supplementary materials


Acta Cryst. (2012). E68, i68-i69    [ doi:10.1107/S1600536812035040 ]

Redetermination of the low-temperature polymorph of Li2MnSiO4 from single-crystal X-ray data

M. Sato, T. Ishigaki, K. Uematsu, K. Toda and H. Okawa

Abstract top

Crystals of dilithium manganese(II) silicate were grown under high-temperature hydrothermal conditions in the system LiOH-MnO2-SiO2. The title compound crystallizes in the [beta]II-Li3PO4 structure type. The coordination polyhedra of all cations are slightly distorted tetrahedra (m symmetry for MnO4 and SiO4), which are linked by corner-sharing to each other. The vertices of the tetrahedra point to the same direction perpendicular to the distorted hexagonal close-packed (hcp) array of O atoms within which half of the tetrahedral voids are occupied by cations. In comparison with the previous refinement from powder X-ray data [Dominko et al. (2006). Electrochem. Commun. 8, 217-222], the present reinvestigation from single-crystal X-ray data allows a more precise determination of the distribution of the Li+ and Mn2+ cations, giving a perfectly site-ordered structure model for both Li+ and Mn2+.

Comment top

Lithium transition metal orthosilicates Li2MSiO4 (M = Mn, Fe, Co) have recently become attracting and received much attention as alternatives for the currently used cathode materials of lithium ion batteries, such as LiCoO2, Li(Ni,Mn,Co)O2, LiMn2O4 and LiMPO4 (M = Fe, Mn), because of the natural abundance of silica, iron, and manganese, but also due to a possible high theoretical capacity through two electron delivery. In order to understand the intercalation mechanism of Li+ ions in Li2MSiO4 cathode materials, their crystal structures have been investigated mainly by means of powder methods using X-ray or synchrotron radiation. Summarized by structural studies up-to-date (Islam et al., 2011; Santamaría-Pérez et al., 2012), it could be concluded that the Li2MSiO4 compounds (M = Fe, Mn, Co) belong to a large family of materials known as derivatives of Li3PO4, where oxygen atoms form arrays with a distorted hexagonal-close-packing (hcp) within which half of the tetrahedral voids are occupied by cations. Depending on which site (up or down) of the array the cations occupy, the material shows a rich polymorphism. Such compounds may be divided into two families, designated as β- and γ-forms after the notations used for Li3PO4 polymorphs (West & Glasser, 1972). The γ-polymorphs are built up of both corner- and edge-sharing tetrahedra with half of the tetrahedra pointing along one direction perpendicular to the hcp array and the other half pointing along the opposite direction, while the β-polymorphs are built up of only corner-sharing tetrahedra, with all the tetrahedra pointing to the same direction perpendicular to the hcp array. Detailed structural information, particularly for electrochemically active Li2MnSiO4, were available from the previous studies. Li2MnSiO4 exhibit three polymorphs, namely a low-temperature form denoted as βII (Pmn21), an intermediate temperature form denoted as γII (Pmnb), and a high-temperature form denoted as γo (P21/n) (Arroyo-de Dompablo et al., 2006, 2008; Belharouak et al., 2009; Dominko et al., 2006; Kokalj et al., 2007; Politaev et al., 2007; Wu et al., 2009; Zhong et al., 2010). It should be noted that in almost all polymorphs a site disorder for cationic sites, particularly for Li+ sites substituted by transition metal ions, was observed. Surprisingly, the structure models proposed for Li2MSiO4 (M = Mn, Fe, Co) have all been determined and refined by powder diffraction methods except for that of Li2CoSiO4 (Yamaguchi et al., 1979). In terms of the fact that lithium has quite low scattering factors for X-rays, this may be true even for neutron diffraction, the crystallographic information obtained for Li sites by powder diffraction should inevitably include ambiguity to some extent. Efforts to obtain single crystals for structure determination have not been rewarded for Li2MSiO4 (M = Mn, Fe). Although Setoguchi (1988) succeeded to grow single crystals of Li2FeSiO4 from a flux method using LiCl at elevated temperatures, he could not determine the structure because of suffering from twinned crystals. Here we describe the single-crystal growth of Li2MnSiO4 by means of a high temperature hydrothermal method and its structure determination using single-crystal X-ray diffraction, confirming a perfectly site-ordered structure for Li2MnSiO4 in its low-temperature βII (Pmn21) polymorph.

