The temperature dependence of the crystal structures of the solid halogens, bromine and chlorine

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Abstract
The neutron powder profile refinement technique has been used to determine the crystal structure of solid Br2 at 5, 80, 170 and 250 K and that of solid Cl2 at 22, 55, 100 and 160 K. The results confirm that the structures of the two halogens are isomorphous, with space group Cmca at all temperatures. For both halogens the lattice parameters a and b increase monotonically with temperature, but the lattice parameter c decreases at the highest temperatures. The data were analysed assuming both isotropic and anisotropic thermal parameters. The intramolecular bond length and its orientation relative to the b-axis show little change with temperature. For Br2 at 5 K the bond length is 2·301(2) Å while a = 6·5672(3), b = 4·4678(2), c = 8·6938(4) Å. For Cl2 at 22 K the bond length is 1·994(2) Å while a = 6·1453(2), b = 4·3954(1), c = 8·1537(2) Å, where the errors quoted are those produced by the profile fitting program.

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