The monoclinic form of trilithium dichromium(III) tris­­(orthophosphate)

The monoclinic form of trilithium dichromium(III) tris­(ortho­phosphate), Li₃Cr₂(PO₄)₃, was prepared by the reactive halide flux method. The structure of the title compound is composed of a three-dimensional anionic framework with composition _∞ ³[Cr₂(PO₄)₃]³⁻ and Li⁺ ions situated in the empty channels. The rigid framework built up from CrO₆ octa­hedra and PO₄ tetra­hedra is the same as that found in other monoclinic Li₃M₂(PO₄)₃ (M = Fe, Sc, V) phases. The three Li⁺ cations of Li₃Cr₂(PO₄)₃ are unequally disordered over six crystallographically different sites. The classical charge balance of the title compound can be represented as [Li⁺]₃[Cr³⁺]₂[P⁵⁺]₃[O²⁻]₁₂. Solid-state UV/Vis spectra indicate that the crystal filed splitting (Δ₀) of the Cr³⁺ ion is around 2.22 eV.

Related literature

For the structures of Li₃M₂(PO₄)₃ (M = Fe, Sc, Cr, V), see: d'Yvoire et al. (1983); Verin et al. (1985); Maksimov et al. (1986). The structures of the ortho­rhom­bic form of Li₃Cr₂(PO₄)₃ have been investigated by Genkina et al. (1991). Structural studies of Li₃V₂(PO₄)₃ based on single-crystal data have been reported previously by Kee & Yun (2013). The general structural features of the monoclinic phases have been discussed by Patoux et al. (2003); Fu et al. (2010); Yang et al. (2010). For ionic radii, see: Shannon (1976).

Experimental

Crystal data

• Li₃Cr₂(PO₄)₃

• M_r = 409.73

• Monoclinic, P 2₁ /c

• a = 8.4625 (4) Å

• b = 8.5560 (3) Å

• c = 14.5344 (5) Å

• β = 125.186 (2)°

• V = 860.08 (6) ų

• Z = 4

• Mo Kα radiation

• μ = 3.16 mm⁻¹

• T = 290 K

• 0.36 × 0.12 × 0.10 mm

Data collection

• Rigaku R-AXIS RAPID S diffractometer

• Absorption correction: multi-scan (ABSCOR; Higashi, 1995) T_min = 0.688, T_max = 1.000

• 8163 measured reflections

• 1962 independent reflections

• 1887 reflections with I > 2σ(I)

• R_int = 0.021

Refinement

• R[F² > 2σ(F²)] = 0.026

• wR(F²) = 0.068

• S = 1.14

• 1962 reflections

• 184 parameters

• 1 restraint

• Δρ_max = 0.71 e Å⁻³

• Δρ_min = −0.71 e Å⁻³

Data collection: RAPID-AUTO (Rigaku, 2006); cell refinement: RAPID-AUTO; data reduction: RAPID-AUTO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012) and publCIF (Westrip, 2010).

Supporting information

S1. Comment

The structures of trilithium dimetal tris(orthophosphates), Li₃M₂(PO₄)₃ (M = Fe, Sc, Cr, V) have been widely investigated due to their ion transport properties and temperature-dependent phase transitions (d'Yvoire et al., 1983; Verin et al., 1985; Maksimov et al., 1986). For Li₃Cr₂(PO₄)₃, the structure of the orthorhombic phase has been studied based on single-crystal diffraction data (Genkina et al., 1991). The monoclinic and rhombohedral phases have been identified by powder diffraction techniques but no detailed structure determinations have been reported yet (d'Yvoire et al., 1983). In attempts to prepare new mixed alkali metal phosphates by using various alkali metal halides, the monoclinic form of Li₃Cr₂(PO₄)₃ has been isolated as single crystals and the detailed structural characterization of this phase is reported here.

