Rb₃[PMo₁₂O₄₀], a compound containing Keggin anions

The cubic structure of trirubidium molybdophosphate, Rb₃[PMo₁₂O₄₀], contains α-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.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](P-O) = 0.005 Å
• R factor = 0.032
• wR factor = 0.086
• Data-to-parameter ratio = 18.3

checkCIF/PLATON results

No syntax errors found



Alert level B
PLAT021_ALERT_1_B Ratio Unique / Expected Reflections too High ...       1.66
PLAT061_ALERT_3_B Tmax/Tmin Range Test RR' too Large .............       0.73

Alert level C
DIFMN02_ALERT_2_C  The minimum difference density is < -0.1*ZMAX*0.75
            _refine_diff_density_min given =     -3.910
            Test value =     -3.150
DIFMN03_ALERT_1_C  The minimum difference density is < -0.1*ZMAX*0.75
            The relevant atom site should be identified.
PLAT062_ALERT_4_C Rescale T(min) & T(max) by .....................       0.25
PLAT098_ALERT_2_C Minimum (Negative) Residual Density ............      -3.91 e/A   
PLAT127_ALERT_1_C Implicit Hall Symbol  Inconsistent with Explicit    -P 4bc 2bc
PLAT141_ALERT_4_C su on a - Axis Small or Missing (x 100000) .....          8 Ang.
PLAT430_ALERT_2_C Short Inter D...A Contact  O5     ..  O7      ..       2.85 Ang.
PLAT731_ALERT_1_C Bond    Calc    1.912(4), Rep  1.9115(13) ......       3.08 su-Ra
              MO2  -O4      1.555  29.555
PLAT731_ALERT_1_C Bond    Calc    1.912(4), Rep  1.9115(13) ......       3.08 su-Ra
              MO2  -O4      1.555   1.555

Alert level G
ABSTM02_ALERT_3_G  When printed, the submitted absorption T values will be
            replaced by the scaled T values. Since the ratio of scaled T's
            is identical to the ratio of reported T values, the scaling
            does not imply a change to the absorption corrections used in
            the study.
            Ratio of Tmax expected/reported      0.248
            Tmax scaled      0.248 Tmin scaled      0.130
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported _diffrn_ambient_temperature .        293 K
PLAT794_ALERT_5_G Check Predicted Bond Valency for Mo2         (6)       5.72

   0 ALERT level A = In general: serious problem
   2 ALERT level B = Potentially serious problem
   9 ALERT level C = Check and explain
   4 ALERT level G = General alerts; check

   7 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   3 ALERT type 2 Indicator that the structure model may be wrong or deficient
   2 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   1 ALERT type 5 Informative message, check

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S1. Comment

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 (K_2.4(H₃O)_0.6)[PW₁₂O₄₀] (Kang et al., 2004).

Related literature

The synthesis of the GaPO₄ precursor is described by Beaurain et al. (2006). For isostructural compounds, see: Boeyens et al. (1976); Goubin et al. (2004); Kang et al. (2004); Parent & Moffat (1996). For properties and applications of compounds with Keggin anions, see: Bonardet et al. (1995); Clemente-Léon et al. (1997); Katsoulis (1998); López et al. (2001); Maestre et al. (2001). Keggin anions were described for the first time by Keggin (1933).

S2. Experimental

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.

S3. Refinement

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.

Computing details

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).

Figures

	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.
	Fig. 2. Polyhedral representation of the α-Keggin [PMo12O40]^3- anion. PO4 tetrahedra are in lime-green, whereas MoO6 octahedra are in violet.
	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.

trirubidium molybdophosphate

Crystal data

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) Å^3	T = 293 K
Z = 2	Prism, yellow transparent
F(000) = 1900	0.20 × 0.20 × 0.15 mm

Data collection

Oxford Diffraction XCALIBUR diffractometer	567 independent reflections
Radiation source: Enhance (Mo) X-ray Source	375 reflections with I > 2.0σ(I)
Graphite monochromator	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

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.033	Method = Modified Sheldrick w = 1/[σ^2(F^2) + ( 0.05P)^2], where P = (max(Fo^2,0) + 2Fc^2)/3
wR(F^2) = 0.086	(Δ/σ)max = 0.000112
S = 0.91	Δρmax = 1.42 e Å^−3
567 reflections	Δρmin = −3.91 e Å^−3
31 parameters

