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 (1 alpha); however, recrystallization is needed for obtaining a low Z', more dense polymorph. Three polymorphs alpha, beta and gamma can be monotonically compressed to 2.0 GPa at least. At ambient pressure 1 crystallizes in space group C2/c, Z' = 4 as polymorph 1 alpha, or as lower-density polymorph 1 beta, of space group P2(1)/c and Z' = 1. Polymorph beta is metastable, and after about one year, it transforms to phase alpha. The isochoric recrystallization above 0.40 GPa yields a new polymorph gamma of space group P2(1)/a and Z' reduced to 1. The polymorph retrieved to ambient conditions for months has showed no signs of transformations. The main motif of OH center dot center dot center dot 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 gamma.