Acta Cryst. (2007). E63, i191 [ doi:10.1107/S1600536807052506 ]
The cubic structure of trirubidium molybdophosphate, Rb3[PMo12O40], contains ![[alpha]](/logos/entities/alpha_rmgif.gif)
-type Keggin [PMo12O40]3- anions and Rb+ counter-ions located in orthogonally intersecting channels. The P and Rb atoms are located on special positions with site symmetries 
m and 
2.m, respectively. The three-dimensional arrangement is isostructural with that found in, for example, X3[PMo12O40], with X = K, NH4 and H3O, or X3[PW12O40] and X4[SiW12O40], with X = Ag and Tl. The compound was crystallized using a typical high-temperature solid-state reaction with GaPO4 and X3Mo3O10 (X = Rb or Li) as starting materials.
The aim of this synthesis was to grow piezoelectric α-GaPO4 single crystals at temperatures below their transition point (1233 K). This was achieved using the high temperature solution growth technique also known as the fluxed-melt technique. In this context, X2Mo3O10 compounds with X = Li, K, Rb, which have relatively low melting points, were used as fluxes. The α-GaPO4 powder was obtained by dissolving 4 N (99.99% purity) Ga metal in nitric acid followed by precipitation with phosphoric acid as described by Beaurain et al. (2006). Rb3Mo3O10 and Li2Mo3O10 used as starting materials were synthesized following the solid-state reaction: X2CO3 + 3MoO3 = X2Mo3O10 + CO2 with X= Li, Rb. 51 wt% of Rb3Mo3O10 and 34 wt% of Li2Mo3O10 were mixed with 15 wt% of α-GaPO4 and homogenized in an agate mortar. The mixture was put in a Pt crucible covered with a lid, heated from room temperature to 1223 K at a ramp rate of 150 K.h−1 in a single temperature zone with a SiC resistance heater furnace, and held at this temperature during 5 h for homogenization. The melted charge was then slowly cooled down at a rate of 2 K.h−1 to 873 K. After 5 h at 873 K, the charge was cooled to room temperature at 200 K.h−1. Yellow transparant crystals of prismatic habit were found, typically of sizes ranging from 0.1 to 1.0 mm, besides α-GaPO4 single crystals.
All atoms were located ab-initio with default values of the charge flipping parameters and an automatic determination of the static charge flip threshold δ (5.246); convergence was reached after 57 iteration cycles. The largest negative residual electron density was found 0.459 Å from P3 and the largest positive residual electron density was found 2.081 Å from O7.
Data collection: CrysAlis CCD (Oxford Diffraction, 2007); cell refinement: CrysAlis RED (Oxford Diffraction, 2007); data reduction: CrysAlis RED (Oxford Diffraction, 2007); program(s) used to solve structure: Superflip (Palatinus & Chapuis, 2007); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996) and DrawXtl (Finger et al., 2007); software used to prepare material for publication: CRYSTALS (Betteridge et al., 2003).
