High-Pressure Preference for the Low Z' Polymorph of a Molecular Crystal.

High pressure destabilizes the high Z' polymorph of 3-hydroxy-4,5-dimethyl-1-phenylpyridazin-6-one (1α); however, recrystallization is needed for obtaining a low Z', more dense polymorph. Three polymorphs α, β, and γ can be monotonically compressed to 2.0 GPa at least. At ambient pressure 1 crystallizes in space group C2/c, Z' = 4 as polymorph 1α, or as lower-density polymorph 1β, of space group P21/c and Z' = 1. Polymorph β is metastable, and after about one year, it transforms to phase α. The isochoric recrystallization above 0.40 GPa yields a new polymorph γ of space group P21/a and Z' reduced to 1. The γ polymorph retrieved to ambient conditions for months has showed no signs of transformations. The main motif of OH…O bonded chains is retained in all three phases, but high pressure enforced identical conformation of closely packed molecules and their identical crystal environment in phase γ. [ABSTRACT FROM AUTHOR]