A new ammonium gallium hydroxyphosphate (NH 4 )Ga(OH)PO 4 was synthesized under mild hydrothermal conditions (200 ยฐ C, ฯ = 168 h). The equimolar content of Ga and P was determined by chemical analysis and electron probe X-ray microanalysis. The presence of NH 4 and OH groups was demonstrated by IR and Raman spectroscopy. An ab initio model of the crystal structure was refined by the Rietveld method (space group P 2 1 / m, Z = 2): a = 4.4832(1) ร
, b = 6.0430(1) ร
, c = 8.5674(1) ร
, ฮฒ = 98.019(1)ยฐ, R p = 0.0552, R wp = 0.0723. A zero SHG signal (T = 300 K) confirmed a centrosymmetric structure of the compound. The structure contains layers composed of GaO 4 (OH) 2 octahedra and PO 4 tetrahedra. The interlayer space accommodates ammonium cations. The layer is based on linear chains of edge-sharing GaO 4 (OH) 2 octahedra with a zigzag trans-arranged -Ga-(OH)-Ga-(OH)-backbone. The construction of the layer in (NH 4 )Ga(OH)PO 4 was found to be topologically related to that in (En) 0.5 Fe(OH)PO 4 . The effect of the gradual F - OH -substitution in the quasi-morphotropic series (NH 4 )GaF 1 -ฮด (OH) ฮด PO 4 (ฮด = 0, 0.5, 1.0) on the degree of polarization of the mixed anionic radical was considered. (NH 4 )Ga(OH)PO 4 is thermally unstable: removal of NH 3 and H 2 O molecules in the range 170-450ยฐC is accompanied by the formation of two polymorphs of GaPO 4 .