![[Figure 1]](./wm2153fig1thm.gif)
| Fig. 1. The coordination environment of the three different cations in the structure of Rb3[PMo12O40]. The Mo atom is in purple, the P atom in light-green, the Rb atom in grey, and the O atoms in red. Displacement ellipsoids are drawn at the 75% level. |
![[Figure 2]](./wm2153fig2thm.gif)
| Fig. 2. Polyhedral representation of the α-Keggin [PMo12O40]3− anion. PO4 tetrahedra are in lime-green, whereas MoO6 octahedra are in violet. |
![[Figure 3]](./wm2153fig3thm.gif)
| Fig. 3. Polyhedral representation of the packing of the [PMo12O40]3− anions, which creates three orthogonally intersecting channels filled with Rb+ counter-cations (in grey). The colours of the polyhedra are as in Fig. 2. |
| Rb3[PMo12O40] | Dx = 4.337 Mg m−3 |
| Mr = 2078.66 | Mo Kα radiation, λ = 0.71073 Å |
| Cubic, Pn3m | Cell parameters from 12049 reflections |
| Hall symbol: -P 4bc 2bc | θ = 3.0–32.5° |
| a = 11.67521 (8) Å | µ = 9.30 mm−1 |
| V = 1591.45 (2) Å3 | T = 293 K |
| Z = 2 | Prism, yellow transparent |
| F(000) = 1900 | 0.20 × 0.20 × 0.15 mm |
| Oxford Diffraction XCALIBUR diffractometer | 567 independent reflections |
| Radiation source: Enhance (Mo) X-ray Source | 375 reflections with I > 2.0σ(I) |
| graphite | Rint = 0.029 |
| Detector resolution: 8.4205 pixels mm-1 | θmax = 32.5°, θmin = 3.0° |
| ω scans | h = −17→16 |
| Absorption correction: multi-scan CrysAlis RED (Oxford Diffraction, 2007) | k = −17→15 |
| Tmin = 0.525, Tmax = 1.000 | l = −17→16 |
| 27390 measured reflections |
| Refinement on F2 | 0 restraints |
| Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
| R[F2 > 2σ(F2)] = 0.033 | Method = Modified Sheldrick
w = 1/[σ2(F2) + ( 0.05P)2], where P = (max(Fo2,0) + 2Fc2)/3 |
| wR(F2) = 0.086 | (Δ/σ)max = 0.0001 |
| S = 0.91 | Δρmax = 1.42 e Å−3 |
| 567 reflections | Δρmin = −3.91 e Å−3 |
| 31 parameters |
| Rb3[PMo12O40] | Z = 2 |
| Mr = 2078.66 | Mo Kα radiation |
| Cubic, Pn3m | µ = 9.30 mm−1 |
| a = 11.67521 (8) Å | T = 293 K |
| V = 1591.45 (2) Å3 | 0.20 × 0.20 × 0.15 mm |
| Oxford Diffraction XCALIBUR diffractometer | 567 independent reflections |
| Absorption correction: multi-scan CrysAlis RED (Oxford Diffraction, 2007) | 375 reflections with I > 2.0σ(I) |
| Tmin = 0.525, Tmax = 1.000 | Rint = 0.029 |
| 27390 measured reflections | θmax = 32.5° |
| R[F2 > 2σ(F2)] = 0.033 | Δρmax = 1.42 e Å−3 |
| wR(F2) = 0.086 | Δρmin = −3.91 e Å−3 |
| S = 0.91 | Absolute structure: ? |
| 567 reflections | Flack parameter: ? |
| 31 parameters | Rogers parameter: ? |
| 0 restraints |
| x | y | z | Uiso*/Ueq | ||
| Rb1 | 0.7500 | 0.2500 | 0.2500 | 0.0233 | |
| Mo2 | 0.24122 (5) | 0.03433 (3) | 0.03433 (3) | 0.0103 | |
