A novel lithium copper iron phosphate with idealized formula Li5Cu22+Fe3+(PO4)4: crystal structure and distribution of defects

Upreti, S.; Yakubovich, O.V.; Chernova, N.A.; Whittingham, M.S.

doi:10.1107/S1600536811011755

inorganic compounds

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ISSN: 2056-9890

Volume 67| Part 5| May 2011| Page i29

doi:10.1107/S1600536811011755

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A novel lithium copper iron phosphate with idealized formula Li₅Cu₂²⁺Fe³⁺(PO₄)₄: crystal structure and distribution of defects

Shailesh Upreti,^a Olga V. Yakubovich,^b Natasha A. Chernova ^a and M. Stanley Whittingham ^a ^*

^aChemistry and Materials, SUNY Binghamton, Binghamton, NY, USA, and ^bDepartment of Geology, Moscow State University, Moscow, Russian Federation
^*Correspondence e-mail: stanwhit@gmail.com

(Received 6 March 2011; accepted 30 March 2011; online 7 April 2011)

Gray–green single crystals were obtained under high-pressure, high-temperature hydrothermal conditions. A refinement of atom occupancies gave the composition Li_3.68Cu²⁺Fe³⁺(Cu_0.55Li_0.45)₂Fe²⁺_0.15(PO₄)₄. The structure is built from triplets of edge-sharing (Cu,Li)O₅–FeO₆–(Cu,Li)O₅ polyhedra, CuO₄ quadrilaterals and PO₄ tetrahedra. In the (Cu,Li)O₅ polyhedra the Cu and Li positions are statistically occupied in a 0.551 (2):0.449 (2) ratio. Both FeO₆ and CuO₄ polyhedra exhibit $[\overline1]$ symmetry. The positions of additional Li atoms with vacancy defects are in the interstices of the framework.

Related literature

For a related structure, see: Yakubovich et al. (2006 ). For related materials with low concentration of Cu atoms at Fe sites, see: Amine et al. (2000 ); Heo et al. (2009 ); Ni et al. (2005 ); Yang et al. (2009 ). For information on bond-valence calculations, see: Pyatenko (1972 ).

Experimental

Crystal data

Cu_2.10Fe_1.15Li_4.59(PO₄)₄
M_r = 609.63
Triclinic, $[P \overline 1]$
a = 4.8950 (14) Å
b = 7.847 (2) Å
c = 8.388 (2) Å
α = 69.472 (5)°
β = 89.764 (6)°
γ = 75.501 (5)°
V = 290.88 (13) Å³
Z = 1
Mo Kα radiation
μ = 5.87 mm⁻¹
T = 298 K
0.27 × 0.23 × 0.19 mm

Data collection

Bruker SMART APEX diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2000 ) T_min = 0.235, T_max = 0.332
100441 measured reflections
1775 independent reflections
1647 reflections with I > 2σ(I)
R_int = 0.023

Refinement

R[F² > 2σ(F²)] = 0.031
wR(F²) = 0.086
S = 1.20
1708 reflections
143 parameters
Δρ_max = 0.66 e Å⁻³
Δρ_min = −0.63 e Å⁻³

Table 1
Selected bond lengths (Å)

Cu1—O4ⁱ	1.936 (2)
Cu1—O6	1.9430 (19)
Fe1—O1	1.931 (2)
Fe1—O5	2.038 (2)
Fe1—O2	2.041 (2)
Cu2—O8	1.969 (2)
Cu2—O7	1.998 (2)
Cu2—O6	2.003 (2)
Cu2—O5	2.075 (2)
Cu2—O2ⁱⁱ	2.171 (2)
Li1—O3	1.967 (8)
Li1—O4ⁱⁱⁱ	2.028 (7)
Li1—O8^iv	2.045 (7)
Li1—O3^v	2.124 (8)
Fe2—O3	1.891 (6)
Fe2—O8^vi	2.063 (6)
Fe2—O7^vii	2.133 (7)
Fe2—O6^vi	2.236 (7)
Fe2—O4ⁱⁱⁱ	2.334 (6)
Li3—O7	1.909 (8)
Li3—O3^viii	1.916 (7)
Li3—O2^viii	2.183 (11)
Li3—O8^ix	2.183 (10)
Symmetry codes: (i) -x-1, -y+2, -z+2; (ii) -x, -y+2, -z+1; (iii) -x, -y+1, -z+2; (iv) -x, -y+2, -z+2; (v) -x+1, -y+1, -z+2; (vi) x+1, y-1, z; (vii) x, y-1, z; (viii) x, y+1, z; (ix) x+1, y, z.

