(Ga0.71B0.29)PO4 with a high-cristobalite-type structure refined from powder data

Gallium boron phosphate, (Ga0.71B0.29)PO4, was synthesized by a high-temperature solid-state reaction method. The crystal structure is isostructural with the tetragonal high-cristobalite structure with space group P which is built from alternating Ga(B)O4 and PO4 tetrahedra interconnected by sharing the common O-atom vertices, resulting in a three-dimensional structure with two-dimensional six-membered-ring tunnels running along the a and b axes.

Gallium boron phosphate, (Ga 0.71 B 0.29 )PO 4 , was synthesized by a high-temperature solid-state reaction method. The crystal structure is isostructural with the tetragonal high-cristobalite structure with space group P4 which is built from alternating Ga(B)O 4 and PO 4 tetrahedra interconnected by sharing the common O-atom vertices, resulting in a three-dimensional structure with two-dimensional six-membered-ring tunnels running along the a and b axes.

Comment
The high-cristobalite boron phosphate has long been used as an effective catalyst for various organic reactions such as hydration, dehydration, oligomerization (Moffat, 1978;Moffat & Schmidtmeyer, 1986;Morey et al., 1983;Tada et al., 1987;Tartarelli et al., 1970). The catalytic activities depend on the ratio of P/B and surface area. In the case of excess B content, BPO 4 catalysts consist predominately of Lewis acid sites, and show catalytic efficiencies for the dehydration. In contrast, in a region consisting of excess phosphorus P content, BPO 4 catalysts have more Brønsted acid sites and exhibit catalytic activities for hydration. Applying trivalent cations to partially substitute boron may vary the ratio of P/B and modify the catalytic property. The possibility of modifying the catalytic properties by varieties of P/B ratio and searching for new phases in the borophosphate system intrigue us to investigate systems containing larger trivalent metal cations. In our previous investigations, a series of compounds with boron partially substituted by transition metals, such as Mn, Fe, Co, Ni, and Cu, has been characterized with low cristobalite type structure (Mi et al., 1999). When we applied a smaller trivalent element Ga to modify the BPO 4 , the occupancy of Ga is more than 50%, while less than 50% for transition metal compounds (M = Mn, Fe, Co, Ni, and Cu). In consequence, the structure of (Ga 0.71 B 0.29 )PO 4 are high-cristobalite structure instead of low-cristobalite type structure.
The crystal structure of (Ga 0.71 B 0.29 )PO 4 is isostructural with the tetragonal high-cristobalite structure (Schulze, 1934;Schmidt et al., 2004) with space group P4 which is built from alternating (Ga, B)O 4 and PO 4 tetrahedra interconnected by sharing the common O-vertices, resulting in a three dimensional network with two dimensional 6-membered ring tunnels running along the a-and b-axis, respectively. Every TO 4 (T = Ga(B), P) tetrahedron connects to four neighboring TO 4 tetrahedra. There are three types of positions for T-atoms. (Ga, B)1 and (Ga, B)2 sit at 1c and 1b, while P at 2g. The long (Schläfli) notation for (Ga, B) nodes is 6 2 .6 2 .6 2 .6 2 .6 2 .6 2 , while 6 2 .6 2 .6 2 .6 2 .6 2 .6 2 for the P nodes, giving the net symbol (6 2 .6 2 .6 2 .6 2 .6 2 .6 2 ) 3 which can be represented by the short symbol (6 6 ) (Achary et al., 2003), indicating that boron and gallium occupy the same position. After refining both the atomic occupation number and displacement parameters, it results in the ratio of Ga:B = 0.71:0.29. In turn, the Ga:P is 1.42:2, which is quite good agreement with that (Ga:P = 3:4) in the reactants for obtaining the pure phase. The introduction of gallium in the compound led to the deformation of all the tetrahedra and quite anisotropic expansion of the structure which results in lowering symmetry from space group I4 of BPO 4 to P4 for the new compound.

Experimental
The title compound has been synthesized via high temperature solid state reaction method and the structure refined from X-ray powder diffraction data. A mixture of H 3 BO 3 , NH 4 H 2 PO 4 , and Ga 2 O 3 with molar ratio of B:Ga:P = 12:3:4 was well supplementary materials sup-2 ground and reacted first at 973 K for 4 h, then cooled down to room temperature and reground again, pressed into pellets and reacted at 1373 K for 8 h, at last shut down the furnace and cooled down to room temperature. The extra B 2 O 3 in the products were washed out by hot water.

Refinement
The cell parameters were obtained by least-square fits of the powder diffractometer data using silicon (a = 5.4308 Å) as an internal standard. Although the powder pattern and cell parameters are quite different from BPO 4 , the starting atomic positional parameters can still be derived from the prototype BPO 4 (Schmidt et al., 2004). During the initial refinement, the unreasonable negative thermal parameters for the B position are indicative of partial substitutions by Ga. The boron position then were assumed to be occupied by two kinds of atoms and the occupacies were allowed to vary during the subsequent refinements. Because it is difficult to refine both the occupation numbers and atomic displacement parameters at the same time, a two-step process was applied to refine the occupancy numbers and atomic displacement parameters. At the begining, all the atomic displacement parameters were set to one value to refine the occupancy number, then fixed the occupany number to refine the displacement parameters. Both processes were performed alternately several times till reasonable values for both atomic occupancies and displacement parameters were obtained. Due to the individual refinement, the standard deviations given by the program are much too small to be a realistic estimate of the uncertainty. Fig. 1. Experimental (points) and calculated (lines) X-ray diffraction patterns of (Ga 0.71 B 0.29 )PO 4 . The difference profile is given at the bottom. The Bragg positions are indicated by the vertical marker below the observed pattern.