| P3 | 0.2500 | 0.2500 | 0.2500 | 0.0059 | |
| O4 | 0.3472 (3) | 0.1528 (3) | −0.0049 (4) | 0.0136 | |
| O5 | 0.2680 (4) | −0.0661 (3) | −0.0661 (3) | 0.0167 | |
| O6 | 0.1741 (4) | 0.1741 (4) | 0.1741 (4) | 0.0078 | |
| O7 | 0.1232 (3) | 0.1232 (3) | −0.0378 (4) | 0.0134 |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Rb1 | 0.0218 (8) | 0.0240 (5) | 0.0240 (5) | 0.0000 | 0.0000 | 0.0000 |
| Mo2 | 0.0106 (3) | 0.01018 (19) | 0.01018 (19) | 0.00047 (14) | 0.00047 (14) | −0.0036 (2) |
| P3 | 0.0059 (10) | 0.0059 (10) | 0.0059 (10) | 0.0000 | 0.0000 | 0.0000 |
| O4 | 0.0143 (15) | 0.0143 (15) | 0.012 (2) | −0.0023 (18) | 0.0014 (14) | −0.0014 (14) |
| O5 | 0.014 (3) | 0.0181 (16) | 0.0181 (16) | 0.0010 (13) | 0.0010 (13) | −0.012 (2) |
| O6 | 0.0078 (16) | 0.0078 (16) | 0.0078 (16) | −0.0004 (18) | −0.0004 (18) | −0.0004 (18) |
| O7 | 0.0147 (14) | 0.0147 (14) | 0.011 (2) | 0.0022 (19) | −0.0034 (14) | −0.0034 (14) |
| Rb1—O4i | 3.281 (5) | Mo2—Mo2x | 3.4161 (9) |
| Rb1—O4ii | 3.281 (5) | Mo2—O7xi | 1.920 (2) |
| Rb1—O4iii | 3.281 (5) | Mo2—O4xi | 1.9115 (13) |
| Rb1—O4iv | 3.281 (5) | Mo2—Mo2xii | 3.7060 (9) |
| Rb1—O5v | 3.044 (5) | Mo2—Mo2xiii | 3.7060 (9) |
| Rb1—O5vi | 3.044 (5) | Mo2—O4 | 1.9115 (13) |
| Rb1—O5vii | 3.044 (5) | Mo2—O5 | 1.687 (5) |
| Rb1—O5viii | 3.044 (5) | Mo2—O6 | 2.438 (5) |
| Rb1—O7i | 3.244 (5) | Mo2—O7 | 1.920 (2) |
| Rb1—O7ii | 3.244 (5) | P3—O6xiv | 1.534 (8) |
| Rb1—O7iii | 3.244 (5) | P3—O6xv | 1.534 (8) |
| Rb1—O7iv | 3.244 (5) | P3—O6iv | 1.534 (8) |
| Mo2—Mo2ix | 3.4161 (9) | P3—O6 | 1.534 (8) |
| O4i—Rb1—O4ii | 139.52 (11) | O7i—Rb1—O7iv | 125.68 (10) |
| O4i—Rb1—O4iii | 58.58 (17) | O7ii—Rb1—O7iv | 80.42 (16) |
| O4ii—Rb1—O4iii | 139.52 (11) | O7iii—Rb1—O7iv | 125.68 (10) |
| O4i—Rb1—O4iv | 139.52 (11) | Mo2ix—Mo2—Mo2x | 60.000 |
| O4ii—Rb1—O4iv | 58.58 (17) | Mo2ix—Mo2—O7xi | 27.16 (12) |
| O4iii—Rb1—O4iv | 139.52 (11) | Mo2x—Mo2—O7xi | 78.62 (14) |
| O4i—Rb1—O5v | 93.45 (8) | Mo2ix—Mo2—O4xi | 86.92 (14) |
| O4ii—Rb1—O5v | 56.75 (12) | Mo2x—Mo2—O4xi | 128.85 (15) |
| O4iii—Rb1—O5v | 93.45 (8) | O7xi—Mo2—O4xi | 88.2 (2) |
| O4iv—Rb1—O5v | 115.33 (12) | Mo2ix—Mo2—Mo2xii | 90.000 |
| O4i—Rb1—O5vi | 56.75 (12) | Mo2x—Mo2—Mo2xii | 120.000 |
| O4ii—Rb1—O5vi | 93.45 (8) | O7xi—Mo2—Mo2xii | 97.22 (14) |
| O4iii—Rb1—O5vi | 115.33 (12) | O4xi—Mo2—Mo2xii | 14.21 (15) |
| O4iv—Rb1—O5vi | 93.45 (8) | Mo2ix—Mo2—Mo2xiii | 120.000 |
| O5v—Rb1—O5vi | 90.273 (12) | Mo2x—Mo2—Mo2xiii | 90.000 |
| O4i—Rb1—O5vii | 115.33 (12) | O7xi—Mo2—Mo2xiii | 144.88 (12) |
| O4ii—Rb1—O5vii | 93.45 (8) | O4xi—Mo2—Mo2xiii | 73.23 (15) |
| O4iii—Rb1—O5vii | 56.75 (12) | Mo2xii—Mo2—Mo2xiii | 60.000 |