The first detailed report on the description of the structure model for Li2MnSiO4 accompanied with numerical crystallographic data is probably that determined by Dominko et al. (2006) who performed Rietveld refinements in the space group Pmn21 with a = 6.3109 (9) Å, b = 5.3800 (9) Å, c = 4.9662 (8) Å and Z = 2. The obtained structure model is presented in Fig. 1. It contains significant site disorders for all cationic sites, though the primary sites for the cations are located within the tetrahedral voids situated among a fairly distorted hexagonal-close-packing (hcp) of oxygen atoms. The MO4 (M = Li, Mn, Si) tetrahedra all point towards the same direction perpendicular to the hcp array, and are linked by corner-sharing. The partially occupied tetrahedral sites are located on the opposite sides of corresponding primary MO4 tetrahedra, the vertices of which point to the opposite direction. The central metal atom pairs are separated by distances of 1.0 (2) Å for Li—Li pairs, 0.52 (10) Å for Mn—Mn pairs and 1.09 (15) Å for Si—Si pairs, forming kinds of (pseudo) trigonal-bipyramidal MO5 polyhedra, as shown in Fig. 1(b). The structure can also be described as a typical βII-Li3PO4 structure if cation sites with low site-occupancies are removed, as shown in Fig. 1(c) and (d). This structure model, which has such excessive disorders, may have somewhat possible deficiencies. Furthermore, the environment around Li+ ions is crucially important for understanding the lithium intercalation behavior during the charge/discharge process. Theroretical studies concerned with expectation of redox potentials and lithium migration paths for Li2MnSiO4 cathodes have been accomplished by several groups (Kokalj et al. 2007; Kuganathan & Islam, 2009; Wu et al. 2009; Mali et al. 2010; Zhong et al. 2010; Duncan et al. 2011) based on the model by Dominko et al. (2006); most of these studies adopted the cation site disorder model or the idealized ordered one only with primary sites.

The structure refined in the present study is a perfectly site-ordered one for all cationic sites (Fig. 2) though the fundamental framework structure is the same as that previously reported. Notably, the displacement ellipsoids are relatively large not only for lithium atoms but also for manganese atoms (Fig. 3). This may reflect a high diffusibility both of Li+ and Mn2+ ions in this cathode material. The ordered structure model found in the present study is consistent with the results of an NMR study by Sirisopanaporn et al. (2011). Unexpectedly, information on atomic coordinates available for Li2MnSiO4 is scarce in the literature, where the data were refined from powder diffraction analyses (Dominko et al., 2006) and obtained from an optimization by atomistic simulation (Arroyo-de Dompablo et al., 2008). Table 2 shows the results of the bond-valence sum (BVS) analysis (Brown & Altermatt, 1985) for cation tetrahedra estimated from refined atomic coordinates in Li2MnSiO4, together with those for the structure models proposed previously for comparison. The deviations from the formal valences of each ion are fairly large for the previous studies, in particularly for Si, while in the present study the values of the BVS calculation for all ions are in very good agreement with the theoretical ones. Moreover, it should be mentioned that the MO4 tetrahedra in the present model have much more regular environments than the previous models.

No structural data based on single-crystal X-ray diffraction data have been reported for Li2MnSiO4, although recent works on positive electrode materials for rechargeable lithium batteries reported the electrochemical characterization of this cathode material. Our present structural study of Li2MnSiO4 provides more accurate information of its crystal structure than has been available up to now.