Data collection: SMART (Bruker, 2001 ); cell refinement: SAINT (Bruker, 2002 ); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006 ); software used to prepare material for publication: publCIF (Westrip, 2010 ).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536811011755/pk2312sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536811011755/pk2312Isup2.hkl

checkCIF report

Comment top

There has been much interest in understanding the chemical and physical behavior of a new class of materials that shows reversible intercalation of lithium in the crystalline lattice for use in the next generation of Li ion batteries. Here we report a new type of Li containing solid which could be of great interest to electrochemists.

The asymmetric unit of the triclinic structure (Fig. 1) includes two tetrahedral P sites, both on the general position. The Cu1 – O distances around the square–planar Cu²⁺ cation at the center of symmetry (1d site) are 1.936 (2) and 1.943 (2) Å. Fe³⁺ cations in 1b Wyckoff site are surrounded by six O atoms, forming octahedral configuration with Fe—O bond lengths in the interval 1.931 (2) – 2.041 (2) Å. The cation-anion distances in five-vertex polyhedra, occupied by Cu and Li atoms in nearly equivalent amounts change from 1.969 (2) to 2.171 (2) Å; thus, the mixed occupation of the polyhedron by Cu²⁺ and Li⁺ cations explains why the Jahn-Teller distorton of the polyhedron is not so evident. Two Li sites with vacancy defects adopt five-vertex coordination, each with four closest oxygen atoms (Table 1), and one oxygen atom at longer distances of 2.739 (11) Å (Li3 –O8) and 2.778 (8) Å (Li1 – O3). In addition, a position of low occupancy for Fe²⁺ (Fe2) atoms has been found at 0.99 (1) Å from the Li3 site. Bond-valance sum data (Pyatenko, 1972) are consistent with the assumed oxidation state of Cu and Fe.

The basic features of the crystal structure consist of triplets of edge sharing (Cu,Li)2 – Fe1 –(Cu,Li)2 polyhedra (Fig.2) and Cu1 quadrilaterals, that form a three-dimensional framework by sharing oxygen vertices. The PO₄ tetrahedra strengthen this framework by sharing all vertices with Fe1 octahedra and/or (Cu,Li)2 polyhedra (P1), while P2 tetrahedron shares one vertex with Fe1 and (Cu,Li)2 polyhedra, two vertices with Cu1 quadrilaterals, and one vertex (O3) remains unshared with the cationic framework and participates in the coordination of Li atoms (Table 2). Li1 and Li3 atoms occupy interstices of the structure; they form tetra groups of five-vertex polyhedra sharing edges (Fig.3). The structure may be described using an idealized formula Li₅Cu²⁺₂Fe³⁺(PO₄)₄; a similar lithium saturated iron phosphate with isomorphous and vacancy defects in the position of Li atoms, having an idealized formula Li₅Fe³⁺(PO₄)₂F₂ was studied in (Yakubovich et al., 2006). There are many reports in literature for crystal structures with low concentration of Cu atoms in Fe sites (Amine et al., 2000; Heo et al., 2009, Yang et al., 2009, Ni et al., 2005), however, the present structure seems to be a rare example of Cu rich three-dimensional matrix with Li⁺ ions in the interstices.

Related literature top

For a related structure, see: Yakubovich et al. (2006). For related materials with low concentration of Cu atoms at Fe sites, see: Amine et al. (2000); Heo et al. (2009); Ni et al. (2005); Yang et al. (2009). For information on bond-valence calculations, see Pyatenko (1972).

Experimental top

Single crystals were grown under high-temperature high-pressure hydrothermal conditions in the LiH₂PO₄—Fe₂O₃—H₃PO₄ system. Fe₂O₃ and LiH₂PO₄, weight ratio 5:1, were placed in a copper ampule of 120 ml volume with 5, 10, 20, 30 or 40% water solution of H₃PO₄. The reaction was conducted at 400 °C, 1000 bar (1 bar = 10 ⁵ Pa) for 100 h. The reaction product was a mixture of brown, grayish-green, and blue-green crystals, white powder and copper chunks in a ratio dependant upon phosphoric acid concentration. The crystals were hand picked under an optical microscope, washed with water and isopropyl alcohol, dried and subjected to single-crystal x-ray diffraction analysis. The obtained crystals were also analyzed with a Jeol 8900 Electron Microprobe for EDS elemental content. The conditions were optimized with an acceleration potential of 15 kV and acurrent of 10 mA. The average result of 15 analyses showed the Cu: Fe: P ratio equal to 0.47: 0.28: 1, which is close to the ratio 0.52: 0.29: 1 determined from single-crystal X-ray refinement.

