A new strontium borophosphate, Sr₆BP₅O₂₀, from synchrotron powder data

Strontium borophosphate, Sr₆BP₅O₂₀, was prepared by a solution synthesis method. The crystal structure was solved ab initio from synchrotron powder data without preliminary knowledge of the chemical formula. The compound crystallizes in space group Ic2. Sr atoms occupy sites coordinated by eight or nine O atoms, and the anionic layer consists of BO₄ and PO₄ tetra­hedra. The eightfold-coordinated Sr atom lies at a site with twofold symmetry, while one P atom and the B atom are located on special positions of site symmetry .

S1. Comment

Borophosphates constitute a poorly studied uncommon class of compounds, whose anions contain both boron–oxygen tetrahedra and phosphorus–oxygen tetrahedra forming in various combinations. In the present study, the structure of the title compound was determined and refined from synchrotron powder diffraction data, as shown in Fig. 1.

The anion of Sr₆BP₅O₂₀ (SBP) is similar to that of Pb₆BP₅O₂₀ (PBP), represented by two groups, viz. the orthophosphate PO₄³⁻ anion of symmetry −4 and the [B(PO₄)₄]⁹⁻ anion. This propeller-like structure consists of a central [BO₄] tetrahedron of symmetry −4, surrounded by an array of four [PO₄] tetrahedra, as shown in Fig. 2 (Belokoneva et al., 2001). Atom Sr1 is coordinated by eight O atoms at distances of 2.425 (4)–2.990 (4) Å, and atom Sr2 is coordinated by nine O atoms at 2.418 (4)–3.169 (4) Å. The Sr—O distances of SBP are more regular than those of PBP, which range from 2.04 (6) to 3.35 (4) Å. The B—O and P—O bond lengths are within usual ranges.

The unit cell of SBP can be obtained by multiplication of that of PBP [a_SBP = 2^1/2a_PBP and c_SBP = 2c_PBP]. However, intense odd index reflections, such as (211), (213), (215) and (321), indicate that the unit cell of SBP cannot be reduced to that of PBP. Fig. 3 shows the differences between the structures of SBP and PBP. Figs. 3(a) and 3(b) are the ab projections of SBP and PBP, respectively. Fig. 3(c) shows (100) and (110) projections, in part, of SBP and PBP, respectively. According to the symmetry of the I-4c2 space group of SBP, there is an inversion symmetry −4 at the center of the ab plane, and there are also b-glide planes (a = 1/4 and 3/4) and a-glide planes (b = 1/4 and 3/4) as a result of the centering translation and c glide. In Fig. 3(a), we can see that two out of four [B(PO₄)₄]⁹⁻ anions are mirrored from the others by the glide planes; this configuration is in sharp contrast to the structure of PBP with only P-4 symmetry (Fig. 3b). Belokoneva et al. (2001) assumed that general position 4h for atom Pb2 in PBP (Z = 1) is randomly occupied (50%). In the present study, however, all the atom sites are fully occupied.

SBP is a new candidate host material for Eu²⁺ activator, an alternative to BaMgAl₁₀O₁₇ (BAM), which has been used as a blue component in plasma display panels (PDPs). SBP has superior thermal stability, and the excellent luminescent intensity of Eu²⁺-doped SBP would make it possible to apply this compound to PDPs in place of BAM. Unlike the BAM structure, involving open layers on which Eu²⁺ ions can be located, SBP provides tight Eu²⁺ sites. Divalent Eu atoms occupy Sr²⁺ sites in SBP and mostly Ba²⁺ sites in BAM. Two possible Sr²⁺ (or Eu²⁺) sites are enclosed by eight and nine O atoms at average distances of 2.6729 (4) and 2.7151 (4) Å, respectively. The thermal degradation in BAM is associated with the loose local structure around the Ba²⁺ (or Eu²⁺) site (nine O atoms at an average distance of 2.9512 (5) Å from the Ba²⁺ center). Eu²⁺ ions can easily escape to other trivalent sites or open spaces by passing through the loosely surrounded O atoms in the case of BAM. On the other hand, both the crystallographic sites for Sr²⁺ ions in SBP make a relatively tight obstacle against the escape of Eu²⁺ ions. This could be the origin of the excellent thermal stability of SBP.

S2. Experimental

The title compound was produced by a solution-based synthesis method. The raw powders strontium nitrate [Sr(NO₃)₂], boric acid (H₃BO₃) and diammonium hydrogen phosphate [(NH₄)₂HPO₄] were dissolved in deionized water, and then a stoichiometric amount of each solution was collected in a quartz container. The solution in the container was stirred and then dried at 373 K for 48 h, followed by further drying at 873 K for 6 h. The dried samples were pulverized and successively fired at 1473 K in a reducing atmosphere. White crystalline powder resulted. A small amount of Sr₃(PO₄)₂ was detected as an impurity.

