(Ga_0.71B_0.29)PO₄ with a high-cristobalite-type structure refined from powder data

Gallium boron phosphate, (Ga_0.71B_0.29)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.

Related literature

For information on cristobalite structures, see: Achary et al. (2003). For borophosphate structures, see: Ewald et al. (2007); Mi et al. (1999); Schmidt et al. (2004); Schulze (1934); Dachille & Glasser (1959); Mackenzie et al. (1959). For the catalytic properties of BPO₄, see: Moffat (1978); Moffat & Schmidtmeyer (1986); Mooney (1956); Morey et al. (1983); Tada et al. (1987); Tartarelli et al. (1970). For crystallographic background, see: Finger et al. (1994)); Thompson et al. (1987).

Experimental

Crystal data

• (Ga_0.71B_0.29)PO₄

• M_r = 147.61

• Tetragonal,

• a = 4.7343 (1) Å

• c = 7.0896 (4) Å

• V = 158.90 (1) ų

• Z = 2

• Cu Kα₁, Cu Kα₂ radiation

• λ = 1.5405, 1.5443 Å

• T = 293 K

• flat sheet, 10 × 10 mm

Data collection

• Rigaku-D/max automatic powder diffractometer

• Specimen mounting: packed powder pellet

• Data collection mode: reflection

• Scan method: step

• 2θ_min = 15.03°, 2θ_max = 100.02°, 2θ_step = 0.01°

Refinement

• R_p = 0.076

• R_wp = 0.129

• R_exp = 0.072

• R(F²) = 0.07586

• χ² = 3.204

• 8500 data points

• 35 parameters

• 2 restraints

Table 1
Selected bond lengths (Å)

(Ga/B)1—O2^i	1.7079 (5)
(Ga/B)2—O1^ii	1.6979 (5)
P1—O1	1.499 (3)
Symmetry codes: (i) x, y, z-1; (ii) x-1, y, z.

Cell refinement: GSAS (Larson & Von Dreele, 2004); data reduction: GSAS; program(s) used to refine structure: GSAS (Larson & Von Dreele, 2004); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: GSAS.

Supporting information

S1. 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, BPO₄ 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, BPO₄ 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 BPO₄, 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_0.71B_0.29)PO₄ are high-cristobalite structure instead of low-cristobalite type structure.

The crystal structure of (Ga_0.71B_0.29)PO₄ 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)O₄ and PO₄ 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 TO₄ (T = Ga(B), P) tetrahedron connects to four neighboring TO₄ 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 6₂.6₂.6₂.6₂.6₂.6₂, while 6₂.6₂.6₂.6₂.6₂.6₂ for the P nodes, giving the net symbol (6₂.6₂.6₂.6₂.6₂.6₂)₃ which can be represented by the short symbol (6⁶)₃.

The (Ga, B)1–O and (Ga, B)2–O bond distances are 1.7079 (5) Å and 1.6979 (5) Å in the (Ga, B)O₄ tetrahedra which are significantly larger than the B–O bond value of 1.463 Å in BPO₄ (Schmidt et al., 2004), but smaller than the bond values of 1.829 Å for Ga–O bond in GaPO₄ (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 BPO₄ to P4 for the new compound.

Related literature

For information on cristobalite structures, see: Achary et al. (2003). For borophosphate structures, see: Ewald et al. (2007); Mi et al. (1999); Schmidt et al. (2004); Schulze (1934). For the catalytic properties of BPO₄, see: Moffat (1978); Moffat & Schmidtmeyer (1986); Mooney (1956); Morey et al. (1983); Tada et al. (1987); Tartarelli et al. (1970). For related literature, see: Dachille & Glasser (1959); Mackenzie et al. (1959).For crystallographic background, see: Finger et al. (1994); Thompson et al. (1987).

S2. 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 H₃BO₃, NH₄H₂PO₄, and Ga₂O₃ with molar ratio of B:Ga:P = 12:3:4 was well 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 B₂O₃ in the products were washed out by hot water.

S3. 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 BPO₄, the starting atomic positional parameters can still be derived from the prototype BPO₄ (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.

Computing details

Data collection: PLEASE SUPPLY; cell refinement: PLEASE SUPPLY; data reduction: GSAS (Larson & Von Dreele, 2004); program(s) used to solve structure: PLEASE SUPPLY; program(s) used to refine structure: GSAS (Larson & Von Dreele, 2004); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: PLEASE SUPPLY.

Figures

	Fig. 1. Experimental (points) and calculated (lines) X-ray diffraction patterns of (Ga0.71B0.29)PO4. The difference profile is given at the bottom. The Bragg positions are indicated by the vertical marker below the observed pattern.
	Fig. 2. The crystal structure of (Ga0.71B0.29)PO4 viewed along the a-axis. (Ga,B)O4 tetrahedra: blue, PO4 tetrahedra: orange.
	Fig. 3. Topological figure for the network of (Ga0.71B0.29)PO4, oxygen atoms were omitted for clarity. Ga(B) atoms: blue spheres, P atoms: purple spheres.

gallium boron phosphate

Crystal data

(Ga0.71B0.29)PO4	Z = 2
Mr = 147.61	F(000) = 140.4
Tetragonal, P4	Dx = 3.084 Mg m^−3
Hall symbol: P -4	Cu Kα1, Cu Kα2 radiation, λ = 1.540500, 1.544300 Å
a = 4.7343 (1) Å	T = 293 K
c = 7.0896 (4) Å	white
V = 158.90 (1) Å^3	flat sheet, 10 × 10 mm

