Strontium borophosphate, Sr
6BP
5O
20, 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 BO
4 and PO
4 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
.
Supporting information
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.
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.
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.
Strontium borophosphate
top
Crystal data top
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) Å3 | Specimen preparation: Prepared at 1473 K |
Z = 4 | |
Data collection top
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 top
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 |
χ2 = 4.884 | Preferred orientation correction: none |
Crystal data top
Sr6BP5O20 | V = 1820.04 (1) Å3 |
Mr = 1011.43 | Z = 4 |
Tetragonal, I4c2 | Synchrotron radiation, λ = 1.54520 Å |
a = 9.78393 (2) Å | T = 298 K |
c = 19.01318 (3) Å | flat sheet, 20 × 20 mm |
Data collection top
Pohang Light Source 8C2 HRPD Beamline diffractometer | Scan method: step |
Specimen mounting: packed powder pellet | 2θmin = 10°, 2θmax = 132°, 2θstep = 0.01° |
Data collection mode: reflection | |
Refinement top
Rp = 0.063 | χ2 = 4.884 |
Rwp = 0.084 | 12201 data points |
Rexp = 0.038 | 50 parameters |
RBragg = 0.037 | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | 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) top
Sr1—O2 | 2.699 (4) | Sr2—O4vii | 2.950 (4) |
Sr1—O2i | 2.990 (4) | Sr2—O5iii | 3.171 (4) |
Sr1—O3ii | 2.574 (4) | Sr2—O5vii | 2.549 (4) |
Sr1—O4iii | 2.426 (4) | P1—O2 | 1.574 (4) |
Sr2—O1 | 2.577 (4) | P2—O1 | 1.583 (4) |
Sr2—O1iv | 2.538 (4) | P2—O3 | 1.505 (4) |
Sr2—O1iii | 2.705 (3) | P2—O4 | 1.512 (4) |
Sr2—O2v | 2.417 (4) | P2—O5 | 1.589 (4) |
Sr2—O3vi | 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. |
Experimental details
Crystal data |
Chemical formula | Sr6BP5O20 |
Mr | 1011.43 |
Crystal system, space group | Tetragonal, I4c2 |
Temperature (K) | 298 |
a, c (Å) | 9.78393 (2), 19.01318 (3) |
V (Å3) | 1820.04 (1) |
Z | 4 |
Radiation type | Synchrotron, λ = 1.54520 Å |
Specimen shape, size (mm) | Flat sheet, 20 × 20 |
|
Data collection |
Diffractometer | Pohang Light Source 8C2 HRPD Beamline diffractometer |
Specimen mounting | Packed powder pellet |
Data collection mode | Reflection |
Scan method | Step |
2θ values (°) | 2θmin = 10 2θmax = 132 2θstep = 0.01 |
|
Refinement |
R factors and goodness of fit | Rp = 0.063, Rwp = 0.084, Rexp = 0.038, RBragg = 0.037, χ2 = 4.884 |
No. of data points | 12201 |
No. of parameters | 50 |
No. of restraints | ? |
Selected bond lengths (Å) topSr1—O2 | 2.699 (4) | Sr2—O4vii | 2.950 (4) |
Sr1—O2i | 2.990 (4) | Sr2—O5iii | 3.171 (4) |
Sr1—O3ii | 2.574 (4) | Sr2—O5vii | 2.549 (4) |
Sr1—O4iii | 2.426 (4) | P1—O2 | 1.574 (4) |
Sr2—O1 | 2.577 (4) | P2—O1 | 1.583 (4) |
Sr2—O1iv | 2.538 (4) | P2—O3 | 1.505 (4) |
Sr2—O1iii | 2.705 (3) | P2—O4 | 1.512 (4) |
Sr2—O2v | 2.417 (4) | P2—O5 | 1.589 (4) |
Sr2—O3vi | 2.597 (4) | B—O5 | 1.471 (4) |
Sr2—O4 | 2.932 (4) | | |
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. |
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 [aSBP = 21/2aPBP and cSBP = 2cPBP]. 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.