| O4iv—Rb1—O5vii | 93.45 (8) | Mo2ix—Mo2—O4 | 128.85 (15) |
| O5v—Rb1—O5vii | 90.273 (12) | Mo2x—Mo2—O4 | 86.92 (14) |
| O4i—Rb1—O5viii | 93.45 (8) | O7xi—Mo2—O4 | 155.75 (19) |
| O4ii—Rb1—O5viii | 115.33 (12) | O4xi—Mo2—O4 | 85.8 (3) |
| O4iii—Rb1—O5viii | 93.45 (8) | Mo2xii—Mo2—O4 | 73.23 (15) |
| O4iv—Rb1—O5viii | 56.75 (12) | Mo2ix—Mo2—O5 | 128.49 (12) |
| O5v—Rb1—O5viii | 172.08 (18) | Mo2x—Mo2—O5 | 128.49 (12) |
| O4i—Rb1—O7i | 48.24 (7) | O7xi—Mo2—O5 | 101.75 (16) |
| O4ii—Rb1—O7i | 110.50 (11) | O4xi—Mo2—O5 | 102.49 (18) |
| O4iii—Rb1—O7i | 48.24 (7) | Mo2xii—Mo2—O5 | 111.12 (13) |
| O4iv—Rb1—O7i | 169.08 (12) | Mo2ix—Mo2—O6 | 45.51 (11) |
| O5v—Rb1—O7i | 53.75 (12) | Mo2x—Mo2—O6 | 45.51 (11) |
| O4i—Rb1—O7ii | 110.50 (11) | O7xi—Mo2—O6 | 72.62 (16) |
| O4ii—Rb1—O7ii | 48.24 (7) | O4xi—Mo2—O6 | 83.34 (18) |
| O4iii—Rb1—O7ii | 169.08 (12) | Mo2xii—Mo2—O6 | 75.75 (13) |
| O4iv—Rb1—O7ii | 48.24 (7) | Mo2ix—Mo2—O7 | 78.62 (14) |
| O5v—Rb1—O7ii | 86.98 (7) | Mo2x—Mo2—O7 | 27.16 (12) |
| O4i—Rb1—O7iii | 48.24 (7) | O7xi—Mo2—O7 | 87.6 (3) |
| O4ii—Rb1—O7iii | 169.08 (12) | O4xi—Mo2—O7 | 155.75 (19) |
| O4iii—Rb1—O7iii | 48.24 (7) | Mo2xii—Mo2—O7 | 144.88 (12) |
| O4iv—Rb1—O7iii | 110.50 (11) | Mo2xiii—Mo2—O4 | 14.21 (15) |
| O5v—Rb1—O7iii | 134.17 (12) | Mo2xiii—Mo2—O5 | 111.12 (13) |
| O4i—Rb1—O7iv | 169.08 (12) | O4—Mo2—O5 | 102.49 (18) |
| O4ii—Rb1—O7iv | 48.24 (7) | Mo2xiii—Mo2—O6 | 75.75 (13) |
| O4iii—Rb1—O7iv | 110.50 (11) | O4—Mo2—O6 | 83.34 (18) |
| O4iv—Rb1—O7iv | 48.24 (7) | O5—Mo2—O6 | 171.9 (2) |
| O5v—Rb1—O7iv | 86.98 (7) | Mo2xiii—Mo2—O7 | 97.22 (14) |
| O5vi—Rb1—O5vii | 172.08 (18) | O4—Mo2—O7 | 88.2 (2) |
| O5vi—Rb1—O5viii | 90.273 (12) | O5—Mo2—O7 | 101.75 (16) |
| O5vii—Rb1—O5viii | 90.273 (12) | O6—Mo2—O7 | 72.62 (16) |
| O5vi—Rb1—O7i | 86.98 (7) | O6xiv—P3—O6xv | 109.471 |
| O5vii—Rb1—O7i | 86.98 (7) | O6xiv—P3—O6iv | 109.471 |
| O5viii—Rb1—O7i | 134.17 (12) | O6xv—P3—O6iv | 109.471 |
| O5vi—Rb1—O7ii | 53.75 (12) | O6xiv—P3—O6 | 109.471 |
| O5vii—Rb1—O7ii | 134.17 (12) | O6xv—P3—O6 | 109.471 |
| O5viii—Rb1—O7ii | 86.98 (7) | O6iv—P3—O6 | 109.471 |
| O7i—Rb1—O7ii | 125.68 (10) | Mo2—O4—Mo2ii | 151.6 (3) |
| O5vi—Rb1—O7iii | 86.98 (7) | Mo2—O6—Mo2ix | 89.0 (2) |
| O5vii—Rb1—O7iii | 86.98 (7) | Mo2—O6—Mo2x | 89.0 (2) |
| O5viii—Rb1—O7iii | 53.75 (12) | Mo2ix—O6—Mo2x | 89.0 (2) |
| O7i—Rb1—O7iii | 80.42 (16) | Mo2—O6—P3 | 125.99 (15) |
| O7ii—Rb1—O7iii | 125.68 (10) | Mo2ix—O6—P3 | 125.99 (15) |
| O5vi—Rb1—O7iv | 134.17 (12) | Mo2x—O6—P3 | 125.99 (15) |
| O5vii—Rb1—O7iv | 53.75 (12) | Mo2x—O7—Mo2 | 125.7 (2) |
| O5viii—Rb1—O7iv | 86.98 (7) |
