Lithium barium silicate, Li₂BaSiO₄, from synchrotron powder data

The structure of lithium barium silicate, Li₂BaSiO₄, has been determined from synchrotron radiation powder data. The title compound was synthesized by high-temperature solid-state reaction and crystallizes in the hexa­gonal space group P6₃cm. It contains two Li atoms, one Ba atom (both site symmetry ..m on special position 6c), two Si atoms [on special positions 4b (site symmetry 3..) and 2a (site symmetry 3.m)] and four O atoms (one on general position 12d, and three on special positions 6c, 4b and 2a). The basic units of the structure are (Li₆SiO₁₃)^5- units, each comprising seven tetra­hedra sharing edges and vertices. These basic units are connected by sharing corners parallel to [001] and through sharing (SiO₄)^4- tetra­hedra in (001). The relationship between the structures and luminescence properties of Li₂SrSiO₄, Li₂CaSiO₄ and the title compound is discussed.

S1. Comment

There are several ways of constituting white light based on the LED technique, each of which involves various phosphors, which emit visible light by absorbing soft UV or blue light produced by LEDs. The representative oxide phosphors are Y₃Al₅O₁₂:Ce³⁺ (YAG) and its variants, and divalent europium-doped orthosilicates. Based on their potential for use as white LEDs, Li₂SrSiO₄ (LSSO), Li₂CaSiO₄ (LCSO) and Li₂BaSiO₄ (LBSO) have recently been reported (Toda et al. 2006). In the present study, the structure of LBSO was determined and refined from synchrotron powder diffraction data (Fig. 1). The structure consists of two types of tetrahedra, (SiO₄)^4- and (LiO₄)^7-, with Ba²⁺ atoms located at the centers of open spaces formed by the tetrahedra (Fig. 2). These tetrahedra share edges to form (Li₆SiO₁₃)^5- units that are connected by sharing corners (O atoms) parallel to [001] and through sharing (SiO₄)^4- tetrahedra in (001) (Fig. 3). The bond-valence sum (Brese & O'Keeffe, 1991) of the Ba²⁺ ion is close to the ideal value [S = 1.95 valence units (v.u.)]. The Si2⁴⁺, Li1¹⁺ and Li2¹⁺ ions also reveal ideal bond-valence sums [S = 3.99, 1.02 and 0.99 v.u.], respectively, while the bond-valence sum of the Si1⁴⁺ ion (S = 3.62 v.u.) deviates from the ideal value. This deviation is due to the longer bond length between atoms Si1 and O4 compared with those with the three O2 atoms (Table 1). Atom O4 is common to the Li1, Li2 and Si1 tetrahedra. Because of the Li—O interactions, the SiO₄ tetrahedron is distorted and the Si1⁴⁺ ion is underbonded.

The structures of LSSO (P3₁21; isostructural with Li₂EuSiO₄; Haferkorn & Meyer, 1998) and LCSO (I42m; Gard & West, 1973) also consist of (SiO₄)^4- and (LiO₄)^7- tetrahedra sharing edges; however, these tetrahedra do not form a compact (Li₆SiO₁₃)^5- unit like that in the title compound.

When LSSO, LCSO and LBSO serve as phosphors, activator ions, such as Eu²⁺ in the present case, have to occupy the alkaline earth sites. In this regard, the local structure around the alkaline earth site has a great influence on the luminescent behavior. Dorenbos (2000a, 2000b, 2003) summarized the crystal field splitting in various Ce³⁺-doped phosphors and suggested a systematic model that describes the crystal field splitting in terms of polyhedron shape and ligand distance and also found that the experimentally measured centroid shift, crystal field splitting, red shift and Stokes shift values for Ce³⁺ ions are in a linear relationship with those of Eu²⁺ ions (Dorenbos, 2003). The crystal field splitting is proportional to 1/R_eff², with a constant designating the polyhedral shape, wherein R_eff is the effective average ligand distance. The Sr, Ca and Ba polyhedra in LSSO, LCSO and LBSO have site symmetries 2, 42m and m, respectively, and different coordination numbers. Measured crystal field splittings of LSSO, LCSO and LBSO phosphors (Kulshreshtha, Sharma & Sohn, 2009) show that higher-symmetry, smaller coordination number and larger polyhedron size (ligand distance) lead to greater crystal field splitting.