The anionic framework of Li₃Cr₂(PO₄)₃ is the same as that of the previously reported monoclinic Li₃V₂(PO₄)₃ structure (Kee & Yun, 2013). The general structural features of this phase have been discussed previously (Patoux et al., 2003; Fu et al., 2010). Figure 1 shows the coordination environment of the Cr and P atoms. CrO₆ octahedra are joined to PO₄ tetrahedra forming a [Cr₂(PO₄)₃] unit. These units share a terminal oxygen atom to construct the anionic three-dimensional framework, ³_∞[Cr₂(PO₄)₃]^3- (Fig. 2). The Cr—O distances (1.9007 (18)–2.0392 (18) Å) are in good agreement with those calculated from their ionic radii (2.00 Å; Shannon, 1976), assuming a valence of +III for Cr.

The Li⁺ ions in the empty channels are surrounded by four O atoms in distorted tetrahedral coordinations. There are six crystallographically independent Li sites for this phase and three Li⁺ ions are unequally disordered over them. It has been reported that the positions of Li⁺ ions in Li₃V₂(PO₄)₃ can vary depending on the synthetic conditions while those of the V, P, and O atoms comprising the rigid framework remain intact (Yang et al., 2010). The positions of the Li1, Li3, and Li4 sites in this work are very close to those of the ordered Li sites found in Li₃V₂(PO₄)₃ (Kee & Yun, 2013).

The classical charge balance of the title compound can be represented as [Li⁺]₃[Cr³⁺]₂[P⁵⁺]₃[O^2-]₁₂. Solid-state UV/Vis spectra indicate that the crystal filed splitting(Δ₀) of the Cr³⁺ ion is around 2.22 eV, which is in agreement with the green color of the crystals.

Related literature

For the structures of Li₃M₂(PO₄)₃ (M = Fe, Sc, Cr, V), see: d'Yvoire et al. (1983); Verin et al. (1985); Maksimov et al. (1986). The structures of the orthorhombic form of Li₃Cr₂(PO₄)₃ have been investigated by Genkina et al. (1991). Structural studies of Li₃V₂(PO₄)₃ based on single-crystal data have been reported previously by Kee & Yun (2013). The general structural features of the monoclinic phases have been discussed by Patoux et al. (2003); Fu et al. (2010); Yang et al. (2010). For ionic radii, see: Shannon (1976).

S2. Experimental

The title compound, Li₃Cr₂(PO₄)₃, was prepared by the reaction of the elements with the use of the reactive halide-flux technique. A combination of the pure elements, Cr powder (Cerac, 99.95%) and P powder (Aldrich, 99%), were mixed in a fused silica tube in a molar ratio of Cr:P = 1:1 and then LiCl (Cerac, 99.8%) and CsCl (Alfa, 99.9%) mixed in molar ratio of LiCl:CsCl = 4:1 were added. The mass ratio of the reactants and the halides was 1:3. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (30 K/h) to 1123 K, where it was kept for 48 h. The tube was cooled to room temperature at a rate of 4 K/h. The excess halide was removed with water and greenish block-shaped crystals were obtained. The crystals are stable in air and water. A qualitative X-ray fluorescence analysis of selected crystals indicated the presence of Cr and P. The final composition of the compound was determined by single-crystal X-ray diffraction.

S3. Refinement

After the positions of heavy elements (Cr, P, and O) had been established, six significant residual peaks suitable for Li⁺ sites were revealed by difference Fourier maps. A model including disorder for these sites was applied. The sum of the Li⁺ occupancies was fixed to 3 to meet the charge balance of the compound; the temperature factors of all Li sites were refined isotropically. The highest peak (0.71 e Å^-3) and the deepest hole (-0.49 e Å^-3) are 0.57 Å and 0.81 Å from atom Li4 and Cr1, respectively.

Computing details

Data collection: RAPID-AUTO (Rigaku, 2006); cell refinement: RAPID-AUTO (Rigaku, 2006); data reduction: RAPID-AUTO (Rigaku, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012) and publCIF (Westrip, 2010).

