Acta Crystallographica Section B

Structural Science

Volume 55, Part 5 (October 1999)

research papers

Acta Cryst. (1999). B55, 799-806 [ doi:10.1107/S0108768199005480 ]

Investigation of the structural phase transitions near 190 K in 4,4'-dichlorobenzophenone

V. V. Mitkevich, V. G. Lirtsman, M. A. Strzhemechny, A. A. Avdeenko and V. V. Eremenko

Abstract: X-ray structure studies have been carried out on single crystals of 4,4'-dichlorobenzophenone at temperatures from 164 to 295 K. The structure determined at 164 K is monoclinic I2/c. Crystal data: = 164 K; a = 24.527 (6), b = 6.064 (1), c = 7.457 (4) Å; [beta] = 100.10 (2)°; V = 1091.9 (6) Å³; Z = 4; $d_{c}$ = 1.528 g cm $^{-3}$ ; $\lambda$ (Mo $K{\alpha}$ ) = 0.71073 $\AA$ ; $\mu$ = 0.565 mm $^{-1}$ ; = 512; = 0.04. The temperature dependence of the unit-cell parameters and of the intensities of some symmetry-sensitive reflections has been measured over the range 164-293 K, both in warm-up and cool-down experiments. It has been established that the phase transformation between the C2/c and I2/c structures occurs as a sequence of two closely spaced first-order phase transitions. In between these phase transitions the crystal is in an intermediate state whose structure is different from both C2/c and I2/c. The experimental data give us ground to suppose that the intermediate state is disordered, having a primitive cell half as large as in the two ordered phases. Within the temperature interval where the said intermediate state exists the thermal expansivity along axis is negative.

Keywords: 4,4'-Dichlorobenzophenone; Phase transitions.