Structure of the high-pressure phase IV of KH2PO4 (KDP).

The high-pressure phase IV of potassium dihydrogen phosphate (KH(2)PO(4), KDP) has been determined at 1.62 GPa/296 K. It is monoclinic, space group P2(1)/c, Z = 12, and on decompression transforms into monoclinic phase I. The interplay of OH···O bonding networks and K(+) coordination types leads to an exceptionally large number of KDP polymorphs, at least 13 according to the literature.