Figures

	Fig. 1. A view showing the local coordination environments of Cr and P atoms with the atom labeling scheme. Displacement ellipsoids for Cr, P, and O atoms are drawn at the 70% probability level. [Symmetry codes: (ii) -x, y+1/2, -z+1/2; (iii) -x+1, y+1/2, -z+1/2; (iv) -x+1, y-1/2, -z+1/2; (v) -x+1, -y+1, -z; (vi) -x+1, -y+1, -z+1; (vii) -x, -y+1, -z; (viii) x, -y+1/2, z-1/2. ].
	Fig. 2. The polyhedral representation of the anionic framework structure built up from [Cr2(PO4)3] units. The disordered Li^+ cations are located in the channels of this framework.

Trilithium dichromium(III) tris(orthophosphate)

Crystal data

Li3Cr2(PO4)3	F(000) = 792
Mr = 409.73	Dx = 3.164 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 8071 reflections
a = 8.4625 (4) Å	θ = 3.4–27.7°
b = 8.5560 (3) Å	µ = 3.16 mm^−1
c = 14.5344 (5) Å	T = 290 K
β = 125.186 (2)°	Block, green
V = 860.08 (6) Å^3	0.36 × 0.12 × 0.10 mm
Z = 4

Data collection

Rigaku R-AXIS RAPID S diffractometer	1962 independent reflections
Radiation source: Sealed X-ray tube	1887 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
ω scans	θmax = 27.5°, θmin = 3.4°
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	h = −10→10
Tmin = 0.688, Tmax = 1.000	k = −11→10
8163 measured reflections	l = −18→18

