Phase transition in (NH4)H-5(PO4)(2)

X-ray single crystal study of (NH4)H-5(PO4)(2) revealed the existence of tw o modifications with similar unit cell dimensions. Low temperature (NH4)(H2 PO4) . H3PO4 (alpha) was investigated at 130 K. The crystal parameters are: monoclinic, a = 10.197(4) Angstrom, b = 7.711(2) Angstrom, c = 9.617(3) An gstrom, beta = 101.69(4)degrees, Z = 4, space group P2(1)/c, R-1 = 0.0244. High temperature (NH4)[H(H-2(PO4)(2)] (beta) was investigated at 220 K. The crystal parameters are: monoclinic, a = 10.332(4) Angstrom, b = 7.702(3) A ngstrom, c = 9.604(4) Angstrom, beta = 101.18(4)degrees, Z = 4, space group C2/c, R-1 = 0.0239. The phase transition alpha <-> beta is characterized b y a hysteresis in the temperature range 180-210 K. The ammonium cation and the H atom of a short hydrogen bond 2.445(2) Angstrom which are ordered in the alpha-modification become disordered in the structure of the beta-form.