inorganic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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The mixed-valent titanium phosphate, Li2Ti2(PO4)3, dilithium dititanium(III/IV) tris­­(orthophosphate)

aDivision of Energy Systems Research and Department of Chemistry, Ajou University, Suwon 443-749, Republic of Korea
*Correspondence e-mail: hsyun@ajou.ac.kr

(Received 12 July 2011; accepted 4 August 2011; online 17 August 2011)

The mixed-valent titanium phosphate, Li2Ti2(PO4)3, has been prepared by the reactive halide flux method. The title compound is isostructural with Li2TiM(PO4)3 (M = Fe, Cr) and Li2FeZr(PO4)3 and has the same 3[Ti2(PO4)3]2− framework as the previously reported Li3-xM2(PO4)3 phases. The framework is built up from corner-sharing TiO6 octa­hedra and PO4 tetra­hedra, one of which has 2 symmetry. The Li+ ions are located on one crystallographic position and reside in the vacancies of the framework. They are surrounded by four O atoms in a distorted tetra­hedral coordination. The classical charge-balance of the title compound can be represented as Li+2(Ti3+/Ti4+)(PO43−)3.

Related literature

The synthesis and structural characterization of stoichiometric Li2TiM(PO4)3 (M = Fe and Cr) and Li2FeZr(PO4)3 have been reported by Patoux et al. (2004[Patoux, S., Rousse, G., Leriche, J.-B. & Masquelier, C. (2004). Solid State Sci. 6, 1113-1120.]) and Catti (2001[Catti, M. (2001). J. Solid State Chem. 156, 305-312.]), respectively. For related phosphates with general formula Li3-xM2(PO4)3 (0 ≤ x ≤ 1), see: Wang & Hwu (1991[Wang, S. & Hwu, S.-J. (1991). J. Solid State Chem. 90, 377-381.]) for Li2.72Ti2(PO4)3. For Li batteries based on Li3-xM2(PO4)3 phases, see: Yin et al. (2003[Yin, S.-C., Grondey, H., Strobel, P., Anne, M. & Nazar, L. F. (2003). J. Am. Chem. Soc. 125, 10402-10411.]). For ionic conductivities of these phases, see: Sato et al. (2000[Sato, M., Ohkawa, H., Yoshida, K., Saito, M., Uematsu, K. & Toda, K. (2000). Solid State Ionics, 135, 137-142.]). For ionic radii, see: Shannon (1976[Shannon, R. D. (1976). Acta Cryst. A32, 751-767.]). For structure validation, see: Spek (2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]).

Experimental

Crystal data
  • Li2Ti2(PO4)3

  • Mr = 394.59

  • Orthorhombic, P b c n

  • a = 12.0344 (5) Å

  • b = 8.5795 (5) Å

  • c = 8.6794 (4) Å

  • V = 896.14 (7) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 2.39 mm−1

  • T = 290 K

  • 0.22 × 0.16 × 0.14 mm

Data collection
  • Rigaku R-AXIS RAPID diffractometer

  • Absorption correction: multi-scan (ABSCOR; Higashi, 1995[Higashi, T. (1995). ABSCOR. Rigaku Corporation, Tokyo, Japan.]) Tmin = 0.802, Tmax = 1.000

  • 6671 measured reflections

  • 1004 independent reflections

  • 974 reflections with I > 2σ(I)

  • Rint = 0.035

Refinement
  • R[F2 > 2σ(F2)] = 0.046

  • wR(F2) = 0.112

  • S = 1.37

  • 1004 reflections

  • 87 parameters

  • Δρmax = 0.49 e Å−3

  • Δρmin = −0.74 e Å−3

Data collection: RAPID-AUTO (Rigaku, 2006[Rigaku (2006). RAPID-AUTO. Rigaku Corporation, Tokyo, Japan.]); cell refinement: RAPID-AUTO; data reduction: RAPID-AUTO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: locally modified version of ORTEP (Johnson, 1965[Johnson, C. K. (1965). ORTEP. Report ORNL-3794. Oak Ridge National Laboratory, Tennessee, USA.]); software used to prepare material for publication: WinGX (Farrugia, 1999[Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837-838.]).

