Relations between compression and thermal contraction in 1,2,4-trichlorobenzene and melting of trichlorobenzene isomers.

The compression and thermal expansion of crystalline 1,2,4-trichlorobenzene, C6H3Cl3, 124TCB, investigated under isobaric and isothermal conditions, are in reverse relation, as for most of crystals, however, the isochoric strain along direction c is clearly different from those along a and b. Single crystals of 124TCB have been in situ grown under isochoric and isobaric conditions, at 270 K/0.1 MPa and 295 K/0.16 GPa, and also at 100 K/0.1 MPa and 295 K/0.64 GPa, when the unit-cell volume is similar. All crystallizations yielded the same phase, of monoclinic space group P21/n, with two symmetry-independent molecules (Z′ = 2). The structure is governed by Cl…Cl and Cl…H interactions and the molecules are all nearly parallel to the [001] direction. Systematic differences of these specific contacts afford rationalizing the isochoric strain of the crystal. The phase diagram of 124TCB has been outlined. [ABSTRACT FROM AUTHOR]