Low temperature phase transition induced biaxial negative thermal expansion of 2,4-dinitroanisole

The crystal structure of 2,4-dinitroanisole (24DNAN) is known to have two polymorphic structures (Z' = 2 and Z' = 1, hereafter referred to as alpha-form and beta-form, respectively) and the oxygen atoms of the nitro group(s) of both polymorphs have been found to be disordered over two positions at 298 K. In the thermal behaviour studies of 24DNAN, the beta-form showed a phase transition at 264.5 K on cooling. To investigate this phase transition variable-temperature single-crystal X-ray diffraction measurements were carried out. The beta angle for the beta-form dramatically changed from 90.190 degrees (298 K) to 96.716 degrees (100 K). The length of the b axis shows negative thermal expansion, 13.741(5) to 13.810(5) angstrom (with a change in temperature from 298 K to 100 K). The thermal expansion coefficients along the principal axes X1, X2, and X3 are -31.6), -15.7) and 262(20) MK-1 (261-100 K), respectively. 24DNAN showed an unusual biaxial NTE induced by a low temperature phase transition.