Lithium cobalt(II) pyrophosphate, Li_1.86CoP₂O₇, from synchrotron X-ray powder data

Structure refinement of high-resolution X-ray powder diffraction data of the title compound gave the composition Li_1.865CoP₂O₇, which is also verified by the ICP measurement. Two Co sites exist in the structure: one is a CoO₅ square pyramid and the other is a CoO₆ octa­hedron. They share edges and are further inter­connected through P₂O₇ groups, forming a three-dimensional framework, which exhibits different kinds of inter­secting tunnels containing Li cations and could be of great inter­est in Li ion battery chemistry. The structure also exhibits cation disorder with 13.5% Co residing at the lithium (Li1) site. Co seems to have an average oxidation state of 2.135, as obtained from the strutural stochiometry that closely supports the magnetic susceptibility findings.

Related literature

For related structures, see: Adam et al. (2008); Nishimura et al. (2010); Zhou et al. (2011). For related materials with Na⁺ and K⁺ cations, see: Erragh et al. (1991); Sanz et al. (1999); Beaury et al. (2004); Gopalakrishna et al. (2005); Bih et al. (2006); Guesmi et al. (2007). For related structural frameworks, see: Beaury et al. (2004); Fagginani & Calvo (1976); Sandström et al. (2003); Etheredge & Hwu (1995); El Maadi et al. (1995); Huang & Hwa (1998); Sanz et al. (1999); Erragh et al. (1998). Pseudovoigt profile coefficients as parameterized in Thompson et al. (1987) and Finger et al. (1994).

Experimental

Crystal data

• CoLi_1.865O₇P₂

• M_r = 245.82

• Monoclinic, P 2₁ /a

• a = 9.76453 (4) Å

• b = 9.69622 (4) Å

• c = 10.95952 (4) Å

• β = 101.7664 (2)°

• V = 1015.83 (1) ų

• Z = 8

• Synchrotron radiation, λ = 0.413988 Å

• μ = 0.89 mm⁻¹

• T = 297 K

• irregular shape, 15 × 13 mm

Data collection

• Advanced Photon Source diffractometer

• Specimen mounting: kapton capillary

• Data collection mode: transmission

• Scan method: continuous

Refinement

• R_p = 0.057

• R_wp = 0.080

• R_exp = 0.049

• R(F²) = 0.04534

• χ² = 2.624

• 24500 data points

• 269 parameters

Data collection: Advance Photon Source Argonne National Laboratory; cell refinement: GSAS (Larson & Von Dreele, 2000); data reduction: Powder4 (Dragoe, 2001); program(s) used to solve structure: GSAS; program(s) used to refine structure: GSAS; molecular graphics: CrystalMaker (Palmer, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

A₂MP₂O₇ is a large family, in which various frameworks are encountered consisting of MO₆ octahedra (Fagginani et al., 1976; Sandström et.al., 2003) and MO₄ polyhedra (Etheredge et al., 1995; Erragh et al., 1998; Sanz et al., 1999) interconnected through P₂O₇ groups (Fig. 1). Dimeric M₂O₁₀ units, built up of two edge-sharing MO₆ octahedra, are only observed for some A₂MP₂O₇ pyrophosphates (El Maadi et al. 1995; Huang et al. 1998) and dimeric M₂O₁₁ units(corner-shared MO₆) seem to be much more rare, just observed for Na₂CoP₂O₇ (Erragh et al. 1991). Two forms of structures were found for Na₂CoP₂O₇ by Erragh et al. 1991: one is triclinic and another one is orthorhombic. The tetragonal structure of Na₂CoP₂O₇ was reported by Sanz et al. 1999 and they found that the tetragonal form could be a derivative of the orthorhombic form, with a higher point symmetry for the former. In addition, the tetragonal structured Na₂CoP₂O₇ was described by Guesmi et al. 2007. To our knowledge, the A₂CoP₂O₇ with Li as cation has never been reported.

