RbGa₃(P₃O₁₀)₂: a new gallium phosphate isotypic with RbAl₃(P₃O₁₀)₂

Rubidium trigallium bis(triphosphate), RbGa₃(P₃O₁₀)₂ has been synthesized by solid-state reaction and studied by single-crystal X-ray diffraction at room temperature. This compound is the first anhydrous gallium phosphate containing both GaO₄ tetra­hedra (Ga1) and GaO₆ octa­hedra (Ga2 and Ga3). The three independent Ga atoms are located on sites with imposed symmetry 2 (Wickoff positions 4a for Ga1 and 4b for Ga2 and Ga3). The GaO₄ and GaO₆ polyhedra are connected through the apices to triphosphate groups and form a three-dimensionnal host lattice. This framework presents inter­secting tunnels running along the [001] and <110> directions, where the Rb²⁺ cations are located on sites with imposed symmetry 2 (Wickoff position 4a). The structure also exhibits remarkable features, such as infinite helical columns created by the junction of GaO₄ and PO₄ tetra­hedra.

S1. Comment

In the perspective of synthesizing original mixed frameworks, we have explored the A₂O–M₂O₃–P₂O₅ pseudo-ternary system by solid-state reaction. M has been chosen to be a trivalent metal such as Al or Ga, which can present octahedral, tetrahedral or bipyramidal coordination, and A is a large alkaline cation such as Cs or Rb. Notably, such compounds can have applications as molecular sieves (Cheetham et al., 1999; Davis, 1997). Our previous experiments close to the cationic composition 10:30:60 for A:M:P led to the characterization of several new phases. In particular, for M = Al, two closely related original structures were discovered, according to the size of the cation, namely CsAl₃(P₃O₁₀)₂ and RbAl₃(P₃O₁₀)₂ (Lesage et al., 2005). The gallium analogue of the caesium aluminium compound has not been synthesized, since our experiments have produced the pentaphosphate, CsGa₂P₅O₁₆ (Lesage et al., 2004). However, the structure of RbGa₃(P₃O₁₀)₂, isotypic with RbAl₃(P₃O₁₀)₂, is presented here.

The projections of the structure along c (Fig. 1) and [110] show that the Rb cation sits at the intersection of tunnels running along [001] and <110> in the [Ga₃P₆O₂₀]_∞ three-dimensionnal framework. The latter is built from P₃O₁₀ triphosphate groups, sharing their corners with both GaO₄ tetrahedra and GaO₆ octahedra. More precisely, one can observe that the connection between triphosphate groups and GaO₄ tetrahedra forms infinite isolated [GaP₆O₂₀]_∞ tetrahedral columns (Fig. 2). In fact, such columns can be decomposed in an elemental GaP₆O₂₂ helical pattern built up of GaO₄ linked through the apices with two P₃O₁₀ triphosphate groups (Fig. 3). Their junction through the 2₁ screw axis parallel to c gives rise to two interlaced infinite helical chains of tetrahedra (Fig. 2). The entire framework results from the assembly of these helical tetrahedral columns through the GaO₆ octahedra (Fig. 1).

The geometry of the P₃O₁₀ group is close to that commonly observed in other triphosphates (Averbuch-Pouchot & Durif, 1996). As expected, the average values for PO₄ tetrahedra are 1.54 Å for P—O bonds and 109.3° for O—P—O angles. Moreover, two sets of distances can be distinguished, since the P—O bonds corresponding to the two P—O—P bridges of the P₃O₁₀ group are significantly larger [1.591 (3)–1.632 (2) Å] than the other P—O bonds [1.485 (2)–1.540 (3) Å]. The geometries of GaO₄ and GaO₆ polyhedra are rather regular, with Ga—O distances of 1.793 (2) and 1.830 (2) Å for GaO₄, and ranging from 1.919 (2) to 1.981 (2) Å for GaO₆. Finally, the Rb cation is surrounded by eight O atoms, with distances ranging from 2.9764 (15) to 3.269 (2) Å. These distances are also in agreement with bond valence sum calculations (Brese & O'Keeffe, 1991), as reported in Table 1, since the Rb, Ga and P cations and O anions have calculated valences close to the theoretical values (1, 3, 5 and 2, respectively).