Supporting information


Comment top

Lithium metal phosphates, Li3-xM2(PO4)3, have been widely investigated as materials for secondary batteries (Yin et al., 2003). It has been reported that the amount of Li can be determined in accordance with the oxidation states of metals (M), and Li occupancies are profoundly related to the ionic conductivity (Sato et al., 2000). In attempts to control the amount of Li ions by using various metals with different oxidation states, a new member of this family with stoichiometric Li (x=1) has been found and we report here the synthesis and structural characterization of the mixed-valent title compound Li2Ti2(PO4)3, (I).

(I) is isostructural with mixed-metallic compounds such as Li2TiM(PO4)3 (M = Fe, Cr; Patoux et al., 2004) and Li2FeZr(PO4)3 (Catti, 2001) for which detailed investigations based on powder diffraction data have been reported. The framework of the title compound is the same as that of the previously reported Li3-xM2(PO4)3 (0 x 1) phases such as Li2.72Ti2(PO4)3 (Wang & Hwu, 1991).

Figure 1 shows the local coordination environment of the Ti and P atoms. Two TiO6 octahedra are joined to three PO4 tetrahedra to form the [Ti2(PO4)3] unit. These units share a terminal oxygen atom, O4, to construct the two-dimensional slabs as shown in Figure 2. The three-dimensional framework, 3[Ti2(PO4)3]2- (Fig. 3) is built up from these slabs which are interconnected along the b axis by sharing terminal oxygen atoms O1 and O6. The Li+ ions reside in the vacancies and are surrounded by four O atoms in a distorted tetrahedral coordination. The Ti—O distances, ranging from 1.913 (4) to 2.045 (4) Å, are in good agreement with that calculated from their ionic radii (1.97 Å, Shannon, 1976), assuming a mixed III/IV valence.

Two crystallographically independent Li+ sites have been reported for Li3-xM2(PO4)3 phases. The Li1 site is fully occupied, whereas the Li2 site is only partially occupied. The oxidation states of each metal have to be adjusted to meet the charge neutrality of these compounds. For example, the average oxidation state of Ti is +3.14 for Li2.72Ti2(PO4)3 assuming Ti to be mixed-valent (86% of TiIII and 14% of TiIV; Wang & Hwu, 1991)). In the title compound, the Li1 site is fully occupied, whereas the Li2 site is vacant. For charge neutrality the Ti site is occupied by 50% of TiIII and 50% of TiIV. Therefore, the classical charge balance of the compound can be represented by Li+2(Ti3+/Ti4+)(PO43-)3.

Related literature top

The synthesis and structural characterization of stoichiometric Li2TiM(PO4)3 (M = Fe and Cr) and Li2FeZr(PO4)3 have been reported by Patoux et al. (2004) and Catti (2001), respectively. For related phosphates with general formula Li3-xM2(PO4)3 (0 x 1), see: Wang & Hwu (1991) for Li2.72Ti2(PO4)3. For Li batteries based on Li3-xM2(PO4)3 phases, see: Yin et al. (2003). For ionic conductivities of these phases, see: Sato et al. (2000). For ionic radii, see: Shannon (1976). For structure validation, see: Spek (2009).

Experimental top

The title compound, Li2Ti2(PO4)3, was prepared by the reaction of elemental Ti (CERAC 99.5%) and P (CERAC 99.5%) powders. The pure elements were mixed and loaded in a silica tube with an elemental ratio of 1:3 in the presence of LiCl (Sigma-Aldrich 99%) as a reactive flux. The mass ratio of the reactants and the flux was 1:5. The tube was kept in air for 3 days for water adsorption. It was then evacuated to 0.133 Pa, sealed, and heated gradually (60 K/h) to 1123 K in a tube furnace, where it was kept for 72 h. The tube was cooled at a rate of 5 K/h to room temperature. Air- and water-stable black block-shaped crystals were isolated after the excess flux was removed with water. Qualitative analysis of the crystals with an XRF indicated the presence of Ti and P.

