Polonium dioxide
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Systematic IUPAC name
Polonium dioxide
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| Properties | |
| PoO2 | |
| Molar mass | 240.98 g/mol |
| Appearance | pale yellow crystalline solid |
| Density | 8.9 g/cm3 |
| Melting point | 500 °C (932 °F; 773 K) (decomposes) sublimes at 885 °C (under oxygen) |
| Structure | |
| fluorite, Pearson symbol cF12 | |
| Fm3m (No 225) | |
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a = 0.5637 nm
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Polonium dioxide (also known as polonium(IV) oxide) is a chemical compound with the formula PoO2. It is one of three oxides of polonium, the other two being polonium monoxide (PoO) and polonium trioxide (PoO3). It is a pale yellow crystalline solid at room temperature. Under lowered pressure (such as a vacuum), it decomposes into elemental polonium and oxygen at 500 °C. It is the most stable oxide of polonium and is an interchalcogen.
At room temperature, polonium dioxide has a face-centered cubic (fluorite) crystal structure; upon heating to high temperatures, it crystallises in the tetragonal crystal system. The cubic form is pale yellow, while the tetragonal form is red. Polonium dioxide darkens upon heating, and is chocolate brown at its sublimation point, 885 °C. The ionic radius of the Po4+
ion is 1.02 or 1.04 Å; thus, the ratio of the ionic radii Po4+
/O2−
is about 0.73, the lower limit of stability for the cubic crystal system, allowing polonium dioxide to have two modifications. When freshly prepared, polonium dioxide is always in the tetragonal form, and changes to the cubic form after being left to stand or after being cooled strongly.
Polonium dioxide does not occur naturally due to the scarcity of polonium in nature and the high temperatures (250 °C) required to form the dioxide.
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