Structure and Properties
Four crystalline forms, (polymorphs), of ZnCl2 are known, α, β, γ, and δ, and in each case the Zn2+ ions are tetrahedrally coordinated to four chloride ions.
| Form | Symmetry | Pearson symbol | Group | No | a (nm) | b (nm) | c (nm) | Z | ρ (g/cm3) |
|---|---|---|---|---|---|---|---|---|---|
| α | Tetragonal | tI12 | I42d | 122 | 0.5398 | 0.5398 | 0.64223 | 4 | 3.00 |
| β | Tetragonal | tP6 | P42/nmc | 137 | 0.3696 | 0.3696 | 1.071 | 2 | 3.09 |
| γ | Monoclinic | mP36 | P21c | 14 | 0.654 | 1.131 | 1.23328 | 12 | 2.98 |
| δ | Orthorhombic | oP12 | Pna21 | 33 | 0.6125 | 0.6443 | 0.7693 | 4 | 2.98 |
Here a, b, and c are lattice constants, Z is the number of structure units per unit cell and ρ is the density calculated from the structure parameters.
The pure anhydrous orthorhombic form (δ) rapidly changes to one of the other forms on exposure to the atmosphere and a possible explanation is that the OH− ions originating from the absorbed water facilitate the rearrangement. Rapid cooling of molten ZnCl2 gives a glass, that is, a rigid amorphous solid and this ability has been related to the structure in the melt.
The covalent character of the anhydrous material is indicated by its relatively low melting point of 275 °C. Further evidence for covalency is provided by the high solubility of the dichloride in ethereal solvents where it forms adducts with the formula ZnCl2L2, where L = ligand such as O(C2H5)2. In the gas phase, ZnCl2 molecules are linear with a bond length of 205 pm.
Molten ZnCl2 has a high viscosity at its melting point and a comparatively low electrical conductivity that increases markedly with temperature. A Raman scattering study of the melt indicated the presence of polymeric structures and a neutron scattering study indicated the presence of tetrahedral {ZnCl4} complexes.
Read more about this topic: Zinc Chloride
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