# 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: AcF$_3$, AcCl$_3$, AcBr$_3$, AcOF, AcOCl, AcOBr, Ac$_2$S$_3$, Ac$_2$O$_3$, AcPO$_4$ and Ac(NO$_3$)$_3$. Except for AcPO$_4$, 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.

## Properties of actinium compounds

| Formula | color | symmetry | space group | No | Pearson symbol | a (pm) | b (pm) | c (pm) | Z | density, g/cm$^3$ |
| Ac | silvery | fcc | Fm3m | 225 | cF4 | 531.1 | 531.1 | 531.1 | 4 | 10.07 |
| AcH$_2$ | unknown | cubic | Fm3m | 225 | cF12 | 567 | 567 | 567 | 4 | 8.35 |
| Ac$_2$O$_3$ | white | trigonal | P3m1 | 164 | hP5 | 408 | 408 | 630 | 1 | 9.18 |
| Ac$_2$S$_3$ | black | cubic | I43d | 220 | cI28 | 778.56 | 778.56 | 778.56 | 4 | 6.71 |
| AcF$_3$ | white | hexagonal | P3c1 | 165 | hP24 | 741 | 741 | 755 | 6 | 7.88 |
| AcCl$_3$ | white | hexagonal | P6$_3$/m | 165 | hP8 | 764 | 764 | 456 | 2 | 4.8 |
| AcBr$_3$ | white | hexagonal | P6$_3$/m | 165 | hP8 | 764 | 764 | 456 | 2 | 5.85 |
| AcOF | white | cubic | Fm3m | | | 593.1 | | | | 8.28 |
| AcOCl | white | tetragonal | | | | 424 | 424 | 707 | | 7.23 |
| AcOBr | white | tetragonal | | | | 427 | 427 | 740 | | 7.89 |
| AcPO$_4$·0.5H$_2$O | 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.

## Oxides

Actinium(III) oxide is the only oxide that actinium can form, with the chemical formula Ac$_2$O$_3$. 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.

## Halides

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.
    AcF$_3$ + 2 NH$_3$ + H$_2$O → AcOF + 2 NH$_4$F
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:
    Ac$_2$O$_3$ + 2 AlBr$_3$ → 2 AcBr$_3$ + Al$_2$O$_3$
and treating it with ammonium hydroxide at 500 °C results in the oxybromide AcOBr.

## Other compounds

Actinium hydride was obtained by reduction of actinium trichloride with potassium at 300 °C, and its structure was deduced by analogy with the corresponding LaH$_2$ hydride. The source of hydrogen in the reaction was uncertain.
Mixing monosodium phosphate (NaH$_2$PO$_4$) with a solution of actinium in hydrochloric acid yields white-colored actinium phosphate hemihydrate (AcPO$_4$·0.5H$_2$O), and heating actinium oxalate with hydrogen sulfide vapors at 1400 °C for a few minutes results in a black actinium sulfide Ac$_2$S$_3$. It may possibly be produced by acting with a mixture of hydrogen sulfide and carbon disulfide on actinium oxide at 1000 °C.