Related literature top

For background to structural studies of Li2MSiO4 (M = Mn, Fe, Co) compounds, see: Islam et al. (2011); Santamaría-Pérez et al. (2012); Setoguchi (1988); Yamaguchi et al. (1979). Polymorphism of Li2MnSiO4 was reported by Arroyo-de Dompablo et al. (2006, 2008); Belharouak et al. (2009); Dominko et al. (2006); Kokalj et al. (2007); Politaev et al. (2007); Wu et al. (2009); Zhong et al. (2010). For notation of Li3PO4 polymorphs, see: West & Glasser (1972). For theoretical studies of the redox potentials and Li migration paths of Li2MnSiO4, see: Kuganathan & Islam (2009); Mali et al. (2010); Duncan et al. (2011), and for NMR studies of this material, see: Sirisopanaporn et al. (2011). For the bond-valence method, see: Brown & Altermatt (1985). For crystallographic background: see: Cooper et al. (2002).

Experimental top

In order to synthesize Li2MnSiO4 single crystals, a high-temperature, high-pressure hydrothermal synthetic method was performed in a silver ampoule contained in a home-made autoclave made of stainless steel (SUS304) with 6 cm in outer diameter, 0.8 cm in inner diameter, and 1.8 cm3 in volume. The pressure was provided by water. High-purity chemical reagents of MnO2, SiO2 and LiOH.2H2O were used as reaction agents. A reaction mixture of LiOH, MnO2 and SiO2 with a molar ratio of Li:Mn:Si = 2:12:2 in a 3 cm long silver ampoule (inside diameter = 0.6 cm) was heated at 823 K for 3 days. The pressure was estimated to be 12 MPa at the reaction temperature according to the pressure-temperature diagram of pure water. The autoclave was then cooled to 323 K at 5 K/h and quenched to room temperature by removing the autoclave from the furnace. The product was filtered off, washed with water, rinsed with ethanol, and dried at ambient temperature. The reaction produced light-green rod-shaped crystals of Li2MnSiO4 that were obtained as a major product along with some quartz crystals.

The surface of the single crystals was observed by using optical (Olympus BX-60) and scanning electron microscopy (SEM, Jeol JSM-5310LVB). The elemental composition of the crystals was characterized by energy dispersive X-ray spectroscopy (EDS) attached to SEM (SEM/EDS, Nippon Denshi JED-2140).

Refinement top

The structure was solved by direct methods and refined by subsequent Fourier syntheses, leading to wR2 = 4.17% in the early stages of refinement. The relatively high value of the Flack parameter, x = 0.167, pointed to a possible twinned crystal. The examination using ROTAX (Cooper et al., 2002) indicated two possible rotation twin axes about [010] and [001]. In addition to these rotation twin formations, a racemic twin (inversion twin) formation can be also possible for the non-centrosymmetric Pmn21 space group. Subsequent refinements using the twin laws for the three cases yielded a satisfactory solution with wR2 = 3.67% for all cases and a Flack parameter x = 0.00 (2) for the rotation twin cases. The twin fraction ratio is 82.9: 17.1. In non-centrosymmetric space groups where mirror planes and/or glide planes exist, an inversion twin is equivalent to the rotation twin through the 2-fold rotation axis perpendicular to the mirror and/or the glide planes. This is true for the present case.

