | 740 | 7.89 | |||||
| AcPO4·0.5H2O | unknown | hexagonal | 721 | 721 | 664 | 5.48 |
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.
See main article: Actinium(III) oxide.
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.[9] [10] 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.[11]
AcF3 + 2 NH3 + H2O → AcOF + 2 NH4F
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:
Ac2O3 + 2 AlBr3 → 2 AcBr3 + Al2O3
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.[12]
Mixing monosodium phosphate (NaH2PO4) with a solution of actinium in hydrochloric acid yields white-colored actinium phosphate hemihydrate (AcPO4·0.5H2O), and heating actinium oxalate with hydrogen sulfide vapors at 1400 °C for a few minutes results in a black actinium sulfide Ac2S3. It may possibly be produced by acting with a mixture of hydrogen sulfide and carbon disulfide on actinium oxide at 1000 °C.