Zirconium tungstate
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| Zirconium(IV) tungstate | |
|---|---|
| Image:Zirconium(IV) tungstate.jpg | |
| General | |
| Other names | zirconium tungsten oxide |
| Molecular formula | Zn(WO4)2 |
| Molar mass | 586.92 g/mol |
| Appearance | white powder |
| CAS number | [16853-74-0] |
| Properties | |
| Density and phase | 5.09 g/cm³, solid |
| Solubility in water | insoluble |
| Melting point | ? |
| Boiling point | ? |
| Hazards | |
| EU classification | not listed |
| NFPA 704 |    |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Zr(WO4)2, or zirconium tungstate is a metal oxide with unusual properties. It exists in several phases, the most well known of which is the metastable cubic phase, which has negative thermal expansion characteristics, namely it shrinks over a wide range of temperatures when heated.
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[edit] Cubic phase
Cubic zirconium tungstate (alpha-ZrW2O8), one of the several known phases of zirconium tungstate (ZrW2O8) is perhaps one of the most studied materials to exhibit negative thermal expansion. It has been shown to contract continuously over a previously unprecedented temperature range of 2 to 1050 K.
This phase is thermodynamically unstable at room temperature with respect to the binary oxides ZrO2 and WO3, but may be synthesised by heating these oxides together and then quenching the material by rapidly cooling it from approximately 900 °C to room temperature.
The structure of cubic zirconium tungstate consists of corner-sharing ZrO6 octahedral and WO4 tetrahedral structural units. Its unusual expansion properties are thought to be due to vibrational modes known as 'Rigid Unit Modes' (RUMs), which involve the coupled rotation of the polyhedral units that make up the structure, and lead to contraction. There is much ongoing research attempting to elucidate why the material exhibits such dramatic negative thermal expansion.
[edit] Detailed crystal structure
The typical accepted unit cell for the cubic (ZrW2O8) consists of 44 atoms aligned in a primitive cubic Bravais lattice with a basis. This simple cubic structure is quite surprising, given the complexity of the underlying structure of the crystal. The dimension of one of these cubic cells is 9.15462 angstroms (0.915462 nm). This is the most interesting crystalline structure for the compound, as it is the form exhibiting negative thermal expansion.
There are four of the octahedral ZrO6 entirely inside each unit cell, and eight of the tetrahedral WO4 units, seven of which cross the unit cell boundary. The octahedra and tetrahedra are not perfect, and are all slightly deformed; there are in fact two distinct types of tetrahedra, differing in the bond angles between the exterior oxygen.
Octahedra and tetrahedra are linked together by sharing an exterior oxygen; geometrically, the two shapes can "pivot" around these corner-sharing oxygens. This pivoting is what is thought to lead to the negative thermal expansion, as in certain low frequency normal modes this leads to the contracting 'RUMs' mentioned above. In addition, in each tetrahedra there is one unconstrained oxygen, an atom that is bonded only to the central tungsten (W) atom.
The image shows the geometric crystal structure of cubic zirconium tungstate, showing explicitly the octahedral and tetrahedral structure. tungsten-centered tetrahedra are in blue, zirconium-centered octahedra in red. This image is not a single unit cell, as seven of the tetrahedra cross cell boundaries. Note the corner-touching between octahedra and tetrahedra; these are the location of the shared oxygen. The vertices of the tetrahedra and octahedra represent the oxygen, which are spread about the central zirconium and tungsten.
[edit] High pressure forms
At high pressure, zirconium tungsate undergoes a series of phase transitions, first to an amorphous phase, and then to a U3O8-type phase, in which the zirconium and tungsten atoms are disordered.
