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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.
Supporting information
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 Uiso,eq ≈ 0.02 Å2 instead of 0.01 Å2 for the O atoms.
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).
rubidium trigallium bis(triphosphate)
top
Crystal data top
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) Å3 | Polyhedra, colourless |
Z = 4 | 0.08 × 0.06 × 0.05 mm |
Data collection top
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 top
Refinement on F | Weighting scheme based on measured s.u.'s w = 1/(σ2(F) + 0.0001F2) |
R[F2 > 2σ(F2)] = 0.035 | (Δ/σ)max = 0.0004 |
wR(F2) = 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) |
Crystal data top
RbGa3(P3O10)2 | V = 1579.42 (17) Å3 |
Mr = 800.5 | Z = 4 |
Orthorhombic, C2221 | Mo Kα radiation |
a = 10.0017 (8) Å | µ = 8.88 mm−1 |
b = 13.0822 (8) Å | T = 298 K |
c = 12.0710 (4) Å | 0.08 × 0.06 × 0.05 mm |
Data collection top
Nonius CCD area-detector diffractometer | 5415 independent reflections |
Absorption correction: gaussian (JANA2000; Petříček & Dušek, 2000) | 2778 reflections with I > 3σ(I) |
Tmin = 0.703, Tmax = 0.849 | Rint = 0.088 |
14620 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.035 | Δρmax = 1.36 e Å−3 |
wR(F2) = 0.034 | Δρmin = −1.24 e Å−3 |
S = 0.90 | Absolute structure: Flack and Bernardinelli (1999), 2364 Friedel pairs |
5415 reflections | Absolute structure parameter: −0.009 (7) |
139 parameters | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | 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 (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
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) top
Rb1—O3i | 2.9764 (15) | Ga3—O3 | 1.959 (2) |
Rb1—O3ii | 2.9764 (15) | Ga3—O3vii | 1.959 (2) |
Rb1—O5 | 3.208 (3) | Ga3—O6 | 1.9401 (17) |
Rb1—O5iii | 3.208 (3) | Ga3—O6vii | 1.9401 (17) |
Rb1—O8iv | 3.051 (2) | Ga3—O10xii | 1.919 (2) |
Rb1—O8v | 3.051 (2) | Ga3—O10xi | 1.919 (2) |
Rb1—O10 | 3.269 (2) | P1—O1 | 1.516 (3) |
Rb1—O10iii | 3.269 (2) | P1—O2 | 1.492 (3) |
Ga1—O1vi | 1.793 (2) | P1—O3 | 1.504 (2) |
Ga1—O1vii | 1.793 (2) | P1—O4 | 1.632 (2) |
Ga1—O8 | 1.830 (2) | P2—O4 | 1.591 (3) |
Ga1—O8viii | 1.830 (2) | P2—O5 | 1.492 (2) |
Ga2—O2ix | 1.941 (2) | P2—O6 | 1.489 (2) |
Ga2—O2ii | 1.941 (2) | P2—O7 | 1.593 (3) |
Ga2—O5 | 1.981 (2) | P3—O7 | 1.618 (3) |
Ga2—O5v | 1.981 (2) | P3—O8 | 1.540 (3) |
Ga2—O9x | 1.945 (2) | P3—O9 | 1.485 (2) |
Ga2—O9xi | 1.945 (2) | P3—O10 | 1.487 (2) |
| | | |
O3i—Rb1—O3ii | 163.71 (6) | O2ii—Ga2—O5v | 92.59 (10) |
O3i—Rb1—O5 | 108.70 (6) | O2ii—Ga2—O9x | 86.91 (10) |
O3i—Rb1—O5iii | 76.29 (6) | O2ii—Ga2—O9xi | 91.36 (10) |