The Rb—O distances are very similar in the two isotypic structures, since they range from 2.958 (3) to 3.306 (2) Å for RbAl₃(P₃O₁₀)₂. However, a significant variation in the cell volume of the two phases is observed [1579.42 (17) ų for RbGa₃(P₃O₁₀)₂, compared with 1516.75 (18) ų for RbAl₃(P₃O₁₀)₂]. This is in agreement with the increase of the ionic radius of the corresponding trivalent element. This is also evidenced by examination of the Ga—O distances, which are indeed significantly larger than the Al—O distances [1.715 (3)–1.746 (2) Å in AlO₄ and 1.847 (3)–1.911 (3) Å in AlO₆].

The triphosphate groups show a close geometry in the two isotypic compounds, with average P—P distances of 2.858 Å and 2.845 Å, respectively, and a P—P—P angle of 126.24° and 126.32°, respectively (Table 2). This geometry is induced by the fact that the P₂O₇ group belonging to the triphosphate shares two apices with the same GaO₆ octahedron in these phases, as discussed by Lesage et al. (2005) for AAl₃(P₃O₁₀)₂ (A = Cs or Rb) structures (Fig. 3).

RbGa₃(P₃O₁₀)₂ is the second gallium triphosphate after Cs₂GaP₃O₁₀ (Guesdon et al., 2002) to be synthesized by a solid-state reaction, i.e. not containing H atoms. Furthermore, it is worth mentioning that it is the first gallium phosphate prepared in this way which presents two types of coordination for Ga in the same structure.

S2. Experimental

The single-crystal used for the determination of RbGa₃(P₃O₁₀)₂ was extracted from a preparation of nominal composition Rb₃Ga₅P₁₂O₃₉, synthesized in two steps. First, RbNO₃ (Chempur, 99.9%), Ga₂O₃ (Alfa Aesar, 99.9%) and (NH₄)₂HPO₄ (Prolabo Normapur, 99.5%) were mixed in an agate mortar. This whitish mixture was placed in a platinum crucible and heated in air at about 770 K for a few hours until the correct weight loss was reached, i.e. when RbNO₃ and (NH₄)₂HPO₄ had decomposed. The resulting powder was finely ground a second time in an agate mortar and placed in a silica tube, which was then evacuated and sealed. The silica tube was heated at 1103 K for 20 h before being slowly cooled at 1 K h⁻¹ down to 1063 K, then at 10 K h⁻¹ down to 863 K. A white powder containing small colourless crystals was thus obtained. Semi-quantitative analysis of a colourless crystal extracted from the preparation was performed with an Oxford 6650 microprobe mounted on a Philips XL30 FEG scanning electron microscope. The cationic composition obtained was in agreement with the expected theoretical value of 10:30:60 for the Rb, Ga and P cations, respectively. Several crystals were then optically selected for testing. Three crystals were studied. The structure was determined using the heavy-atom method and successive difference synthesis and Fourier synthesis for the first crystal, then starting from the data previously determined for the other two crystals. The existence conditions hkl: h + k = 2n and 00l: l = 2n are consistent with the non-centrosymmetric space group C222₁ (n°20) [Not expected nomenclature - please check]. As a consequence, the compound may adopt two enantiomorphic structures. Structure determinations and refinements showed that one of the crystals studied is a pure enantiomorph, whereas the other two are twinned by inversion. We present here only the results for the pure enantiomorph. The Flack parameter (Flack & Bernardinelli, 1999) was refined to −0.009 (7). For the other two crystals, the results of which are not presented here, the Flack parameter is close to 0.5. The harmonic displacement parameters also have significantly higher values in the twinned crystals than in the pure one, with U_iso,eq ≈ 0.02 Ų instead of 0.01 Ų for the O atoms.

Computing details

Data collection: EVALCCD (Duisenberg et al., 2003); cell refinement: EVALCCD (Duisenberg et al., 2003); data reduction: Jana2000 (Petricek and Dusek, 2000); program(s) used to solve structure: Jana2000 (Petricek and Dusek, 2000); program(s) used to refine structure: Jana2000 (Petricek and Dusek, 2000); molecular graphics: Diamond version 2.1e (Brandenburg, 2001); software used to prepare material for publication: Jana2000 (Petricek and Dusek, 2000).

