CRYSTAL-STRUCTURE OF A NEW MONOCLINIC FORM OF POTASSIUM DIHYDROGEN PHOSPHATE-CONTAINING ORTHOPHOSPHACIDIUM ION, [H4PO4](+)

The crystal structure of a new monoclinic form of potassium dihydrogen phosphate, KH2PO4, has been determined by single crystal X-ray diffraction. Crystals are monoclinic, space group P2/c with a = 7.4399(7), b = 7.2634(9), c = 9.3629(13) Angstrom, beta = 127.696(8)degrees, V = 400.35(9) Angstrom(3), d(m) = 2.25, d(c) = 2.257 Mg m(-3) for Z = 4, mu = 17.9 cm(-1), F(000) = 272, T = 293K. The structure was refined to R = 0.022 and R(w) = 0.038 for 740 reflections with I greater than or equal to 3 sigma(I). The crystal structure consists of a compact assembly of potassium and phosphate ions, arranged in columns along the b-axis, bound by K...O ionic bonds and O-H...O hydrogen bonds in a three dimensional network as in all other forms of KH2PO4 and KD2PO4. The two crystallographically nonequivalent phosphate groups appear to exist as an orthophosphacidium ion, [H4PO4](+) and a PO4-3 ion rather than as the expected H2PO4 ions. This study provides the first example of crystal structure determination of the [P(OH)(4)](+) ion. (C) 1995 Academic Press, Inc.