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top

	Fig. 1. The main structural elements of the title compound. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) x, y - 1, z; (ii) -x, 1 - y, 1 - z; (v) 1 - x, 1 - y, 1 - z].
	Fig. 2. The crystal structure of the title compound projected onto the plane cb.
	Fig. 3. The structure fragment showing the groups of five-vertex Li polyhedra sharing edges.

Pentalithium dicopper iron tetraphosphate top

Crystal data top

Cu_2.10Fe_1.15Li_4.59(PO₄)₄	Z = 1
M_r = 609.63	F(000) = 293.9
Triclinic, P1	D_x = 3.480 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 4.8950 (14) Å	Cell parameters from 8918 reflections
b = 7.847 (2) Å	θ = 2.4–28.3°
c = 8.388 (2) Å	µ = 5.87 mm⁻¹
α = 69.472 (5)°	T = 298 K
β = 89.764 (6)°	Block, green
γ = 75.501 (5)°	0.27 × 0.23 × 0.19 mm
V = 290.88 (13) Å³

Data collection top

Bruker SMART APEX diffractometer	1775 independent reflections
Radiation source: fine-focus sealed tube	1647 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.023
ϕ scans, and ω scans	θ_max = 30.6°, θ_min = 2.6°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −7→7
T_min = 0.235, T_max = 0.332	k = −11→11
100441 measured reflections	l = −12→12

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.031	w = 1/[σ²(F_o²) + (0.040P)² + 0.5P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.086	(Δ/σ)_max = 0.001
S = 1.20	Δρ_max = 0.66 e Å⁻³
1708 reflections	Δρ_min = −0.63 e Å⁻³
143 parameters	Extinction correction: SHELXL97 (Sheldrick, 2010), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.008 (3)

Crystal data top

Cu_2.10Fe_1.15Li_4.59(PO₄)₄	γ = 75.501 (5)°
M_r = 609.63	V = 290.88 (13) Å³
Triclinic, P1	Z = 1
a = 4.8950 (14) Å	Mo Kα radiation
b = 7.847 (2) Å	µ = 5.87 mm⁻¹
c = 8.388 (2) Å	T = 298 K
α = 69.472 (5)°	0.27 × 0.23 × 0.19 mm
β = 89.764 (6)°

Data collection top

Bruker SMART APEX diffractometer	1775 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2000)	1647 reflections with I > 2σ(I)
T_min = 0.235, T_max = 0.332	R_int = 0.023
100441 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.031	143 parameters
wR(F²) = 0.086	0 restraints
S = 1.20	Δρ_max = 0.66 e Å⁻³
1708 reflections	Δρ_min = −0.63 e Å⁻³