O3i—Rb1—O8iv | 93.21 (6) | O5—Ga2—O5v | 87.15 (10) |
O3i—Rb1—O8v | 72.02 (6) | O5—Ga2—O9x | 175.68 (9) |
O3i—Rb1—O10 | 50.90 (6) | O5—Ga2—O9xi | 91.15 (10) |
O3i—Rb1—O10iii | 136.01 (6) | O5v—Ga2—O5 | 87.15 (10) |
O3ii—Rb1—O3i | 163.71 (6) | O5v—Ga2—O9x | 91.15 (10) |
O3ii—Rb1—O5 | 76.29 (6) | O5v—Ga2—O9xi | 175.68 (9) |
O3ii—Rb1—O5iii | 108.70 (6) | O9x—Ga2—O9xi | 90.81 (10) |
O3ii—Rb1—O8iv | 72.02 (6) | O9xi—Ga2—O9x | 90.81 (10) |
O3ii—Rb1—O8v | 93.21 (6) | O3—Ga3—O3vii | 177.02 (6) |
O3ii—Rb1—O10 | 136.01 (6) | O3—Ga3—O6 | 89.41 (9) |
O3ii—Rb1—O10iii | 50.90 (6) | O3—Ga3—O6vii | 88.55 (9) |
O5—Rb1—O5iii | 145.71 (5) | O3—Ga3—O10xii | 88.10 (9) |
O5—Rb1—O8iv | 126.06 (6) | O3—Ga3—O10xi | 94.07 (9) |
O5—Rb1—O8v | 86.39 (6) | O3vii—Ga3—O3 | 177.02 (6) |
O5—Rb1—O10 | 59.75 (6) | O3vii—Ga3—O6 | 88.55 (9) |
O5—Rb1—O10iii | 108.60 (6) | O3vii—Ga3—O6vii | 89.41 (9) |
O5iii—Rb1—O5 | 145.71 (5) | O3vii—Ga3—O10xii | 94.07 (9) |
O5iii—Rb1—O8iv | 86.39 (6) | O3vii—Ga3—O10xi | 88.10 (9) |
O5iii—Rb1—O8v | 126.06 (6) | O6—Ga3—O6vii | 93.52 (7) |
O5iii—Rb1—O10 | 108.60 (6) | O6—Ga3—O10xii | 175.57 (9) |
O5iii—Rb1—O10iii | 59.75 (6) | O6—Ga3—O10xi | 90.08 (8) |
O8iv—Rb1—O8v | 53.85 (6) | O6vii—Ga3—O6 | 93.52 (7) |
O8iv—Rb1—O10 | 132.72 (6) | O6vii—Ga3—O10xii | 90.08 (8) |
O8iv—Rb1—O10iii | 83.14 (6) | O6vii—Ga3—O10xi | 175.57 (9) |
O8v—Rb1—O8iv | 53.85 (6) | O10xii—Ga3—O10xi | 86.44 (10) |
O8v—Rb1—O10 | 83.14 (6) | O10xi—Ga3—O10xii | 86.44 (10) |
O8v—Rb1—O10iii | 132.72 (6) | O1—P1—O2 | 115.18 (14) |
O10—Rb1—O10iii | 143.47 (5) | O1—P1—O3 | 113.30 (12) |
O10iii—Rb1—O10 | 143.47 (5) | O1—P1—O4 | 103.51 (13) |
O1vi—Ga1—O1vii | 110.97 (10) | O2—P1—O3 | 111.98 (13) |
O1vi—Ga1—O8 | 116.55 (11) | O2—P1—O4 | 105.91 (12) |
O1vi—Ga1—O8viii | 107.23 (11) | O3—P1—O4 | 105.82 (13) |
O1vii—Ga1—O1vi | 110.97 (10) | O4—P2—O5 | 107.57 (13) |
O1vii—Ga1—O8 | 107.23 (11) | O4—P2—O6 | 111.36 (13) |
O1vii—Ga1—O8viii | 116.55 (11) | O4—P2—O7 | 101.22 (13) |
O8—Ga1—O8viii | 98.02 (10) | O5—P2—O6 | 118.23 (13) |
O8viii—Ga1—O8 | 98.02 (10) | O5—P2—O7 | 109.08 (13) |
O2ix—Ga2—O2ii | 177.54 (11) | O6—P2—O7 | 108.04 (11) |
O2ix—Ga2—O5 | 92.59 (10) | O7—P3—O8 | 105.86 (14) |
O2ix—Ga2—O5v | 89.20 (10) | O7—P3—O9 | 108.13 (13) |
O2ix—Ga2—O9x | 91.36 (10) | O7—P3—O10 | 103.40 (13) |
O2ix—Ga2—O9xi | 86.91 (10) | O8—P3—O9 | 108.53 (13) |
O2ii—Ga2—O2ix | 177.54 (11) | O8—P3—O10 | 110.63 (13) |
O2ii—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. |
Experimental details
Crystal data |
Chemical formula | RbGa3(P3O10)2 |
Mr | 800.5 |
Crystal system, space group | Orthorhombic, C2221 |
Temperature (K) | 298 |
a, b, c (Å) | 10.0017 (8), 13.0822 (8), 12.0710 (4) |
V (Å3) | 1579.42 (17) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 8.88 |