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

	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

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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)

Geometric parameters (Å, º)

Rb1—O4^i	3.281 (5)	Mo2—Mo2^x	3.4161 (9)
Rb1—O4^ii	3.281 (5)	Mo2—O7^xi	1.920 (2)
Rb1—O4^iii	3.281 (5)	Mo2—O4^xi	1.9115 (13)
Rb1—O4^iv	3.281 (5)	Mo2—Mo2^xii	3.7060 (9)
Rb1—O5^v	3.044 (5)	Mo2—Mo2^xiii	3.7060 (9)
Rb1—O5^vi	3.044 (5)	Mo2—O4	1.9115 (13)
Rb1—O5^vii	3.044 (5)	Mo2—O5	1.687 (5)
Rb1—O5^viii	3.044 (5)	Mo2—O6	2.438 (5)
Rb1—O7^i	3.244 (5)	Mo2—O7	1.920 (2)
Rb1—O7^ii	3.244 (5)	P3—O6^xiv	1.534 (8)
Rb1—O7^iii	3.244 (5)	P3—O6^xv	1.534 (8)
Rb1—O7^iv	3.244 (5)	P3—O6^iv	1.534 (8)
Mo2—Mo2^ix	3.4161 (9)	P3—O6	1.534 (8)
O4^i—Rb1—O4^ii	139.52 (11)	O7^i—Rb1—O7^iv	125.68 (10)
O4^i—Rb1—O4^iii	58.58 (17)	O7^ii—Rb1—O7^iv	80.42 (16)
O4^ii—Rb1—O4^iii	139.52 (11)	O7^iii—Rb1—O7^iv	125.68 (10)
O4^i—Rb1—O4^iv	139.52 (11)	Mo2^ix—Mo2—Mo2^x	60.000
O4^ii—Rb1—O4^iv	58.58 (17)	Mo2^ix—Mo2—O7^xi	27.16 (12)
O4^iii—Rb1—O4^iv	139.52 (11)	Mo2^x—Mo2—O7^xi	78.62 (14)
O4^i—Rb1—O5^v	93.45 (8)	Mo2^ix—Mo2—O4^xi	86.92 (14)
O4^ii—Rb1—O5^v	56.75 (12)	Mo2^x—Mo2—O4^xi	128.85 (15)
O4^iii—Rb1—O5^v	93.45 (8)	O7^xi—Mo2—O4^xi	88.2 (2)
O4^iv—Rb1—O5^v	115.33 (12)	Mo2^ix—Mo2—Mo2^xii	90.000
O4^i—Rb1—O5^vi	56.75 (12)	Mo2^x—Mo2—Mo2^xii	120.000
O4^ii—Rb1—O5^vi	93.45 (8)	O7^xi—Mo2—Mo2^xii	97.22 (14)
O4^iii—Rb1—O5^vi	115.33 (12)	O4^xi—Mo2—Mo2^xii	14.21 (15)
O4^iv—Rb1—O5^vi	93.45 (8)	Mo2^ix—Mo2—Mo2^xiii	120.000
O5^v—Rb1—O5^vi	90.273 (12)	Mo2^x—Mo2—Mo2^xiii	90.000
O4^i—Rb1—O5^vii	115.33 (12)	O7^xi—Mo2—Mo2^xiii	144.88 (12)
O4^ii—Rb1—O5^vii	93.45 (8)	O4^xi—Mo2—Mo2^xiii	73.23 (15)
O4^iii—Rb1—O5^vii	56.75 (12)	Mo2^xii—Mo2—Mo2^xiii	60.000
O4^iv—Rb1—O5^vii	93.45 (8)	Mo2^ix—Mo2—O4	128.85 (15)
O5^v—Rb1—O5^vii	90.273 (12)	Mo2^x—Mo2—O4	86.92 (14)
O4^i—Rb1—O5^viii	93.45 (8)	O7^xi—Mo2—O4	155.75 (19)
O4^ii—Rb1—O5^viii	115.33 (12)	O4^xi—Mo2—O4	85.8 (3)
O4^iii—Rb1—O5^viii	93.45 (8)	Mo2^xii—Mo2—O4	73.23 (15)
O4^iv—Rb1—O5^viii	56.75 (12)	Mo2^ix—Mo2—O5	128.49 (12)