Figures

	Fig. 1. A projection of RbGa3(P3O10)2 along c.
	Fig. 2. A perspective view of an isolated [GaP6O20]∞ tetrahedral column. One GaP6O22 helical elemental pattern is highlighted by bold lines.
	Fig. 3. A perspective view of one GaP6O22 helical elemental pattern (black lines) linked to two GaO6 octahedra (white lines). For clarity, only one triphosphate group has been labelled. Displacement ellipsoids are drawn at the 70% probability level. [Symmetry codes: (vi) 1/2 − x, 1/2 + y, 1/2 − z; (viii) 1 − x, y, 1/2 − z; (ix) 1/2 + x, 1/2 + y, z; (x) 1/2 − x, 1/2 − y, 1/2 + z; (xi) 1 − x, 1 − y, 1/2 + z. Please clarify - the codes given in the figure are (i), (ii), (iii), (iv) and (v).]

rubidium trigallium bis(triphosphate)

Crystal data

RbGa3(P3O10)2	F(000) = 1520
Mr = 800.5	Dx = 3.365 Mg m^−3
Orthorhombic, C2221	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: C 2c 2	Cell parameters from 14620 reflections
a = 10.0017 (8) Å	θ = 6.0–42.0°
b = 13.0822 (8) Å	µ = 8.88 mm^−1
c = 12.0710 (4) Å	T = 298 K
V = 1579.42 (17) Å^3	Polyhedra, colourless
Z = 4	0.08 × 0.06 × 0.05 mm

Data collection

Nonius CCD area-detector diffractometer	5415 independent reflections
Radiation source: fine-focus sealed X-ray tube	2778 reflections with I > 3σ(I)
Graphite monochromator	Rint = 0.088
Detector resolution: 9.09 pixels mm^-1	θmax = 42.0°, θmin = 6.0°
ϕ and ω scans	h = −18→18
Absorption correction: gaussian (JANA2000; Petříček & Dušek, 2000)	k = −24→24
Tmin = 0.703, Tmax = 0.849	l = −22→19
14620 measured reflections

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
R[F^2 > 2σ(F^2)] = 0.035	(Δ/σ)max = 0.0004
wR(F^2) = 0.034	Δρmax = 1.36 e Å^−3
S = 0.90	Δρmin = −1.24 e Å^−3
5415 reflections	Absolute structure: Flack and Bernardinelli (1999), 2364 Friedel pairs
139 parameters	Absolute structure parameter: −0.009 (7)

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

	x	y	z	Uiso*/Ueq
Rb1	0.47254 (5)	0	0.5	0.02601 (15)
Ga1	0.13548 (4)	0	0	0.00854 (12)
Ga2	0.5	0.25501 (3)	0.25	0.00550 (11)
Ga3	0	0.29389 (3)	0.25	0.00649 (11)
P1	0.05518 (7)	0.20469 (6)	0.48826 (7)	0.00646 (18)
P2	0.23061 (7)	0.13610 (6)	0.31723 (6)	0.00584 (18)
P3	0.23825 (7)	−0.06473 (6)	0.22343 (6)	0.00584 (19)
O1	−0.0339 (2)	0.11134 (17)	0.47918 (19)	0.0168 (7)
O2	0.0796 (2)	0.24184 (18)	0.60341 (17)	0.0105 (6)
O3	0.0147 (2)	0.29005 (15)	0.41175 (16)	0.0091 (5)
O4	0.1994 (2)	0.16421 (17)	0.44281 (17)	0.0098 (6)
O5	0.3780 (2)	0.14535 (16)	0.30075 (17)	0.0089 (6)
O6	0.1408 (2)	0.19230 (15)	0.24000 (18)	0.0094 (5)
O7	0.19002 (19)	0.01837 (15)	0.31429 (17)	0.0096 (5)
O8	0.25546 (19)	−0.00520 (18)	0.11425 (15)	0.0095 (5)
O9	0.1292 (2)	−0.14057 (17)	0.20867 (17)	0.0123 (6)
O10	0.3701 (2)	−0.09922 (16)	0.2664 (2)	0.0122 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb1	0.0228 (2)	0.0222 (2)	0.0329 (3)	0	0	−0.0052 (2)
Ga1	0.00916 (19)	0.0086 (2)	0.0079 (2)	0	0	0.00030 (17)
Ga2	0.00598 (18)	0.00624 (18)	0.00427 (19)	0	−0.00015 (17)	0
Ga3	0.00625 (19)	0.00669 (18)	0.0065 (2)	0	−0.00102 (18)	0
P1	0.0076 (3)	0.0071 (3)	0.0047 (4)	−0.0007 (2)	0.0007 (3)	−0.0002 (3)
P2	0.0062 (3)	0.0056 (3)	0.0057 (3)	0.0005 (3)	0.0008 (3)	−0.0008 (3)
P3	0.0062 (3)	0.0044 (3)	0.0069 (4)	0.0001 (2)	−0.0004 (2)	0.0007 (3)
O1	0.0210 (11)	0.0167 (11)	0.0127 (13)	−0.0091 (9)	0.0016 (9)	−0.0008 (9)
O2	0.0128 (10)	0.0159 (11)	0.0029 (10)	−0.0022 (8)	0.0010 (7)	−0.0030 (9)
O3	0.0107 (10)	0.0099 (8)	0.0066 (9)	0.0038 (9)	0.0005 (8)	−0.0008 (7)
O4	0.0116 (10)	0.0130 (10)	0.0047 (10)	0.0036 (8)	0.0008 (8)	−0.0012 (8)
O5	0.0067 (9)	0.0089 (10)	0.0113 (11)	0.0001 (8)	0.0008 (8)	0.0004 (8)
O6	0.0117 (9)	0.0120 (9)	0.0043 (10)	0.0059 (7)	−0.0009 (8)	0.0001 (9)
O7	0.0133 (9)	0.0053 (9)	0.0102 (10)	−0.0021 (7)	0.0045 (7)	−0.0025 (8)
O8	0.0100 (8)	0.0116 (10)	0.0069 (9)	−0.0037 (8)	−0.0004 (7)	0.0037 (8)
O9	0.0135 (10)	0.0106 (10)	0.0129 (11)	−0.0071 (8)	−0.0039 (9)	0.0029 (8)
O10	0.0098 (9)	0.0123 (9)	0.0147 (13)	0.0032 (7)	−0.0036 (8)	−0.0024 (8)

