Promethium compounds
Promethium compounds are compounds containing the element promethium, which normally take the +3 oxidation state. Promethium belongs to the cerium group of lanthanides and is chemically very similar to the neighboring elements.[1] Because of its instability, chemical studies of promethium are incomplete. Even though a few compounds have been synthesized, they are not fully studied; in general, they tend to be pink or red in color.[2][3] Treatment of acidic solutions containing Pm3+ ions with ammonia results in a gelatinous light-brown sediment of hydroxide, Pm(OH)3, which is insoluble in water.[4] When dissolved in hydrochloric acid, a water-soluble yellow salt, PmCl3, is produced;[4] similarly, when dissolved in nitric acid, a nitrate results, Pm(NO3)3. The latter is also well-soluble; when dried, it forms pink crystals, similar to Nd(NO3)3.[4] The electron configuration for Pm3+ is [Xe] 4f4, and the color of the ion is pink. The ground state term symbol is 5I4.[5] The sulfate is slightly soluble, like the other cerium group sulfates. Cell parameters have been calculated for its octahydrate; they lead to conclusion that the density of Pm2(SO4)3·8 H2O is 2.86 g/cm3.[6] The oxalate, Pm2(C2O4)3·10 H2O, has the lowest solubility of all lanthanide oxalates.[7]
Unlike the nitrate, the oxide is similar to the corresponding samarium salt and not the neodymium salt. As-synthesized, e.g. by heating the oxalate, it is a white or lavender-colored powder with disordered structure.[4] This powder crystallizes in a cubic lattice upon heating to 600 °C. Further annealing at 800 °C and then at 1750 °C irreversibly transforms it to monoclinic and hexagonal phases, respectively, and the last two phases can be interconverted by adjusting the annealing time and temperature.[8]
| Formula | symmetry | space group | No | Pearson symbol | a (pm) | b (pm) | c (pm) | Z | density, g/cm3 |
|---|---|---|---|---|---|---|---|---|---|
| α-Pm | dhcp[9][10] | P63/mmc | 194 | hP4 | 365 | 365 | 1165 | 4 | 7.26 |
| β-Pm | bcc[10] | Fm3m | 225 | cF4 | 410 | 410 | 410 | 4 | 6.99 |
| Pm2O3 | cubic[8] | Ia3 | 206 | cI80 | 1099 | 1099 | 1099 | 16 | 6.77 |
| Pm2O3 | monoclinic[8] | C2/m | 12 | mS30 | 1422 | 365 | 891 | 6 | 7.40 |
| Pm2O3 | hexagonal[8] | P3m1 | 164 | hP5 | 380.2 | 380.2 | 595.4 | 1 | 7.53 |
Promethium forms only one stable oxidation state, +3, in the form of ions; this is in line with other lanthanides. According to its position in the periodic table, the element cannot be expected to form stable +4 or +2 oxidation states; treating chemical compounds containing Pm3+ ions with strong oxidizing or reducing agents showed that the ion is not easily oxidized or reduced.[1]
| Formula | color | coordination number |
symmetry | space group | No | Pearson symbol | m.p. (°C) |
|---|---|---|---|---|---|---|---|
| PmF3 | Purple-pink | 11 | hexagonal | P3c1 | 165 | hP24 | 1338 |
| PmCl3 | Lavender | 9 | hexagonal | P63/mc | 176 | hP8 | 655 |
| PmBr3 | Red | 8 | orthorhombic | Cmcm | 63 | oS16 | 624 |
| α-PmI3 | Red | 8 | orthorhombic | Cmcm | 63 | oS16 | α→β |
| β-PmI3 | Red | 6 | rhombohedral | R3 | 148 | hR24 | 695 |
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