| Symmetry codes: (i) z+1, x, y; (ii) −z+1/2, −x+1/2, y; (iii) z+1, −x+1/2, −y+1/2; (iv) −z+1/2, x, −y+1/2; (v) −x+1, −y, −z; (vi) x+1/2, y+1/2, −z; (vii) x+1/2, −y, z+1/2; (viii) −x+1, y+1/2, z+1/2; (ix) y, z, x; (x) z, x, y; (xi) x, z, y; (xii) −y+1/2, z, −x+1/2; (xiii) −y+1/2, −x+1/2, z; (xiv) −x+1/2, −y+1/2, z; (xv) y, −z+1/2, −x+1/2. |
| Rb1—O4i | 3.281 (5) | Mo2—O5 | 1.687 (5) |
| Rb1—O5ii | 3.044 (5) | Mo2—O6 | 2.438 (5) |
| Rb1—O7i | 3.244 (5) | Mo2—O7 | 1.920 (2) |
| Mo2—O7iii | 1.920 (2) | P3—O6 | 1.534 (8) |
| Mo2—O4 | 1.9115 (13) |
| Symmetry codes: (i) z+1, x, y; (ii) −x+1, −y, −z; (iii) x, z, y. |
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Structures containing Keggin anions (Keggin, 1933), [XM12O40]n- with X a main group or a transition metal ion and M a transition metal ion, have been largely studied from a fundamental point of view (López et al., 2001; Maestre et al., 2001) and also because of of their applications in view of their interesting redox properties in relation to their micro-porosity in the salts of Keggin anions with various cations (Bonardet et al., 1995; Clemente-Léon et al., 1997; Katsoulis et al., 1998).
The structure of Rb3[PMo12O40] is comprised of [PMo12O40]3− anions and Rb+ counter-cations located in one-dimensional square channels. The anions have the α-Keggin structure type, i.e. they are composed of a central tetrahedrally coordinated hetero-atom, in the present case a phosphorus atom. The PO4 tetrahedron is caged by 12 octahedral MoO6-units linked to one another by neighboring oxygen atoms. It is noted that the MoO6 octahedra are rather distorted, having one short distance Mo2—O5 of 1.687 (5) Å, one long distance Mo2—O6 of 2.438 (5) Å, and four intermediate distances of 1.9115 (13) Å and 1.920 (2) Å, each appearing twice. The PO4 tetrahedron is by symmetry regular (P on a 43m site; d(P–O) = 1.534 (8) Å). The Rb atom is in 12-fold coordination by oxygen, with three different Rb–O distances of 3.044 (5) Å, 3.244 (5) Å, and 3.281 (5) Å, respectively, each distance appearing 4 times. Fig. 1 shows the coordination environment of the three cations.
Fig. 2 shows a polyhedral representation of the α-Keggin anion [PMo12O40]3−. A view of the three-dimensional structure is shown in Fig. 3; there are two square channels that are occupied by Rb cations. Due to the cubic symmetry there are in fact three orthogonally intersecting Rb-containing channels.
It is noted that the title compound is isostructural with a number of other structures of rather different composition such as (K, NH4, H3O)3[PMo12O40] (Boeyens et al., 1976), (Ag, Tl)3[PW12O40] and (Ag, Tl)4[SiW12O40] (Parent & Moffat, 1996), K3[PMo12O40] (Goubin et al., 2004) and (K2.4(H3O)0.6)[PW12O40] (Kang et al., 2004).