Computing details top

Data collection: CrystalClear (Rigaku, 2010); cell refinement: CrystalClear (Rigaku, 2010); data reduction: CrystalClear (Rigaku, 2010); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. Structure model of Li2MnSiO4 determined by Dominko et al. (2006). The original structure model is projected (a) along [001] and (b) along [100], while the structure model where cation sites with low occupancies are removed is projected (c) along [001] and (d) along [100]. Polyhedra are indicated by red color for LiO4, blue color for MnO4, and green color for SiO4.
[Figure 2] Fig. 2. Structure model of Li2MnSiO4 determined by the present study with (a) projection view along [001] and (b) projection view along [100]. Polyhedra are indicated by red color for LiO4, blue color for MnO4, and green color for SiO4.
[Figure 3] Fig. 3. A perspective view of a part of the Li2MnSiO4 structure with the unit cell outlined. Displacement ellipsoids are drawn at the 70% probability level. Polyhedra are indicated by red color for LiO4, blue color for MnO4, and green color for SiO4.
dilithium manganese(II) silicate top
Crystal data top
Li2MnSiO4F(000) = 154
Mr = 160.91Dx = 3.174 Mg m3
Orthorhombic, Pmn21Mo Kα radiation, λ = 0.71069 Å
Hall symbol: P 2ac -2Cell parameters from 1684 reflections
a = 6.3133 (16) Åθ = 3.2–27.5°
b = 5.3677 (14) ŵ = 4.11 mm1
c = 4.9685 (12) ÅT = 295 K
V = 168.37 (7) Å3Prism, light green
Z = 20.26 × 0.19 × 0.18 mm
Data collection top
Rigaku Mercury375R
diffractometer
423 independent reflections
Radiation source: Sealed Tube419 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.019
Detector resolution: 13.6612 pixels mm-1θmax = 27.4°, θmin = 3.8°
profile data from ω–scansh = 88
Absorption correction: multi-scan
(REQAB; Rigaku, 1998)
k = 66
Tmin = 0.377, Tmax = 0.477l = 66
1636 measured reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0217P)2]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.015(Δ/σ)max < 0.001
wR(F2) = 0.037Δρmax = 0.25 e Å3
S = 1.14Δρmin = 0.62 e Å3
423 reflectionsExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
45 parametersExtinction coefficient: 0.392 (13)
1 restraintAbsolute structure: Flack (1983), 189 Friedel pairs
0 constraintsFlack parameter: 0.171 (15)
Primary atom site location: structure-invariant direct methods
Crystal data top
Li2MnSiO4V = 168.37 (7) Å3
Mr = 160.91Z = 2
Orthorhombic, Pmn21Mo Kα radiation
a = 6.3133 (16) ŵ = 4.11 mm1
b = 5.3677 (14) ÅT = 295 K
c = 4.9685 (12) Å0.26 × 0.19 × 0.18 mm
Data collection top
Rigaku Mercury375R
diffractometer
423 independent reflections
Absorption correction: multi-scan
(REQAB; Rigaku, 1998)
419 reflections with I > 2σ(I)
Tmin = 0.377, Tmax = 0.477Rint = 0.019
1636 measured reflectionsθmax = 27.4°
Refinement top
R[F2 > 2σ(F2)] = 0.0151 restraint
wR(F2) = 0.037Δρmax = 0.25 e Å3
S = 1.14Δρmin = 0.62 e Å3
423 reflectionsAbsolute structure: Flack (1983), 189 Friedel pairs
45 parametersFlack parameter: 0.171 (15)
Special details top