O5^v—Rb1—O5^viii	172.08 (18)	Mo2^x—Mo2—O5	128.49 (12)
O4^i—Rb1—O7^i	48.24 (7)	O7^xi—Mo2—O5	101.75 (16)
O4^ii—Rb1—O7^i	110.50 (11)	O4^xi—Mo2—O5	102.49 (18)
O4^iii—Rb1—O7^i	48.24 (7)	Mo2^xii—Mo2—O5	111.12 (13)
O4^iv—Rb1—O7^i	169.08 (12)	Mo2^ix—Mo2—O6	45.51 (11)
O5^v—Rb1—O7^i	53.75 (12)	Mo2^x—Mo2—O6	45.51 (11)
O4^i—Rb1—O7^ii	110.50 (11)	O7^xi—Mo2—O6	72.62 (16)
O4^ii—Rb1—O7^ii	48.24 (7)	O4^xi—Mo2—O6	83.34 (18)
O4^iii—Rb1—O7^ii	169.08 (12)	Mo2^xii—Mo2—O6	75.75 (13)
O4^iv—Rb1—O7^ii	48.24 (7)	Mo2^ix—Mo2—O7	78.62 (14)
O5^v—Rb1—O7^ii	86.98 (7)	Mo2^x—Mo2—O7	27.16 (12)
O4^i—Rb1—O7^iii	48.24 (7)	O7^xi—Mo2—O7	87.6 (3)
O4^ii—Rb1—O7^iii	169.08 (12)	O4^xi—Mo2—O7	155.75 (19)
O4^iii—Rb1—O7^iii	48.24 (7)	Mo2^xii—Mo2—O7	144.88 (12)
O4^iv—Rb1—O7^iii	110.50 (11)	Mo2^xiii—Mo2—O4	14.21 (15)
O5^v—Rb1—O7^iii	134.17 (12)	Mo2^xiii—Mo2—O5	111.12 (13)
O4^i—Rb1—O7^iv	169.08 (12)	O4—Mo2—O5	102.49 (18)
O4^ii—Rb1—O7^iv	48.24 (7)	Mo2^xiii—Mo2—O6	75.75 (13)
O4^iii—Rb1—O7^iv	110.50 (11)	O4—Mo2—O6	83.34 (18)
O4^iv—Rb1—O7^iv	48.24 (7)	O5—Mo2—O6	171.9 (2)
O5^v—Rb1—O7^iv	86.98 (7)	Mo2^xiii—Mo2—O7	97.22 (14)
O5^vi—Rb1—O5^vii	172.08 (18)	O4—Mo2—O7	88.2 (2)
O5^vi—Rb1—O5^viii	90.273 (12)	O5—Mo2—O7	101.75 (16)
O5^vii—Rb1—O5^viii	90.273 (12)	O6—Mo2—O7	72.62 (16)
O5^vi—Rb1—O7^i	86.98 (7)	O6^xiv—P3—O6^xv	109.471
O5^vii—Rb1—O7^i	86.98 (7)	O6^xiv—P3—O6^iv	109.471
O5^viii—Rb1—O7^i	134.17 (12)	O6^xv—P3—O6^iv	109.471
O5^vi—Rb1—O7^ii	53.75 (12)	O6^xiv—P3—O6	109.471
O5^vii—Rb1—O7^ii	134.17 (12)	O6^xv—P3—O6	109.471
O5^viii—Rb1—O7^ii	86.98 (7)	O6^iv—P3—O6	109.471
O7^i—Rb1—O7^ii	125.68 (10)	Mo2—O4—Mo2^ii	151.6 (3)
O5^vi—Rb1—O7^iii	86.98 (7)	Mo2—O6—Mo2^ix	89.0 (2)
O5^vii—Rb1—O7^iii	86.98 (7)	Mo2—O6—Mo2^x	89.0 (2)
O5^viii—Rb1—O7^iii	53.75 (12)	Mo2^ix—O6—Mo2^x	89.0 (2)
O7^i—Rb1—O7^iii	80.42 (16)	Mo2—O6—P3	125.99 (15)
O7^ii—Rb1—O7^iii	125.68 (10)	Mo2^ix—O6—P3	125.99 (15)
O5^vi—Rb1—O7^iv	134.17 (12)	Mo2^x—O6—P3	125.99 (15)
O5^vii—Rb1—O7^iv	53.75 (12)	Mo2^x—O7—Mo2	125.7 (2)
O5^viii—Rb1—O7^iv	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.