Refinement

Refinement on F^2	1 restraint
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.026	Secondary atom site location: difference Fourier map
wR(F^2) = 0.068	w = 1/[σ^2(Fo^2) + (0.0319P)^2 + 1.8772P] where P = (Fo^2 + 2Fc^2)/3
S = 1.14	(Δ/σ)max < 0.001
1962 reflections	Δρmax = 0.71 e Å^−3
184 parameters	Δρmin = −0.71 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Li1	0.4750 (10)	0.2119 (8)	0.1752 (6)	0.022 (2)*	0.71 (2)
Li2	0.104 (2)	0.208 (2)	0.3364 (14)	0.022 (6)*	0.30 (2)
Li3	0.1164 (11)	0.5885 (9)	0.1920 (7)	0.017 (3)*	0.59 (2)
Li4	0.1935 (14)	0.1892 (12)	0.2627 (8)	0.036 (3)*	0.64 (3)
Li5	0.672 (3)	0.227 (2)	0.2586 (15)	0.019 (6)*	0.27 (2)
Li6	0.273 (3)	0.059 (3)	0.1835 (19)	0.072 (8)*	0.48 (3)
Cr1	0.36305 (5)	0.53352 (4)	0.11142 (3)	0.00608 (11)
Cr2	0.13604 (5)	0.53218 (5)	0.38801 (3)	0.00787 (11)
P1	0.46068 (8)	0.39077 (7)	0.35382 (5)	0.00707 (13)
P2	0.75261 (8)	0.38724 (7)	0.14717 (5)	0.00693 (13)
P3	0.04107 (8)	0.25059 (7)	0.00513 (5)	0.00717 (13)
O1	0.6057 (2)	0.4162 (2)	0.17588 (15)	0.0129 (4)
O2	0.2909 (3)	0.3809 (2)	0.36484 (16)	0.0138 (4)
O3	0.5955 (3)	0.0119 (2)	0.23933 (16)	0.0181 (4)
O4	0.0766 (3)	0.0017 (2)	0.27885 (15)	0.0124 (3)
O5	0.6674 (3)	0.4170 (2)	0.02547 (15)	0.0135 (4)
O6	0.3515 (3)	0.5558 (2)	0.54233 (16)	0.0197 (4)
O7	0.1224 (3)	0.6330 (2)	0.06579 (16)	0.0135 (4)
O8	0.0340 (2)	0.1757 (2)	0.09856 (15)	0.0135 (4)
O9	0.2391 (2)	0.3295 (2)	0.06304 (17)	0.0166 (4)
O10	0.0217 (3)	0.3652 (2)	0.41922 (16)	0.0156 (4)
O11	0.4784 (2)	0.2282 (2)	0.31441 (14)	0.0106 (3)
O12	0.1852 (3)	0.7155 (2)	0.32091 (15)	0.0140 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr1	0.00427 (18)	0.00702 (19)	0.00645 (19)	0.00016 (12)	0.00280 (15)	0.00019 (13)
Cr2	0.00503 (18)	0.0106 (2)	0.00828 (19)	−0.00115 (13)	0.00402 (15)	−0.00151 (14)
P1	0.0064 (3)	0.0078 (3)	0.0061 (3)	0.0019 (2)	0.0030 (2)	0.0007 (2)
P2	0.0054 (3)	0.0085 (3)	0.0063 (3)	0.0014 (2)	0.0031 (2)	0.0000 (2)
P3	0.0053 (3)	0.0063 (3)	0.0100 (3)	−0.0001 (2)	0.0045 (2)	−0.0001 (2)
O1	0.0089 (8)	0.0216 (9)	0.0095 (8)	0.0060 (7)	0.0060 (7)	0.0022 (7)
O2	0.0112 (8)	0.0133 (8)	0.0197 (9)	−0.0001 (7)	0.0106 (7)	−0.0023 (7)
O3	0.0348 (11)	0.0105 (8)	0.0154 (9)	−0.0066 (8)	0.0182 (9)	−0.0043 (7)
O4	0.0105 (8)	0.0128 (8)	0.0095 (8)	0.0013 (7)	0.0033 (7)	0.0010 (7)
O5	0.0140 (8)	0.0171 (9)	0.0089 (8)	0.0027 (7)	0.0062 (7)	0.0018 (7)
O6	0.0117 (9)	0.0235 (10)	0.0146 (9)	−0.0025 (8)	0.0023 (8)	−0.0065 (8)
O7	0.0117 (8)	0.0122 (8)	0.0186 (9)	0.0053 (7)	0.0099 (7)	0.0053 (7)
O8	0.0100 (8)	0.0180 (9)	0.0108 (8)	−0.0009 (7)	0.0050 (7)	0.0038 (7)
O9	0.0076 (8)	0.0083 (8)	0.0302 (11)	−0.0018 (7)	0.0087 (8)	−0.0017 (8)
O10	0.0115 (8)	0.0191 (9)	0.0157 (9)	−0.0037 (7)	0.0075 (7)	0.0043 (7)
O11	0.0125 (8)	0.0081 (8)	0.0086 (8)	0.0038 (6)	0.0045 (7)	0.0002 (6)
O12	0.0216 (9)	0.0100 (8)	0.0151 (8)	−0.0057 (7)	0.0133 (8)	−0.0041 (7)

Geometric parameters (Å, º)