Here, we report a new Li containing solid with a three-dimensional framework (Fig. 2) crystallizing in the monoclinic space group P2₁/a. Its structure is similar to the recently reported Li₂MnP₂O₇ (Adam et al. 2008), a new member of the A₂MP₂O₇ family: original M₂O₉ units, built up of one MO₅ trigonal bipyramid sharing one edge with one MO₆ octahedron, sharing corners with P₂O₇ pyrophosphate groups to form undulating (M₄P₈O₃₂)_∞ layers. A 3-D framework results from the interconnection between metal oxide and pyrophosphate groups, and the lithium cations are located in the tunnels thus formed (Fig 2). The structure of the related Fe-compound has been studied by us (Zhou et al. 2011) and Nishimura et al. (2010), as well as the electrochemical properties, which showed that it is a good candidate for the cathode material of lithium-ion batteries. The title compound also has the potential to work as the cathode material for lithium-ion batteries. We present here its crystal structure, as determined and refined from synchrotron powder X-ray diffraction data (Fig. 3).

Related literature

For related structures, see: Adam et al. (2008); Nishimura et al. (2010); Zhou et al. (2011). For related materials with Na and K cations, see: Erragh et al. (1991); Sanz et al. (1999); Beaury et al. (2004); Gopalakrishna et al. (2005); Bih et al. (2006); Guesmi et al. (2007). For related structural frameworks, see: Beaury et al. (2004); Fagginani & Calvo (1976); Sandström et al. (2003); Etheredge & Hwu (1995); El Maadi et al. (1995); Huang et al. (1998); Sanz et al. (1999); Erragh et al. (1998);

S2. Experimental

The powder sample was synthesized through a "wet" method based on mixing stoichiometric aqueous solutions of the precursors followed by thermal treatments. The general procedure involves the mixing of soluble precursors in distilled water followed by a slow evaporation through continuous stirring to dryness before annealing the resultant solids. The precursors for the synthesis were Li(CH₃COO),Co(CH₃COO)₂. 4H₂O,and NH₄H₂PO₄, which were dissolved in 100 ml distilled water in a molar ratio of 2:1:2 (1.32 g, 2.49 g and 2.3 g respectively) to give a 0.02 molar lithium solution. The self-adjusted pH of all the solutions were found to be around 4.5. The solution was stirred and evaporated on a hot-plate in the hood followed by vacuum oven drying overnight at 363 K. The resulting solid was preheated in a H2/He (8.5°/91.5° by volume) atmosphere at 673 K for 4h to decompose the precursors followed by reheating under the same atmosphere up to 873 K for 16h with intermittent grinding to obtain the pink colored powder as final product. The sample was also analyzed with a Perkin-Elmer ICP-OES Optima 7000 DV for the elemental content. The average result of 3 analyses showed that the ratio of Li: Co: P is 1.85: 0.996:2. In addition, the SQUID magnetic study on the sample using a Quantum Design MPMS XL SQUID magnetometer showed that the effective magnetic moment of it is 5.23m_Bwhich is typical divalent Co.

S3. Refinement

During structural refinement, occupancy factor for Li1 and Co3 were refined using constrains for atomic coordinate, atomic displacement parameter, and keeping the sum of occupancy facter equals to unity, which later were fixed to their close refined values as 0.73 and 0.27 respectively. Occupancy for Co1 was also observed to be deficient and fixed to it's closely refined value of 0.739 to 0.73, in final refinement cycles.

Computing details

Data collection: Advance Photon Source Argonne National Laboratory; cell refinement: GSAS (Larson & Von Dreele, 2000); data reduction: Powder4 (Dragoe, 2001); program(s) used to solve structure: GSAS (Larson & Von Dreele, 2000); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000); molecular graphics: publCIF (Westrip, 2010); software used to prepare material for publication: CrystalMaker (Palmer, 2005).

Figures

	Fig. 1. Thermal ellipsoid view of Li1.86CoP2O7 framework, having edge shared CoO5 and CoO6 interconnected through P2O7 moities.
	Fig. 2. Polyhedral view of unit cell packing showing tunneled structures containing Li ions, viewed along [100].
	Fig. 3. X-ray Rietveld refinement profiles for Li1.86CoP2O7, data recorded at room temperature. Triangles mark the experimental points (red), solid line is the calculated profile (blue) and bottom trace shows the difference curve (green).

