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

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: T = 164 K; a = 24.527 (6), b = 6.064 (1), c = 7.457 (4) Å; beta = 100.10 (2) degrees; V = 1091.9 (6) Å(3); Z = 4; d(c) = 1.528 g cm(-3); lambda(Mo Kalpha) = 0.71073 Å; µ = 0.565 mm(-1); F(000) = 512; R = 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 c is negative.