On the other hand, the centroid shift is proportional to Nα_sp/R_eff⁶ according to the ligand polarization model, where N is the coordination number and α_sp is the anion polarizability of the nearest anion neighbors (Dorenbos, 2000a, 2000b). Using experimentally measured centroid shift and ligand distance data for LSSO, LCSO and LBSO (Kulshreshtha, Sharma & Sohn, 2009), we confirmed that the greater the cation size, the higher the polarizability.

The energy transfer among the Eu²⁺ ions located at the alkaline earth site is also affected significantly by the local structure around the site. The angular class model proposed by Vasquez (1996) was found to be in a good agreement with the energy transfer rate obtained from the decay measurement for LSSO, LCSO and LBSO phosphors (Kulshreshtha, Shin & Sohn, 2009). The site symmetry information was incorporated into the model via several parameters, such as position generating vector of an angular class, maximum number of activators in an angular class and Riemann's zeta function of dipole–dipole interaction. It was found that the higher the site symmetry of the activator sites, the higher the energy transfer rate.

Related literature

For related literature, see: Dorenbos (2000a, 2000b, 2003); Kulshreshtha, Sharma & Sohn (2009); Kulshreshtha, Shin & Sohn (2009); Rodriguez-Carvajal (1990); Sakata & Sato (1990); Vasquez (1996); Werner et al. (1985).

S2. Experimental

LBSO was synthesized by a high-temperature solid-state reaction using stoichiometric quantities of high purity oxides, namely lithium carbonate (Li₂CO₃, Sigma Aldrich, 99.99%), barium carbonate (BaCO₃, Kojundo Chemicals Ltd, 99.9%) and silicon oxide (SiO₂, Kojundo Chemical Ltd, 99.9%). A stoichiometric mixture was weighed and mixed thoroughly in an agate mortar with the addition of acetone, which was then triturated several times for homogenization. The preparation was then placed in alumina crucibles and preheated in air at 573 K for 3 h, then ground again and heated at 1123 K under a reducing atmosphere of a 25%H₂–75%N₂ stream in a horizontal tube furnace for 16 h, so that the desired oxidation state of the activator ion could be attained. After firing, the samples were cooled to room temperature in the furnace and were ground again with an agate mortar for further use. The powder pattern for the obtained powder was collected using synchrotron radiation at the 8 C2 high-resolution powder diffraction beamline of the Pohang Light Source. Small amounts of Ba₂SiO₄, Li₂SiO₃ and Li₄SiO₄ were detected as impurities.

S3. Refinement

The diffraction pattern also includes peaks originated from Ba₂SiO₄ (Pmcn, a = 5.8115 Å, b = 10.2135 Å and c =7.5035 Å), Li₂SiO₃ (Cmc2₁, a = 9.4008 Å, b = 5.4073 Å and c = 4.6529 Å) and Li₄SiO₄ (P2₁/m, a = 5.1469 Å, b = 6.1051 Å, c = 5.2960 Å and β = 90.3546°), but the peaks of Li₂BaSiO₄ and those of the impurities are well separated from one another in the high-resolution synchrotron radiation powder pattern, at least in the low and medium scattering angle region (ICSD, 2008). We could easily exclude the peaks of impurities from the peaks used for unit-cell determination. The Li₂BaSiO₄ powder diffraction pattern was indexed in the hexagonal system using TREOR (Werner et al., 1985) with merit M30 = 271.0 (F30 = 448.0). The 2θ difference between the positions of observed and calculated peaks was less than 0.002°. The space group P6₃cm (No. 185) was chosen from the systematic absences and confirmed by the subsequent structure refinement. The positions of Ba and Si atoms were determined by direct methods using the integrated intensities in the FULLPROF suite (Rodriguez-Carvajal, 1990). The positions of the O atoms were determined by the simulated annealing method in FULLPROF. The positions of the Li atoms were determined by the maximum entropy method using the ENIGMA package (Sakata & Sato, 1990). Fig. 4 shows the positions of Li atoms on the plane with z = 0.06. The Rietveld refinement was initiated by using the atomic positions obtained from the combination of the direct method, the simulated annealing method and the maximum entropy method.