Lithium cobalt(II) pyrophosphate

Crystal data

CoLi1.865O7P2	F(000) = 948.7
Mr = 245.82	Dx = 3.214 Mg m^−3
Monoclinic, P21/a	Melting point: 1023 K
Hall symbol: -P 2yab	Synchrotron radiation, λ = 0.413988 Å
a = 9.76453 (4) Å	µ = 0.89 mm^−1
b = 9.69622 (4) Å	T = 297 K
c = 10.95952 (4) Å	Particle morphology: block
β = 101.7664 (2)°	pink
V = 1015.83 (1) Å^3	irregular, 15 × 13 mm
Z = 8	Specimen preparation: Prepared at 873 K and 101.325 kPa

Data collection

Advanced Photon Source diffractometer	Specimen mounting: kapton capillary
Radiation source: Synchrotron	Data collection mode: transmission
Si monochromator	Scan method: continuous

Refinement

Least-squares matrix: full	Excluded region(s): Reflections exceeding 2-theta 30 were omitted for the ease of refinement.
Rp = 0.057	Profile function: CW Profile function number 3 with 19 terms Pseudovoigt profile coefficients as parameterized in Thompson et al., (1987) and Finger et al. (1994). #1(GU) = 6.454 #2(GV) = -0.998 #3(GW) = 0.075 #4(GP) = 0.000 #5(LX) = 0.327 #6(LY) = 0.000 #7(S/L) = 0.0011 #8(H/L) = 0.0014 #9(trns) = 0.00 #10(shft)= 0.0000 #11(stec)= 0.00 #12(ptec)= 0.00 #13(sfec)= 0.00 #14(L11) = 0.067 #15(L22) = 0.070 #16(L33) = 0.058 #17(L12) = 0.010 #18(L13) = 0.010 #19(L23) = -0.004 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.080	269 parameters
Rexp = 0.049	0 restraints
R(F^2) = 0.04534	(Δ/σ)max = 0.03
24500 data points	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 165.338 2: -31.4210 3: -8.19118 4: 6.28432 5: -10.8524 6: 14.3842 7: -8.22810 8: -0.949190 9: 13.3092 10: -14.8044 11: 4.75285 12: -1.09442 13: -0.739293 14: 5.47743 15: -2.87265 16: 1.05489 17: -3.22764 18: 1.21714 19: 0.537209 20: -3.25962 21: 2.18736 22: -1.12437 23: -2.15219 24: 3.41374 25: -3.88852 26: 1.14500 27: 3.06741 28: -3.05038 29: 0.529274 30: 0.298855 31: -4.06399 32: 1.50867 33: 1.15056 34: -2.20673 35: 0.550944 36: 5.540140E-02