Refinement top

The highest residual electron density (0.49 e Å-3) is 1.24 Å from the O3 site and the deepest hole (-0.74 e Å-3) is 2.29 Å from the O1 site. No additional symmetry, as tested by PLATON (Spek, 2009), was detected in this structure.

Computing details top

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: locally modified version of ORTEP (Johnson, 1965); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. A view showing the local coordination environments of Ti and P atoms with the atom labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. (Symmetry codes: (i) 0.5–x, 0.5–y, 1/2 + z; (iv) 1–x, y, 0.5–z; (v) x, 1–y, 1/2 + z; (vi) 1–x, 1–y, –z; (ix) x, –y, 1/2 + z).
[Figure 2] Fig. 2. The polyhedral representation of the slab structure built up from [Ti2(PO4)3] units. Li atoms are located in the vacancies.
[Figure 3] Fig. 3. A stereoscopic view of Li2Ti2(PO4)3, viewed down the c axis.
dilithium dititanium(III/IV) tris(orthophosphate) top
Crystal data top
Li2Ti2(PO4)3F(000) = 764
Mr = 394.59Dx = 2.925 Mg m3
Orthorhombic, PbcnMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2abCell parameters from 6390 reflections
a = 12.0344 (5) Åθ = 3.3–27.4°
b = 8.5795 (5) ŵ = 2.39 mm1
c = 8.6794 (4) ÅT = 290 K
V = 896.14 (7) Å3Block, black
Z = 40.22 × 0.16 × 0.14 mm
Data collection top
Rigaku R-AXIS RAPID
diffractometer
1004 independent reflections
Radiation source: sealed tube974 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.035
ω scansθmax = 27.4°, θmin = 3.4°
Absorption correction: multi-scan
(ABSCOR; Higashi, 1995)
h = 1414
Tmin = 0.802, Tmax = 1.000k = 1111
6671 measured reflectionsl = 1111
Refinement top
Refinement on F287 parameters
Least-squares matrix: full0 restraints
R[F2 > 2σ(F2)] = 0.046 w = 1/[σ2(Fo2) + (0.P)2 + 10.7789P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.112(Δ/σ)max < 0.001
S = 1.37Δρmax = 0.49 e Å3
1004 reflectionsΔρmin = 0.74 e Å3
Crystal data top
Li2Ti2(PO4)3V = 896.14 (7) Å3
Mr = 394.59Z = 4
Orthorhombic, PbcnMo Kα radiation
a = 12.0344 (5) ŵ = 2.39 mm1
b = 8.5795 (5) ÅT = 290 K
c = 8.6794 (4) Å0.22 × 0.16 × 0.14 mm
Data collection top
Rigaku R-AXIS RAPID
diffractometer
1004 independent reflections
Absorption correction: multi-scan
(ABSCOR; Higashi, 1995)
974 reflections with I > 2σ(I)
Tmin = 0.802, Tmax = 1.000Rint = 0.035
6671 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0460 restraints
wR(F2) = 0.112 w = 1/[σ2(Fo2) + (0.P)2 + 10.7789P]
where P = (Fo2 + 2Fc2)/3
S = 1.37Δρmax = 0.49 e Å3
1004 reflectionsΔρmin = 0.74 e Å3
87 parameters
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Li0.1812 (9)0.2861 (13)0.2191 (12)0.020 (2)
Ti0.38824 (7)0.25295 (11)0.03788 (11)0.0077 (2)