Data collection

Rigaku-D/max automatic powder diffractometer	Data collection mode: reflection
Graphite monochromator	Scan method: step
Specimen mounting: packed powder pellet	2θmin = 15.026°, 2θmax = 100.016°, 2θstep = 0.01°

Refinement

Least-squares matrix: full	Profile function: Thompson et al. (1987); Finger et al. (1994); Stephens et al. (1999)
Rp = 0.076	35 parameters
Rwp = 0.129	2 restraints
Rexp = 0.072	(Δ/σ)max = 0.001
R(F^2) = 0.07586	Background function: The background function is a cosine Fourier series with a leading constant term. Ib= B1+ΣBjcos[P*(j-1)] (j=2-9), here P = 2θ, Bj (j = 1-9) values are given below: 1: 935.903 2: -1634.98 3: 1422.47 4: -1094.93 5: 681.394 6: -358.046 7: 116.953 8: -17.2104 9: -22.9558
8500 data points

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ga1	0.5	0.5	0.0	0.0347 (12)*	0.7002 (4)
Ga2	0.0	0.0	0.5	0.0406 (11)*	0.7179 (3)
P1	0.5	0.0	0.7456 (5)	0.0447 (12)*
O1	0.7299 (3)	0.1326 (3)	0.6304 (5)	0.0493 (15)*
O2	0.6286 (3)	0.7718 (4)	0.8669 (5)	0.0511 (16)*
B1	0.5	0.5	0.0	0.0347 (12)*	0.2998 (4)
B2	0.0	0.0	0.5	0.0406 (11)*	0.2821 (3)

Geometric parameters (Å, º)

(Ga/B)1—P1^i	2.9759 (8)	(Ga/B)2—O1^ix	1.6979 (5)
(Ga/B)1—P1^ii	2.9759 (8)	(Ga/B)2—O1^x	1.6979 (5)
(Ga/B)1—P1^iii	2.9759 (8)	P1—O1	1.499 (3)
(Ga/B)1—P1^iv	2.9759 (8)	P1—O1^ix	1.499 (3)
(Ga/B)1—O2^i	1.7079 (5)	P1—O2^xi	1.509 (3)
(Ga/B)1—O2^iii	1.7079 (5)	P1—O2^xii	1.509 (3)
(Ga/B)1—O2^v	1.7079 (5)	B1—O2^i	1.7079 (5)
(Ga/B)1—O2^vi	1.7079 (5)	B1—O2^iii	1.7079 (5)
(Ga/B)2—P1^vii	2.9386 (8)	B1—O2^v	1.7079 (5)
(Ga/B)2—P1	2.9386 (8)	B1—O2^vi	1.7079 (5)
(Ga/B)2—P1^viii	2.9386 (8)	B2—O1^vii	1.6979 (5)
(Ga/B)2—P1^iii	2.9386 (8)	B2—O1^iii	1.6979 (5)
(Ga/B)2—O1^vii	1.6979 (5)	B2—O1^ix	1.6979 (5)
(Ga/B)2—O1^iii	1.6979 (5)	B2—O1^x	1.6979 (5)
O2^i—(Ga/B)1—O2^iii	107.765 (2)	O2^xi—P1—O2^xii	110.502 (4)
O2^i—(Ga/B)1—O2^v	112.941 (3)	(Ga/B)2^xiii—O1—P1	133.541 (1)
O2^i—(Ga/B)1—O2^vi	107.765 (2)	P1—O1—B2^xiii	133.541 (1)
O2^iii—(Ga/B)1—O2^v	107.765 (2)	(Ga/B)1^xiv—O2—P1^xv	135.267 (1)
O2^iii—(Ga/B)1—O2^vi	112.941 (3)	P1^xv—O2—B1^xiv	135.267 (1)
O2^v—(Ga/B)1—O2^vi	107.765 (2)	O2^i—B1—O2^iii	107.765 (2)
O1^vii—(Ga/B)2—O1^iii	107.239 (2)	O2^i—B1—O2^v	112.941 (3)
O1^vii—(Ga/B)2—O1^ix	114.035 (3)	O2^i—B1—O2^vi	107.765 (2)
O1^vii—(Ga/B)2—O1^x	107.239 (2)	O2^iii—B1—O2^v	107.765 (2)
O1^iii—(Ga/B)2—O1^ix	107.239 (2)	O2^iii—B1—O2^vi	112.941 (3)
O1^iii—(Ga/B)2—O1^x	114.035 (3)	O2^v—B1—O2^vi	107.765 (2)
O1^ix—(Ga/B)2—O1^x	107.239 (2)	O1^vii—B2—O1^iii	107.239 (2)
O1—P1—O1^ix	113.942 (3)	O1^vii—B2—O1^ix	114.035 (3)
O1—P1—O2^xi	108.506 (2)	O1^vii—B2—O1^x	107.239 (2)
O1—P1—O2^xii	107.696 (2)	O1^iii—B2—O1^ix	107.239 (2)
O1^ix—P1—O2^xi	107.696 (2)	O1^iii—B2—O1^x	114.035 (3)
O1^ix—P1—O2^xii	108.506 (2)	O1^ix—B2—O1^x	107.239 (2)

Symmetry codes: (i) x, y, z−1; (ii) x, y+1, z−1; (iii) y, −x+1, −z+1; (iv) y+1, −x+1, −z+1; (v) −x+1, −y+1, z−1; (vi) −y+1, x, −z+1; (vii) x−1, y, z; (viii) y, −x, −z+1; (ix) −x+1, −y, z; (x) −y, x−1, −z+1; (xi) x, y−1, z; (xii) −x+1, −y+1, z; (xiii) x+1, y, z; (xiv) x, y, z+1; (xv) x, y+1, z.