Computing details

Data collection: PLS HRPD Beamline Software (reference?); cell refinement: FULLPROF (Rodriguez-Carvajal, 1990); data reduction: FULLPROF (Rodriguez-Carvajal, 1990); program(s) used to solve structure: FULLPROF(Rodriguez-Carvajal, 1990) and ENIGMA (Sakata & Sato, 1990); program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal, 1990); molecular graphics: Balls&Sticks (Ozawa & Kang, 2004); software used to prepare material for publication: FULLPROF (Rodriguez-Carvajal, 1990).

Figures

	Fig. 1. A comparison between the observed and calculated diffraction patterns and the difference curve.
	Fig. 2. A view of the asymmetric unit and some symmetry-related atoms of the title compound. [Symmetry code: (i) y, 0, z - 1; (ii) 2/3, 1/3, z - 1/2; (iii) y - x + 1, -x + 1, z; (iv) -y + 1, x - y, z; (v) x - y, -y, z; (vi) -x + 1, y - x + 1, z; (vii) -y + 1, 0, z - 1/2; (viii) -y + 1, -y, z; (ix) y + 1, 0, z; (x) 0, 0, z - 1/2.]
	Fig. 3. The structure of Li2BaSiO4. Ba atoms (green spheres in the electronic version of the paper), [SiO4] tetrahedra (yellow) and [LiO4] tetrahedra (blue) are shown.
	Fig. 4. The charge density map obtained by the maximum entropy method in the (001) plane at z = 0.06.

lithium barium silicate

Crystal data

Li2BaSiO4	F(000) = 648
Mr = 243.3	Dx = 4.023 (1) Mg m^−3
Hexagonal, P63CM	Synchrotron radiation, λ = 1.54960 Å
Hall symbol: P 6c -2	T = 298 K
a = 8.10040 (1) Å	white
c = 10.60052 (1) Å	flat sheet, 20 × 0.5 mm
V = 602.38 (1) Å^3	Specimen preparation: Prepared at 1123 K
Z = 6

Data collection

Pohang Light Source 8C2 HRPD Beamline diffractometer	Data collection mode: reflection
Radiation source: synchrotron, synchrotron	Scan method: step
Si 111 monochromator	2θmin = 10°, 2θmax = 131°, 2θstep = 0.005°
Specimen mounting: packed powder pellet

Refinement

Refinement on Inet	24201 data points
Least-squares matrix: full with fixed elements per cycle	Profile function: pseudo-Voigt
Rp = 0.090	50 parameters
Rwp = 0.121	Weighting scheme based on measured s.u.'s
Rexp = 0.072	(Δ/σ)max < 0.001
RBragg = 0.037	Background function: spline
χ^2 = 2.890	Preferred orientation correction: none

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

	x	y	z	Uiso*/Ueq
Ba	0.57143 (6)	0.00000	0.2283 (9)	1.104 (8)*
Si1	1.00000	0.00000	0.3117 (9)	0.67 (9)*
Si2	0.66666	0.33333	0.4788 (11)	0.60 (4)*
O1	0.7774 (5)	0.2260 (5)	0.0332 (9)	1.20 (9)*
O2	1.00000	0.1934 (5)	0.2648 (10)	0.86 (12)*
O3	0.66666	0.33333	0.3282 (11)	1.74 (15)*
O4	0.00000	0.00000	0.475 (2)	0.66 (13)*
Li1	1.00000	0.19438	0.07271	1.00000*
Li2	0.00000	0.75289	0.94752	1.00000*

Geometric parameters (Å, º)

Ba—O1	2.713 (10)	Li1—O1	1.989 (5)
Ba—O3	2.631 (6)	Li1—O2	2.036 (11)
Si1—O2	1.644 (6)	Li1—O4^iii	1.885 (12)
Si1—O4^i	1.73 (2)	Li2—O1^iv	1.963 (5)
Si2—O1^ii	1.635 (6)	Li2—O2^v	1.985 (10)
Si2—O3	1.596 (16)	Li2—O4^vi	2.023 (3)
O1^vii—Ba—O3	74.54 (10)

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