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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
xyzUiso*/Ueq
Li10.7503 (4)0.1688 (4)0.995 (5)0.0090 (7)
Mn10.50.33172 (5)0.99680.00733 (15)
Si110.32090 (9)0.9851 (3)0.00350 (17)
O10.50.6868 (3)1.1569 (5)0.0071 (5)
O20.50.3867 (3)0.5804 (4)0.0081 (4)
O30.7887 (2)0.1799 (2)1.0979 (4)0.0075 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Li10.0084 (17)0.0061 (15)0.0125 (15)0.0009 (7)0.0000 (14)0.002 (2)
Mn10.0055 (2)0.0072 (2)0.0093 (2)000.0000 (2)
Si10.0035 (3)0.0028 (3)0.0042 (4)000.0003 (3)
O10.0075 (9)0.0083 (8)0.0056 (11)000.0004 (6)
O20.0098 (8)0.0048 (7)0.0096 (10)000.0001 (7)
O30.0068 (5)0.0067 (6)0.0089 (7)0.0009 (4)0.0008 (7)0.0000 (5)
Geometric parameters (Å, º) top
Li1—O1i1.936 (10)Mn1—O12.065 (2)
Li1—O31.956 (6)Mn1—O22.090 (2)
Li1—O3ii1.99 (2)Si1—O1v1.631 (3)
Li1—O2iii2.009 (6)Si1—O31.6331 (17)
Mn1—O3iv2.0585 (16)Si1—O3vi1.6331 (17)
Mn1—O32.0585 (15)Si1—O2vii1.639 (2)
O1i—Li1—O3112.0 (7)O3iv—Mn1—O2107.31 (6)
O1i—Li1—O3ii107.5 (5)O3—Mn1—O2107.31 (6)
O3—Li1—O3ii107.7 (7)O1—Mn1—O2104.54 (8)
O1i—Li1—O2iii108.6 (6)O1v—Si1—O3109.35 (10)
O3—Li1—O2iii113.9 (5)O1v—Si1—O3vi109.35 (10)
O3ii—Li1—O2iii106.8 (6)O3—Si1—O3vi109.58 (13)
O3iv—Mn1—O3124.58 (8)O1v—Si1—O2vii108.23 (13)
O3iv—Mn1—O1105.74 (5)O3—Si1—O2vii110.16 (10)
O3—Mn1—O1105.74 (5)O3vi—Si1—O2vii110.16 (10)
Symmetry codes: (i) x, y1, z; (ii) x+3/2, y, z1/2; (iii) x+3/2, y, z+1/2; (iv) x+1, y, z; (v) x+3/2, y+1, z1/2; (vi) x+2, y, z; (vii) x+3/2, y+1, z+1/2.
Selected geometric parameters (Å, º) top
Li1—O1i1.936 (10)Mn1—O12.065 (2)
Li1—O31.956 (6)Mn1—O22.090 (2)
Li1—O3ii1.99 (2)Si1—O1v1.631 (3)
Li1—O2iii2.009 (6)Si1—O31.6331 (17)
Mn1—O3iv2.0585 (16)Si1—O2vi1.639 (2)
O1i—Li1—O3112.0 (7)O3iv—Mn1—O1105.74 (5)
O1i—Li1—O3ii107.5 (5)O3iv—Mn1—O2107.31 (6)
O3—Li1—O3ii107.7 (7)O1—Mn1—O2104.54 (8)
O1i—Li1—O2iii108.6 (6)O1v—Si1—O3109.35 (10)
O3—Li1—O2iii113.9 (5)O3—Si1—O3vii109.58 (13)
O3ii—Li1—O2iii106.8 (6)O1v—Si1—O2vi108.23 (13)
O3iv—Mn1—O3124.58 (8)O3—Si1—O2vi110.16 (10)
Symmetry codes: (i) x, y1, z; (ii) x+3/2, y, z1/2; (iii) x+3/2, y, z+1/2; (iv) x+1, y, z; (v) x+3/2, y+1, z1/2; (vi) x+3/2, y+1, z+1/2; (vii) x+2, y, z.
Bond-valence parameters derived from the present model and the previous studies. top
AtomSitePresent workDominko et al.1)Arroyo-de Dompablo et al.2)
Li4b1.02 (6)1.0 (1)0.9
Mn2a1.89 (5)2.1 (1)1.77
Si2a3.89 (7)3.6 (2)3.65
O12a2.02 (9)1.9 (3)1.75
O24b1.97 (7)1.9 (2)1.86
O32a1.87 (7)2.0 (2)1.90
1) The data, referred to Dominko et al. (2006), are based on the coordinates for primary MO4 (M = Li, Mn, Si) tetrahedra. 2) The data, referred to Arroyo-de Dompablo et al. (2008), are based on the coordinates for primary MO4 (M = Li, Mn, Si) tetrahedra optimized by density functional theory (DFT) methods.
Acknowledgements top

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references
References top

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