Special details

Experimental. Data was collected with powder sample packed in Kapton capillary

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Co1	0.25281 (15)	0.71550 (15)	0.18010 (14)	0.0137 (4)*	0.73
Co2	0.30165 (11)	0.43043 (11)	0.32719 (10)	0.0095 (3)*
P1	0.3790 (2)	0.6536 (2)	0.5771 (2)	0.0078 (5)*
P2	0.0613 (2)	0.9265 (2)	0.24116 (18)	0.0089 (5)*
P3	0.0210 (2)	0.4551 (2)	0.75861 (19)	0.0103 (5)*
P4	0.6144 (2)	0.7956 (2)	−0.1113 (2)	0.0117 (6)*
O1	0.1615 (5)	0.8210 (5)	0.3113 (5)	0.0130 (14)*
O2	0.4709 (5)	0.7806 (5)	−0.0792 (4)	0.0106 (12)*
O3	0.3911 (5)	0.8599 (4)	0.1472 (5)	0.0107 (13)*
O4	0.0302 (5)	0.8437 (5)	0.5592 (4)	0.0116 (14)*
O5	0.4330 (5)	0.4292 (5)	−0.1033 (4)	0.0168 (13)*
O6	0.1790 (5)	0.8356 (5)	−0.1511 (4)	0.0115 (13)*
O7	0.0189 (5)	0.4120 (4)	0.6224 (4)	0.0087 (13)*
O8	0.1866 (5)	0.2976 (4)	0.4146 (4)	0.0047 (12)*
O9	0.1013 (5)	0.6017 (4)	0.7747 (4)	0.0116 (13)*
O10	0.3827 (5)	0.5734 (5)	0.7087 (4)	0.0061 (12)*
O11	0.4152 (5)	0.5900 (4)	0.2661 (4)	0.0085 (13)*
O12	0.2775 (5)	0.5646 (5)	0.4803 (4)	0.0134 (13)*
O13	0.3713 (5)	1.0314 (5)	−0.2124 (5)	0.0205 (15)*
O14	0.2204 (5)	0.6299 (5)	−0.0018 (4)	0.0094 (13)*
Li1	1.3433 (3)	0.9255 (3)	−0.0397 (3)	0.0087 (7)*	0.73
Li2	0.0807 (17)	0.1083 (15)	0.0261 (15)	0.009 (4)*
Li3	0.6728 (15)	0.0711 (14)	0.5465 (14)	0.029 (4)*
Li4	0.400 (2)	0.246 (2)	0.5725 (17)	0.065 (7)*
Co3	1.3433 (3)	0.9255 (3)	−0.0397 (3)	0.0087 (7)*	0.27

Geometric parameters (Å, º)

Co1—O1	2.105 (5)	P2—O5^v	1.524 (4)
Co1—O3	2.028 (4)	P2—O10^iv	1.584 (5)
Co1—O11	2.067 (4)	P2—O11^vi	1.516 (5)
Co1—O13^i	2.226 (5)	P3—O3^ii	1.514 (5)
Co1—O14	2.123 (5)	P3—O7	1.546 (5)
Co2—O4^ii	2.030 (5)	P3—O9	1.616 (4)
Co2—O6^i	2.180 (5)	P3—O13^vii	1.563 (5)
Co2—O8	2.068 (4)	P4—O2	1.519 (5)
Co2—O11	2.091 (4)	P4—O6^iii	1.524 (4)
Co2—O12	2.173 (4)	P4—O9^viii	1.582 (5)
Co2—O13^i	2.128 (5)	P4—O14^iii	1.589 (5)
P1—O4^iii	1.529 (5)	Co3—O2^ix	1.983 (5)
P1—O8^iv	1.547 (4)	Co3—O3^ix	2.104 (6)
P1—O10	1.633 (5)	Co3—O6^ix	2.006 (6)
P1—O12	1.556 (4)	Co3—O13^ix	2.220 (5)
P2—O1	1.512 (5)	Co3—O14^x	2.154 (5)
O1—Co1—O3	100.14 (19)	O8—Co2—O11	169.24 (17)
O1—Co1—O11	111.53 (19)	O8—Co2—O13^i	96.85 (19)
O1—Co1—O14	145.9 (2)	O11—Co2—O13^i	83.05 (18)
O3—Co1—O11	90.63 (18)	O8^iv—P1—O10	108.2 (3)
O3—Co1—O14	94.6 (2)	O8^iv—P1—O12	109.1 (3)
O11—Co1—O14	98.73 (18)	O10—P1—O12	103.6 (3)
O4^ii—Co2—O8	84.60 (18)	O1—P2—O5^v	111.4 (3)
O4^ii—Co2—O11	95.03 (18)	O1—P2—O10^iv	106.9 (3)
O4^ii—Co2—O13^i	177.0 (2)	O5^v—P2—O10^iv	104.4 (3)

Symmetry codes: (i) −x+1/2, y−1/2, −z; (ii) −x+1/2, y−1/2, −z+1; (iii) x+1/2, −y+3/2, z; (iv) −x+1/2, y+1/2, −z+1; (v) −x+1/2, y+1/2, −z; (vi) x−1/2, −y+3/2, z; (vii) x−1/2, −y+3/2, z+1; (viii) x+1/2, −y+3/2, z−1; (ix) x+1, y, z; (x) −x+3/2, y+1/2, −z.