Acta Cryst. (2002). B58, 116-124 [ doi:10.1107/S0108768101014744 ]
Abstract: At room temperature the two title compounds are isomorphous. Both have a disordered structure that includes a major component, one of the title compounds, and a minor component, the 6-halo isomer, occurring at the same site in the crystal. When the chloro compound is cooled to as low as 98 K, there is no change in the structure and no significant change in the degree of disorder. When the bromo compound is cooled, it undergoes a second-order phase transition at 282 K to a second polymorph, structurally very similar, but with two molecules in the asymmetric unit. In both isomorphs and both polymorphs the molecules occur in two-dimensional layers that are essentially the same. In the low-temperature form of the bromo compound the layers have shifted relative to each other by approximately 0.7 Å in y. In the low-temperature form the disorder, which is still present after the transition, decreases as the temperature is lowered.
Keywords: disorder; phase transitions; polymorphism.
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