Geometric parameters (Å, º)

Rb1—O3^i	2.9764 (15)	Ga3—O3	1.959 (2)
Rb1—O3^ii	2.9764 (15)	Ga3—O3^vii	1.959 (2)
Rb1—O5	3.208 (3)	Ga3—O6	1.9401 (17)
Rb1—O5^iii	3.208 (3)	Ga3—O6^vii	1.9401 (17)
Rb1—O8^iv	3.051 (2)	Ga3—O10^xii	1.919 (2)
Rb1—O8^v	3.051 (2)	Ga3—O10^xi	1.919 (2)
Rb1—O10	3.269 (2)	P1—O1	1.516 (3)
Rb1—O10^iii	3.269 (2)	P1—O2	1.492 (3)
Ga1—O1^vi	1.793 (2)	P1—O3	1.504 (2)
Ga1—O1^vii	1.793 (2)	P1—O4	1.632 (2)
Ga1—O8	1.830 (2)	P2—O4	1.591 (3)
Ga1—O8^viii	1.830 (2)	P2—O5	1.492 (2)
Ga2—O2^ix	1.941 (2)	P2—O6	1.489 (2)
Ga2—O2^ii	1.941 (2)	P2—O7	1.593 (3)
Ga2—O5	1.981 (2)	P3—O7	1.618 (3)
Ga2—O5^v	1.981 (2)	P3—O8	1.540 (3)
Ga2—O9^x	1.945 (2)	P3—O9	1.485 (2)
Ga2—O9^xi	1.945 (2)	P3—O10	1.487 (2)
O3^i—Rb1—O3^ii	163.71 (6)	O2^ii—Ga2—O5^v	92.59 (10)
O3^i—Rb1—O5	108.70 (6)	O2^ii—Ga2—O9^x	86.91 (10)
O3^i—Rb1—O5^iii	76.29 (6)	O2^ii—Ga2—O9^xi	91.36 (10)
O3^i—Rb1—O8^iv	93.21 (6)	O5—Ga2—O5^v	87.15 (10)
O3^i—Rb1—O8^v	72.02 (6)	O5—Ga2—O9^x	175.68 (9)
O3^i—Rb1—O10	50.90 (6)	O5—Ga2—O9^xi	91.15 (10)
O3^i—Rb1—O10^iii	136.01 (6)	O5^v—Ga2—O5	87.15 (10)
O3^ii—Rb1—O3^i	163.71 (6)	O5^v—Ga2—O9^x	91.15 (10)
O3^ii—Rb1—O5	76.29 (6)	O5^v—Ga2—O9^xi	175.68 (9)
O3^ii—Rb1—O5^iii	108.70 (6)	O9^x—Ga2—O9^xi	90.81 (10)
O3^ii—Rb1—O8^iv	72.02 (6)	O9^xi—Ga2—O9^x	90.81 (10)
O3^ii—Rb1—O8^v	93.21 (6)	O3—Ga3—O3^vii	177.02 (6)
O3^ii—Rb1—O10	136.01 (6)	O3—Ga3—O6	89.41 (9)
O3^ii—Rb1—O10^iii	50.90 (6)	O3—Ga3—O6^vii	88.55 (9)
O5—Rb1—O5^iii	145.71 (5)	O3—Ga3—O10^xii	88.10 (9)
O5—Rb1—O8^iv	126.06 (6)	O3—Ga3—O10^xi	94.07 (9)
O5—Rb1—O8^v	86.39 (6)	O3^vii—Ga3—O3	177.02 (6)
O5—Rb1—O10	59.75 (6)	O3^vii—Ga3—O6	88.55 (9)
O5—Rb1—O10^iii	108.60 (6)	O3^vii—Ga3—O6^vii	89.41 (9)
