Hydrothermal synthesis and crystal structure of a new ammonium gallium hydroxyphosphate (NH4)Ga(OH)PO4

A new ammonium gallium hydroxyphosphate (NH4)Ga(OH)PO4 was synthesized under mild hydrothermal conditions (200 degrees C, tau = 168 h). The equimolar content of Ga and P was determined by chemical analysis and electron probe X-ray microanalysis. The presence of NH4 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 P2(1)/m, Z = 2): a = 4.4832(1) angstrom, b = 6.0430(1) angstrom, c = 8.5674(1) angstrom, beta = 98.019(1)degrees, 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 GaO4(OH)(2) octahedra and PO4 tetrahedra. The interlayer space accommodates ammonium cations. The layer is based on linear chains of edge-sharing GaO4(OH)(2) octahedra with a zigzag trans-arranged-Ga-(OH)-Ga-(OH)-backbone. The construction of the layer in (NH4)Ga(OH)PO4 was found to be topologically related to that in (En)(0.5)Fe(OH)PO4. The effect of the gradual F- -> OH- substitution in the quasi-morphotropic series (NH4)GaF1-delta(OH)(delta)PO4 (delta = 0, 0.5, 1.0) on the degree of polarization of the mixed anionic radical was considered. (NH4)Ga(OH)PO4 is thermally unstable: removal of NH3 and H2O molecules in the range 170-450 degrees C is accompanied by the formation of two polymorphs of GaPO4.