Crystal size (mm) | 0.08 × 0.06 × 0.05 |
|
Data collection |
Diffractometer | Nonius CCD area-detector diffractometer |
Absorption correction | Gaussian (JANA2000; Petříček & Dušek, 2000) |
Tmin, Tmax | 0.703, 0.849 |
No. of measured, independent and observed [I > 3σ(I)] reflections | 14620, 5415, 2778 |
Rint | 0.088 |
(sin θ/λ)max (Å−1) | 0.941 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.035, 0.034, 0.90 |
No. of reflections | 5415 |
No. of parameters | 139 |
No. of restraints | ? |
Δρmax, Δρmin (e Å−3) | 1.36, −1.24 |
Absolute structure | Flack and Bernardinelli (1999), 2364 Friedel pairs |
Absolute structure parameter | −0.009 (7) |
Selected bond lengths (Å) topRb1—O3i | 2.9764 (15) | P1—O1 | 1.516 (3) |
Rb1—O5 | 3.208 (3) | P1—O2 | 1.492 (3) |
Rb1—O8ii | 3.051 (2) | P1—O3 | 1.504 (2) |
Rb1—O10 | 3.269 (2) | P1—O4 | 1.632 (2) |
Ga1—O1iii | 1.793 (2) | P2—O4 | 1.591 (3) |
Ga1—O8 | 1.830 (2) | P2—O5 | 1.492 (2) |
Ga2—O2iv | 1.941 (2) | P2—O6 | 1.489 (2) |
Ga2—O5 | 1.981 (2) | P2—O7 | 1.593 (3) |
Ga2—O9v | 1.945 (2) | P3—O7 | 1.618 (3) |
Ga3—O3 | 1.959 (2) | P3—O8 | 1.540 (3) |
Ga3—O6 | 1.9401 (17) | P3—O9 | 1.485 (2) |
Ga3—O10vi | 1.919 (2) | P3—O10 | 1.487 (2) |
Symmetry codes: (i) x+1/2, y−1/2, z; (ii) −x+1, −y, z+1/2; (iii) −x, −y, z−1/2; (iv) −x+1/2, −y+1/2, z−1/2; (v) x+1/2, y+1/2, z; (vi) x−1/2, y+1/2, z. |
Bond valence sum calculations for RbGa3(P3O10)2 top | Rb1 | Ga1 | Ga2 | Ga3 | P1 | P2 | P3 | Σ(ν-) |
O1 | | 0.843 | | | 1.269 | | | 2.11 |
| | 0.843 | | | | | | |
O2 | | | 0.565 | | 1.354 | | | 1.92 |
| | | 0.565 | | | | | |
O3 | 0.144 | | | 0.539 | 1.310 | | | 1.99 |
| 0.144 | | | 0.539 | | | | |
O4 | | | | | 0.927 | 1.036 | | 1.96 |
O5 | 0.077 | | 0.507 | | | 1.354 | | 1.94 |
| 0.077 | | 0.507 | | | | | |
O6 | | | | 0.567 | | 1.365 | | 1.93 |
| | | | 0.567 | | | | |
O7 | | | | | | 1.030 | 0.963 | 1.99 |
O8 | 0.118 | 0.763 | | | | | 1.189 | 2.07 |
| 0.118 | 0.763 | | | | | | |
O9 | | | 0.559 | | | | 1.379 | 1.94 |
| | | 0.559 | | | | | |
O10 | 0.065 | | | 0.600 | | | 1.372 | 2.04 |
| 0.065 | | | 0.600 | | | | |
Σ(ν+) | 0.81 | 3.21 | 3.26 | 3.41 | 4.86 | 4.78 | 4.90 | |
Comparison of the geometry of the P3O10 triphosphate groups in RbGa3(P3O10)2 and RbAl3(P3O10)2. top | RbGa3(P3O10)2a | RbAl3(P3O10)2b |
| | |
P1—P2 (Å) | 2.8541 (11) | 2.8344 (11) |
P2—P3vii (Å) | 2.8619 (11) | 2.8556 (12) |
| | |
P1—P2—P3vii (°) | 126.24 (3) | 126.32 (4) |
Notes: (a) this work; (b) Lesage et al. (2005). Symmetry code: (vii) 1/2 − x, −1/2 + y, 1/2 − z. |
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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) Å3 for RbGa3(P3O10)2, compared with 1516.75 (18) Å3 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.