P10.50.5399 (2)0.250.0086 (4)
P20.35246 (11)0.10469 (15)0.39437 (15)0.0069 (3)
O10.4200 (3)0.3537 (5)0.1627 (5)0.0186 (9)
O20.4304 (4)0.4408 (5)0.1413 (5)0.0219 (10)
O30.5306 (4)0.1556 (5)0.0618 (5)0.0217 (10)
O40.2282 (3)0.3271 (5)0.0130 (4)0.0152 (8)
O50.3221 (3)0.1629 (5)0.2322 (4)0.0137 (8)
O60.3443 (3)0.0729 (4)0.1040 (5)0.0146 (8)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Li0.019 (5)0.025 (5)0.017 (5)0.002 (4)0.005 (4)0.004 (4)
Ti0.0076 (4)0.0079 (4)0.0078 (4)0.0006 (3)0.0001 (3)0.0007 (4)
P10.0114 (9)0.0075 (8)0.0071 (8)00.0023 (7)0
P20.0060 (6)0.0079 (6)0.0067 (6)0.0001 (5)0.0003 (5)0.0001 (5)
O10.018 (2)0.025 (2)0.0126 (18)0.0112 (17)0.0036 (17)0.0017 (17)
O20.031 (3)0.018 (2)0.016 (2)0.0095 (18)0.0074 (19)0.0006 (17)
O30.021 (2)0.019 (2)0.025 (2)0.0074 (17)0.0096 (19)0.0075 (18)
O40.017 (2)0.020 (2)0.0087 (17)0.0098 (16)0.0012 (16)0.0001 (15)
O50.0107 (18)0.021 (2)0.0096 (17)0.0007 (15)0.0008 (16)0.0035 (16)
O60.0131 (19)0.0084 (18)0.022 (2)0.0007 (14)0.0049 (17)0.0013 (16)
Geometric parameters (Å, º) top
Li—O41.909 (11)P1—O2iv1.521 (4)
Li—O6i1.979 (11)P1—O1v1.528 (4)
Li—O1i1.994 (12)P1—O1vi1.528 (4)
Li—O52.001 (11)P1—Livii3.048 (11)
Li—Tii2.909 (10)P1—Liviii3.048 (11)
Li—Ti2.960 (10)P2—O3iv1.522 (4)
Li—P22.997 (11)P2—O6ix1.527 (4)
Li—P2ii2.998 (11)P2—O4i1.531 (4)
Li—P1iii3.048 (11)P2—O51.537 (4)
Ti—O21.913 (4)P2—Lii2.998 (11)
Ti—O31.917 (4)O1—P1vi1.528 (4)
Ti—O11.981 (4)O1—Liii1.994 (12)
Ti—O52.019 (4)O3—P2iv1.522 (4)
Ti—O42.039 (4)O4—P2ii1.531 (4)
Ti—O62.045 (4)O6—P2x1.527 (4)
Ti—Liii2.909 (10)O6—Liii1.979 (11)
P1—O21.521 (4)
O4—Li—O6i131.3 (6)O2—P1—O1vi111.9 (2)
O4—Li—O1i140.7 (6)O2iv—P1—O1vi107.1 (2)
O6i—Li—O1i82.7 (4)O1v—P1—O1vi106.6 (4)
O4—Li—O584.2 (4)O3iv—P2—O6ix110.1 (2)
O6i—Li—O5114.2 (5)O3iv—P2—O4i108.0 (2)
O1i—Li—O599.8 (5)O6ix—P2—O4i109.5 (2)
O2—Ti—O394.55 (19)O3iv—P2—O5110.8 (2)
O2—Ti—O189.61 (18)O6ix—P2—O5108.5 (2)
O3—Ti—O196.49 (19)O4i—P2—O5109.9 (2)
O2—Ti—O592.03 (18)P1vi—O1—Ti144.9 (3)
O3—Ti—O595.45 (18)P1vi—O1—Liii119.3 (4)
O1—Ti—O5167.79 (17)Ti—O1—Liii94.1 (3)
O2—Ti—O492.14 (19)P1—O2—Ti155.2 (3)
O3—Ti—O4172.33 (19)P2iv—O3—Ti168.1 (3)
O1—Ti—O487.31 (17)P2ii—O4—Li120.8 (4)
O5—Ti—O480.54 (16)P2ii—O4—Ti142.1 (2)
O2—Ti—O6170.86 (18)Li—O4—Ti97.1 (4)
O3—Ti—O688.11 (17)P2—O5—Li115.2 (4)
O1—Ti—O681.39 (17)P2—O5—Ti142.7 (2)
O5—Ti—O696.43 (16)Li—O5—Ti94.8 (4)
O4—Ti—O685.86 (16)P2x—O6—Liii127.8 (4)
O2—P1—O2iv112.1 (3)P2x—O6—Ti138.0 (3)
O2—P1—O1v107.1 (2)Liii—O6—Ti92.6 (4)
O2iv—P1—O1v111.9 (2)
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x+1/2, y+1/2, z1/2; (iii) x1/2, y1/2, z+1/2; (iv) x+1, y, z+1/2; (v) x, y+1, z+1/2; (vi) x+1, y+1, z; (vii) x+1/2, y+1/2, z+1/2; (viii) x+1/2, y+1/2, z; (ix) x, y, z+1/2; (x) x, y, z1/2.

