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 0.3 to 1050 K (at higher temperatures the material decomposes). Since the structure is cubic, as described below, the thermal contraction is isotropic - equal in all directions. There is much ongoing research attempting to elucidate why the material exhibits such dramatic negative thermal expansion.
This phase is thermodynamically unstable at room temperature with respect to the binary oxides ZrO2 and WO3, but may be synthesised by heating stoichiometric quantities of 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.
Read more about this topic: Zirconium Tungstate
Other articles related to "phases, cubic, cubic phase, phase":
... The solid can exist in five phases, listing from high temperature to low temperature hexagonal, cubic, tetragonal, orthorhombic, and rhombohedral crystal structure ... All of the phases exhibit the ferroelectric effect except the cubic phase ... The high temperature cubic phase is easiest to describe, consisting of octahedral TiO6 centres that define a cube with Ti vertices and Ti-O-Ti edges ...
... See also Lyotropic liquid crystal and Columnar phase A lyotropic liquid crystal consists of two or more components that exhibit liquid-crystalline properties ... In the lyotropic phases, solvent molecules fill the space around the compounds to provide fluidity to the system ... enables them to induce a variety of different phases ...
... The arrangement of the groups in the structure of cubic ZrW2O8 is analogous to the simple NaCl structure, with ZrO6 octahedra at the Na sites, and W2O8 groups at the Cl sites ... The unit cell consists of 44 atoms aligned in a primitive cubic Bravais lattice, with unit cell length 9.15462 Angstroms ... by the disordering of the orientation of tungstate groups, and the space group above the phase transition temperature (~180C) is Pa ...
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