Special details top

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Cu1	−0.5000	1.0000	1.0000	0.00809 (14)
Fe1	0.0000	1.0000	0.5000	0.00691 (15)
Cu2	−0.09925 (13)	1.25406 (8)	0.71583 (7)	0.0080 (2)	0.551 (2)
Li2	−0.09925 (13)	1.25406 (8)	0.71583 (7)	0.0080 (2)	0.449 (2)
P1	−0.37011 (14)	1.70119 (10)	0.53149 (9)	0.00622 (16)
P2	0.08696 (15)	0.80671 (10)	0.91211 (8)	0.00607 (16)
Li1	0.3243 (15)	0.3973 (10)	1.1070 (9)	0.023 (2)	0.92 (3)
Fe2	0.3834 (13)	0.4186 (9)	0.8189 (9)	0.014 (2)	0.076 (3)
Li3	0.262 (2)	1.5161 (15)	0.7187 (13)	0.040 (2)	0.924 (3)
O1	0.2556 (4)	1.1578 (3)	0.4209 (3)	0.0106 (4)
O2	0.3006 (4)	0.7643 (3)	0.5056 (3)	0.0090 (4)
O3	0.2355 (5)	0.6208 (3)	0.8948 (3)	0.0132 (4)
O4	−0.2135 (4)	0.7997 (3)	0.9644 (3)	0.0111 (4)
O5	0.0858 (5)	0.9747 (3)	0.7461 (3)	0.0104 (4)
O6	−0.2511 (4)	1.1666 (3)	0.9440 (2)	0.0076 (4)
O7	0.2528 (4)	1.3249 (3)	0.6291 (3)	0.0108 (4)
O8	−0.2947 (5)	1.5157 (3)	0.6891 (3)	0.0133 (4)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cu1	0.0056 (2)	0.0073 (2)	0.0128 (2)	−0.00214 (17)	0.00253 (17)	−0.00512 (18)
Fe1	0.0061 (3)	0.0056 (3)	0.0090 (3)	−0.00124 (19)	0.00114 (19)	−0.0029 (2)
Cu2	0.0102 (3)	0.0052 (3)	0.0078 (3)	−0.0007 (2)	0.0029 (2)	−0.0023 (2)
Li2	0.0102 (3)	0.0052 (3)	0.0078 (3)	−0.0007 (2)	0.0029 (2)	−0.0023 (2)
P1	0.0052 (3)	0.0056 (3)	0.0076 (3)	−0.0011 (2)	0.0007 (2)	−0.0023 (2)
P2	0.0054 (3)	0.0060 (3)	0.0067 (3)	−0.0009 (2)	0.0008 (2)	−0.0026 (2)
Li1	0.026 (4)	0.019 (4)	0.025 (4)	−0.002 (3)	−0.001 (3)	−0.011 (3)
Fe2	0.014 (3)	0.012 (3)	0.018 (4)	−0.004 (2)	0.006 (2)	−0.008 (3)
Li3	0.058 (6)	0.050 (6)	0.036 (5)	−0.024 (5)	0.013 (5)	−0.037 (5)
O1	0.0101 (9)	0.0133 (10)	0.0120 (9)	−0.0062 (8)	0.0023 (7)	−0.0069 (8)
O2	0.0052 (9)	0.0095 (9)	0.0134 (9)	−0.0009 (7)	−0.0001 (7)	−0.0063 (8)
O3	0.0153 (10)	0.0115 (10)	0.0134 (9)	0.0000 (8)	0.0017 (8)	−0.0079 (8)
O4	0.0066 (9)	0.0084 (9)	0.0186 (10)	−0.0015 (7)	0.0033 (8)	−0.0055 (8)
O5	0.0160 (10)	0.0091 (9)	0.0069 (8)	−0.0058 (8)	−0.0003 (7)	−0.0023 (7)
O6	0.0075 (9)	0.0101 (9)	0.0080 (8)	−0.0053 (7)	0.0013 (7)	−0.0044 (7)
O7	0.0105 (9)	0.0128 (10)	0.0127 (9)	−0.0055 (8)	0.0056 (7)	−0.0075 (8)
O8	0.0139 (10)	0.0088 (10)	0.0117 (9)	0.0013 (8)	0.0017 (8)	−0.0003 (8)

Geometric parameters (Å, º) top

Cu1—O4ⁱ	1.936 (2)	P2—O5	1.545 (2)
Cu1—O6	1.9430 (19)	P2—O6^v	1.554 (2)
Fe1—O1	1.931 (2)	Li1—O3	1.967 (8)
Fe1—O5	2.038 (2)	Li1—O4^vi	2.028 (7)
Fe1—O2	2.041 (2)	Li1—O8^v	2.045 (7)
Cu2—O8	1.969 (2)	Li1—O3^vii	2.124 (8)
Cu2—O7	1.998 (2)	Fe2—O3	1.891 (6)
Cu2—O6	2.003 (2)	Fe2—O8^viii	2.063 (6)
Cu2—O5	2.075 (2)	Fe2—O7^ix	2.133 (7)
Cu2—O2ⁱⁱ	2.171 (2)	Fe2—O6^viii	2.236 (7)
P1—O7ⁱⁱⁱ	1.521 (2)	Fe2—O4^vi	2.334 (6)
P1—O1ⁱⁱⁱ	1.524 (2)	Li3—O7	1.909 (8)
P1—O8	1.545 (2)	Li3—O3^x	1.916 (7)
P1—O2^iv	1.555 (2)	Li3—O2^x	2.183 (11)
P2—O3	1.515 (2)	Li3—O8^xi	2.183 (10)
P2—O4	1.542 (2)