S3. Refinement

The diffraction pattern also includes peaks from Sr₃(PO₄)₂ (R-3m, a = 5.3889 Å and c = 19.7909 Å) but the peaks of SBP and Sr₃(PO₄)₂ are well separated from each other in the high-resolution synchrotron powder pattern, at least in the low and medium scattering-angle region. We could easily exclude the peaks of Sr₃(PO₄)₂ from the rest of the peaks used for unit-cell determination. The SBP powder diffraction pattern was indexed in the tetragonal system using the TREOR program (Werner et al., 1985) with merit M(30) = 145.0 (F30 = 277.0) and checked with the DICVOL program (Boultif & Louer, 1991) with merit M(30) = 154.3 (F30 = 308.3). The 2θ difference between the positions of observed and calculated peaks was less than 0.002°. Space group I-4c2 (No. 120) was chosen from the systematic absences and confirmed by the subsequent structure refinement. The position of Sr and P atoms were determined by direct methods using the integrated intensities in the Fullprof suite (Rodriguez-Carvajal, 1990). The positions of the O atoms and B atom were determined by the simulated annealing method with Fullprof2k. The Rietveld refinement was initiated by using the atomic positions obtained from the simulated annealing method.

Computing details

Data collection: PLS HRPD Beamline Software (reference?); cell refinement: FULLPROF2k (Rodriguez-Carvajal, 1990); data reduction: FULLPROF2k; program(s) used to solve structure: FULLPROF2k; program(s) used to refine structure: FULLPROF2k; molecular graphics: STRUPLO (reference?); software used to prepare material for publication: FULLPROF2k.

Figures

	Fig. 1. Comparison between the observed and calculated patterns and the difference curve for the Rietveld refinement.
	Fig. 2. The structure of Sr6BP5O20. Sr atoms (spheres), and [PO4] tetrahedra and [BO4] tetrahedra (dotted), are shown.
	Fig. 3. The crystal structures of Sr6BP5O20 and Pb6BP5O20: ab projection of (a) Sr6BP5O20 and (b) Pb6BP5O20, and (c) selective projection to (100) and (110) of Sr6BP5O20 and Pb6BP5O20, respectively. Sr and Pb atoms are shown as spheres.

Strontium borophosphate

Crystal data

Sr6BP5O20	F(000) = 1872
Mr = 1011.43	Dx = 3.691 (1) Mg m^−3
Tetragonal, I4c2	Synchrotron radiation, λ = 1.54520 Å
Hall symbol: I -4 -2c	T = 298 K
a = 9.78393 (2) Å	white
c = 19.01318 (3) Å	flat sheet, 20 × 20 mm
V = 1820.04 (1) Å^3	Specimen preparation: Prepared at 1473 K
Z = 4

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 = 132°, 2θstep = 0.01°
Specimen mounting: packed powder pellet

Refinement

Refinement on Inet	12201 data points
Least-squares matrix: full with fixed elements per cycle	Profile function: pseudo-Voigt
Rp = 0.063	50 parameters
Rwp = 0.084	Weighting scheme based on measured s.u.'s
Rexp = 0.038	(Δ/σ)max < 0.001
RBragg = 0.037	Background function: spline
χ^2 = 4.884	Preferred orientation correction: none

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

	x	y	z	Uiso*/Ueq
Sr1	0.22151 (6)	0.27849 (6)	0.00000	0.0117 (2)*
Sr2	0.49664 (6)	0.30031 (7)	0.33472 (3)	0.0112 (1)*
P1	0.00000	0.00000	0.00000	0.0079 (8)*
P2	0.2052 (2)	0.4776 (2)	0.34889 (7)	0.0087 (4)*
O1	0.3410 (3)	0.5053 (4)	0.3059 (2)	0.0097 (11)*
O2	0.1318 (5)	0.0323 (4)	0.0444 (2)	0.0213 (13)*
O3	0.1478 (4)	0.6010 (4)	0.3858 (2)	0.0152 (13)*
O4	0.2268 (4)	0.3544 (4)	0.3956 (2)	0.0178 (12)*
O5	0.1011 (4)	0.4228 (4)	0.2912 (2)	0.0071 (11)*
B	0.00000	0.50000	0.25000	0.014 (4)*

Geometric parameters (Å, º)

Sr1—O2	2.699 (4)	Sr2—O4^vii	2.950 (4)
Sr1—O2^i	2.990 (4)	Sr2—O5^iii	3.171 (4)
Sr1—O3^ii	2.574 (4)	Sr2—O5^vii	2.549 (4)
Sr1—O4^iii	2.426 (4)	P1—O2	1.574 (4)
Sr2—O1	2.577 (4)	P2—O1	1.583 (4)
Sr2—O1^iv	2.538 (4)	P2—O3	1.505 (4)
Sr2—O1^iii	2.705 (3)	P2—O4	1.512 (4)
Sr2—O2^v	2.417 (4)	P2—O5	1.589 (4)
Sr2—O3^vi	2.597 (4)	B—O5	1.471 (4)
Sr2—O4	2.932 (4)
P1—Sr1—P1	166.99 (13)

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