O5^iii—Rb1—O5	145.71 (5)	O3^vii—Ga3—O10^xii	94.07 (9)
O5^iii—Rb1—O8^iv	86.39 (6)	O3^vii—Ga3—O10^xi	88.10 (9)
O5^iii—Rb1—O8^v	126.06 (6)	O6—Ga3—O6^vii	93.52 (7)
O5^iii—Rb1—O10	108.60 (6)	O6—Ga3—O10^xii	175.57 (9)
O5^iii—Rb1—O10^iii	59.75 (6)	O6—Ga3—O10^xi	90.08 (8)
O8^iv—Rb1—O8^v	53.85 (6)	O6^vii—Ga3—O6	93.52 (7)
O8^iv—Rb1—O10	132.72 (6)	O6^vii—Ga3—O10^xii	90.08 (8)
O8^iv—Rb1—O10^iii	83.14 (6)	O6^vii—Ga3—O10^xi	175.57 (9)
O8^v—Rb1—O8^iv	53.85 (6)	O10^xii—Ga3—O10^xi	86.44 (10)
O8^v—Rb1—O10	83.14 (6)	O10^xi—Ga3—O10^xii	86.44 (10)
O8^v—Rb1—O10^iii	132.72 (6)	O1—P1—O2	115.18 (14)
O10—Rb1—O10^iii	143.47 (5)	O1—P1—O3	113.30 (12)
O10^iii—Rb1—O10	143.47 (5)	O1—P1—O4	103.51 (13)
O1^vi—Ga1—O1^vii	110.97 (10)	O2—P1—O3	111.98 (13)
O1^vi—Ga1—O8	116.55 (11)	O2—P1—O4	105.91 (12)
O1^vi—Ga1—O8^viii	107.23 (11)	O3—P1—O4	105.82 (13)
O1^vii—Ga1—O1^vi	110.97 (10)	O4—P2—O5	107.57 (13)
O1^vii—Ga1—O8	107.23 (11)	O4—P2—O6	111.36 (13)
O1^vii—Ga1—O8^viii	116.55 (11)	O4—P2—O7	101.22 (13)
O8—Ga1—O8^viii	98.02 (10)	O5—P2—O6	118.23 (13)
O8^viii—Ga1—O8	98.02 (10)	O5—P2—O7	109.08 (13)
O2^ix—Ga2—O2^ii	177.54 (11)	O6—P2—O7	108.04 (11)
O2^ix—Ga2—O5	92.59 (10)	O7—P3—O8	105.86 (14)
O2^ix—Ga2—O5^v	89.20 (10)	O7—P3—O9	108.13 (13)
O2^ix—Ga2—O9^x	91.36 (10)	O7—P3—O10	103.40 (13)
O2^ix—Ga2—O9^xi	86.91 (10)	O8—P3—O9	108.53 (13)
O2^ii—Ga2—O2^ix	177.54 (11)	O8—P3—O10	110.63 (13)
O2^ii—Ga2—O5	89.20 (10)	O9—P3—O10	119.36 (14)

Symmetry codes: (i) x+1/2, y−1/2, z; (ii) x+1/2, −y+1/2, −z+1; (iii) x, −y, −z+1; (iv) −x+1, −y, z+1/2; (v) −x+1, y, −z+1/2; (vi) −x, −y, z−1/2; (vii) −x, y, −z+1/2; (viii) x, −y, −z; (ix) −x+1/2, −y+1/2, z−1/2; (x) x+1/2, y+1/2, z; (xi) −x+1/2, y+1/2, −z+1/2; (xii) x−1/2, y+1/2, z.