O4ⁱ—Cu1—O6	88.35 (9)	O5—Cu2—O2ⁱⁱ	78.90 (8)
O1—Fe1—O5ⁱⁱ	88.97 (8)	O7ⁱⁱⁱ—P1—O1ⁱⁱⁱ	111.94 (12)
O1—Fe1—O2	92.27 (9)	O7ⁱⁱⁱ—P1—O8	112.64 (12)
O5ⁱⁱ—Fe1—O2	82.88 (8)	O1ⁱⁱⁱ—P1—O8	106.40 (12)
O8—Cu2—O7	92.29 (9)	O7ⁱⁱⁱ—P1—O2^iv	109.52 (12)
O8—Cu2—O6	88.84 (9)	O1ⁱⁱⁱ—P1—O2^iv	110.70 (12)
O7—Cu2—O6	136.25 (9)	O8—P1—O2^iv	105.43 (12)
O8—Cu2—O5	176.89 (9)	O3—P2—O4	108.60 (12)
O7—Cu2—O5	90.75 (9)	O3—P2—O5	111.10 (12)
O6—Cu2—O5	89.40 (9)	O4—P2—O5	112.78 (12)
O8—Cu2—O2ⁱⁱ	99.79 (9)	O3—P2—O6^v	109.26 (12)
O7—Cu2—O2ⁱⁱ	102.81 (8)	O4—P2—O6^v	108.23 (12)
O6—Cu2—O2ⁱⁱ	120.07 (8)	O5—P2—O6^v	106.78 (11)

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

Experimental details

Crystal data
Chemical formula	Cu_2.10Fe_1.15Li_4.59(PO₄)₄
M_r	609.63
Crystal system, space group	Triclinic, P1
Temperature (K)	298
a, b, c (Å)	4.8950 (14), 7.847 (2), 8.388 (2)
α, β, γ (°)	69.472 (5), 89.764 (6), 75.501 (5)
V (Å³)	290.88 (13)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	5.87
Crystal size (mm)	0.27 × 0.23 × 0.19

Data collection
Diffractometer	Bruker SMART APEX diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2000)
T_min, T_max	0.235, 0.332
No. of measured, independent and observed [I > 2σ(I)] reflections	100441, 1775, 1647
R_int	0.023
(sin θ/λ)_max (Å⁻¹)	0.716

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.031, 0.086, 1.20
No. of reflections	1708
No. of parameters	143
Δρ_max, Δρ_min (e Å⁻³)	0.66, −0.63

Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2002), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2006), publCIF (Westrip, 2010).

Selected bond lengths (Å) top

Cu1—O4ⁱ	1.936 (2)	Li1—O8^iv	2.045 (7)
Cu1—O6	1.9430 (19)	Li1—O3^v	2.124 (8)
Fe1—O1	1.931 (2)	Fe2—O3	1.891 (6)
Fe1—O5	2.038 (2)	Fe2—O8^vi	2.063 (6)
Fe1—O2	2.041 (2)	Fe2—O7^vii	2.133 (7)
Cu2—O8	1.969 (2)	Fe2—O6^vi	2.236 (7)
Cu2—O7	1.998 (2)	Fe2—O4ⁱⁱⁱ	2.334 (6)
Cu2—O6	2.003 (2)	Li3—O7	1.909 (8)
Cu2—O5	2.075 (2)	Li3—O3^viii	1.916 (7)
Cu2—O2ⁱⁱ	2.171 (2)	Li3—O2^viii	2.183 (11)
Li1—O3	1.967 (8)	Li3—O8^ix	2.183 (10)
Li1—O4ⁱⁱⁱ	2.028 (7)

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

Acknowledgements

We thank the US Department of Energy, Office of Vehicle Technologies, for their financial support through the BATT program at LBNL. Financial support from the National Science Foundation, DMR 0705657, is greatly appreciated. Olga Yakubovich thanks the Russian Fund for Basic Researches (Grant N 10–05-01068a) for the financial support.

References

Amine, K., Yasuda, H. & Yamachi, M. (2000). Proc. Electrochem. Soc. 99, 311–325. Google Scholar
Brandenburg, K. (2006). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2000). SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Bruker (2001). SMART. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Bruker (2002). SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Heo, J. B., Lee, S. B., Cho, S. H., Kim, J., Park, S. H. & Lee, Y. S. (2009). Mater. Lett. 63, 581–583. CrossRef CAS Google Scholar
Ni, J. F., Zhou, H. H., Chen, J. T. & Zhang, X. X. (2005). Mater. Lett. 59, 2361–2365. CrossRef CAS Google Scholar
Pyatenko, Yu. A. (1972). Sov. Phys. Crystallogr. 17, 677–682. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
Yakubovich, O. V., Massa, W., Kireev, V. V. & Urusov, V. S. (2006). Dokl. Phys. 51, 474-480. CrossRef CAS Google Scholar
Yang, R., Song, X., Zhao, M. & Wang, F. (2009). J. Alloys Compd, 468, 365–369. CrossRef CAS Google Scholar

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