Li1—O3	1.934 (7)	Cr1—O5^v	1.9007 (18)
Li1—O9	1.977 (7)	Cr1—O7	1.9380 (17)
Li1—O11	2.011 (7)	Cr1—O9	1.9471 (18)
Li1—O1	2.066 (7)	Cr1—O1	1.9709 (17)
Li2—O4	1.908 (17)	Cr1—O3^iii	1.9965 (18)
Li2—O2	2.028 (17)	Cr1—O11^iii	2.0172 (17)
Li2—O10	2.170 (17)	Cr2—O6	1.9192 (19)
Li2—O12^i	2.184 (17)	Cr2—O10	1.9208 (18)
Li3—O7	1.902 (8)	Cr2—O8^ii	1.9847 (18)
Li3—O12	1.943 (8)	Cr2—O2	2.0041 (18)
Li3—O4^ii	2.049 (8)	Cr2—O12	2.0129 (18)
Li3—O3^iii	2.137 (8)	Cr2—O4^ii	2.0392 (18)
Li4—O8	1.953 (10)	P1—O6^vi	1.4994 (19)
Li4—O4	1.969 (10)	P1—O2	1.5368 (18)
Li4—O2	2.040 (10)	P1—O11	1.5443 (17)
Li4—O11	2.098 (10)	P1—O3^iii	1.5463 (19)
Li5—O1	1.900 (18)	P2—O5	1.4979 (18)
Li5—O3	1.916 (18)	P2—O12^iv	1.5394 (18)
Li5—O12^iv	2.103 (18)	P2—O1	1.5424 (17)
Li5—O11	2.206 (18)	P2—O4^iii	1.5532 (18)
Li5—O7^iv	2.252 (18)	P3—O7^vii	1.5248 (18)
Li6—O8	1.93 (2)	P3—O10^viii	1.5268 (19)
Li6—O1^iv	2.08 (2)	P3—O9	1.5319 (18)
Li6—O11	2.22 (2)	P3—O8	1.5337 (18)
Li6—O3	2.39 (2)
O5^v—Cr1—O7	93.57 (8)	O2—Cr2—O12	94.84 (8)
O5^v—Cr1—O9	95.83 (9)	O6—Cr2—O4^ii	175.15 (8)
O7—Cr1—O9	91.63 (8)	O10—Cr2—O4^ii	88.07 (8)
O5^v—Cr1—O1	94.97 (8)	O8^ii—Cr2—O4^ii	90.18 (7)
O7—Cr1—O1	171.08 (8)	O2—Cr2—O4^ii	86.09 (8)
O9—Cr1—O1	84.97 (8)	O12—Cr2—O4^ii	79.09 (8)
O5^v—Cr1—O3^iii	172.27 (8)	O6^vi—P1—O2	115.09 (11)
O7—Cr1—O3^iii	84.53 (8)	O6^vi—P1—O11	112.02 (11)
O9—Cr1—O3^iii	91.72 (9)	O2—P1—O11	106.60 (10)
O1—Cr1—O3^iii	87.34 (8)	O6^vi—P1—O3^iii	106.84 (12)
O5^v—Cr1—O11^iii	91.32 (8)	O2—P1—O3^iii	107.11 (11)
O7—Cr1—O11^iii	93.70 (8)	O11—P1—O3^iii	108.98 (10)
O9—Cr1—O11^iii	170.80 (8)	O5—P2—O12^iv	111.51 (10)
O1—Cr1—O11^iii	88.66 (8)	O5—P2—O1	112.18 (10)
O3^iii—Cr1—O11^iii	81.34 (8)	O12^iv—P2—O1	105.13 (10)
O6—Cr2—O10	94.07 (9)	O5—P2—O4^iii	109.45 (11)
O6—Cr2—O8^ii	94.27 (8)	O12^iv—P2—O4^iii	111.89 (10)
O10—Cr2—O8^ii	86.83 (8)	O1—P2—O4^iii	106.55 (10)
O6—Cr2—O2	89.50 (8)	O7^vii—P3—O10^viii	104.06 (10)
O10—Cr2—O2	91.50 (8)	O7^vii—P3—O9	111.22 (10)
O8^ii—Cr2—O2	175.97 (8)	O10^viii—P3—O9	107.65 (11)
O6—Cr2—O12	99.31 (8)	O7^vii—P3—O8	112.88 (10)
O10—Cr2—O12	165.23 (8)	O10^viii—P3—O8	114.36 (11)
O8^ii—Cr2—O12	85.96 (8)	O9—P3—O8	106.63 (11)

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