Experimental details

Crystal data
Chemical formulaLi2Ti2(PO4)3
Mr394.59
Crystal system, space groupOrthorhombic, Pbcn
Temperature (K)290
a, b, c (Å)12.0344 (5), 8.5795 (5), 8.6794 (4)
V3)896.14 (7)
Z4
Radiation typeMo Kα
µ (mm1)2.39
Crystal size (mm)0.22 × 0.16 × 0.14
Data collection
DiffractometerRigaku R-AXIS RAPID
diffractometer
Absorption correctionMulti-scan
(ABSCOR; Higashi, 1995)
Tmin, Tmax0.802, 1.000
No. of measured, independent and
observed [I > 2σ(I)] reflections
6671, 1004, 974
Rint0.035
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.046, 0.112, 1.37
No. of reflections1004
No. of parameters87
w = 1/[σ2(Fo2) + (0.P)2 + 10.7789P]
where P = (Fo2 + 2Fc2)/3
Δρmax, Δρmin (e Å3)0.49, 0.74

Computer programs: RAPID-AUTO (Rigaku, 2006), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), locally modified version of ORTEP (Johnson, 1965), WinGX (Farrugia, 1999).

 

Acknowledgements

This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2010-0029617). Use was made of the X-ray facilities supported by Ajou University.

References

First citationCatti, M. (2001). J. Solid State Chem. 156, 305–312.
First citationFarrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838.  CrossRef CAS IUCr Journals
First citationHigashi, T. (1995). ABSCOR. Rigaku Corporation, Tokyo, Japan.
First citationJohnson, C. K. (1965). ORTEP. Report ORNL-3794. Oak Ridge National Laboratory, Tennessee, USA.
First citationPatoux, S., Rousse, G., Leriche, J.-B. & Masquelier, C. (2004). Solid State Sci. 6, 1113–1120.
First citationRigaku (2006). RAPID-AUTO. Rigaku Corporation, Tokyo, Japan.
First citationSato, M., Ohkawa, H., Yoshida, K., Saito, M., Uematsu, K. & Toda, K. (2000). Solid State Ionics, 135, 137–142.  Web of Science CrossRef CAS
First citationShannon, R. D. (1976). Acta Cryst. A32, 751–767.  CrossRef CAS IUCr Journals Web of Science
First citationSheldrick, G. M. (2008). Acta Cryst. A64, 112–122.  Web of Science CrossRef CAS IUCr Journals
First citationSpek, A. L. (2009). Acta Cryst. D65, 148–155.  Web of Science CrossRef CAS IUCr Journals
First citationWang, S. & Hwu, S.-J. (1991). J. Solid State Chem. 90, 377–381.
First citationYin, S.-C., Grondey, H., Strobel, P., Anne, M. & Nazar, L. F. (2003). J. Am. Chem. Soc. 125, 10402–10411.

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