research papers
The crystal structure of a strontium europium aluminate, Sr0.864Eu0.136Al2O4, with a novel hexagonal form was investigated by a combination of Rietveld analysis and the maximum-entropy method (MEM) with synchrotron X-ray powder diffraction data. The electron density image calculated by the MEM/Rietveld method revealed that the apical oxygen ion in the AlO4 tetrahedron has a broad distribution corresponding to an extraordinarily large atomic displacement parameter. This structure could be expressed by a split-atom model, with which the Rietveld refinement gave Rwp = 2.99% and RB = 4.16%. Subsequently, MEM-based pattern fitting (MPF) decreased the R factors to Rwp = 2.81% and RB = 2.34% and the electron density image clearly showed that the apical oxygen ions of the AlO4 tetrahedra are split over three sites around a threefold axis involving an elongated distribution of the residual O ions along the c axis. These results suggest that AlO4 tetrahedra in Sr0.864Eu0.136Al2O4 are orientationally disordered.
Keywords: strontium europium aluminate; Rietveld refinement; maximum entropy method; orientational disorder; electron density distribution; charge density distribution.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S002188980401430X/pd5014sup1.cif |
Computing details top
Cell refinement: PowderX (Dong, 1999) developed by C. Dong; program(s) used to refine structure: RIETAN-2000 (Izumi et al., 2000) developed by F. Izumi; molecular graphics: VENUS (Izumi & Dilanian, 2002) developed by F. Izumi et al..
strontium europium aluminate top
Crystal data top
Al2Eu0.136O4Sr0.864 | Dx = 3.73 Mg m−3 |
Mr = 214.33 | ? radiation, λ = 0.77571 Å |
Hexagonal, P6322 | µ = 4.8 mm−1 |
Hall symbol: P6c 2c | T = 297 K |
a = 5.11635 (10) Å | Particle morphology: spherical particle |
c = 8.3673 (2) Å | white |
V = 189.69 (1) Å3 | cylinder, ? × ? × ? mm |
Z = 2 | Specimen preparation: Prepared at 1523 K |
F(000) = 198 |
Data collection top
SPring-8 BL02B2 diffractometer | Data collection mode: transmission |
Radiation source: synchrotron radiation, synchrotoron radiation | Scan method: step |
Si(111) double-crystal monochrometor | 2θmin = 2.50°, 2θmax = 70.00°, 2θstep = 0.01° |
Specimen mounting: Sample was installed into Lindeman glass capillary. |
Refinement top
Refinement on Inet | 7700 data points |
Rp = 0.020 | Profile function: split psudo-Voigt of Toraya |
Rwp = 0.030 | 28 parameters |
Rexp = 0.017 | 1 constraint |
RBragg = 0.041 | |
χ2 = 3.240 | Background function: an 6th-order Legendre polynomial multiplied by a set of numerical values |
Crystal data top
Al2Eu0.136O4Sr0.864 | Z = 2 |
Mr = 214.33 | ? radiation, λ = 0.77571 Å |
Hexagonal, P6322 | µ = 4.8 mm−1 |
a = 5.11635 (10) Å | T = 297 K |
c = 8.3673 (2) Å | cylinder, ? × ? × ? mm |
V = 189.69 (1) Å3 |
Data collection top
SPring-8 BL02B2 diffractometer | Scan method: step |
Specimen mounting: Sample was installed into Lindeman glass capillary. | 2θmin = 2.50°, 2θmax = 70.00°, 2θstep = 0.01° |
Data collection mode: transmission |
Refinement top
Rp = 0.020 | χ2 = 3.240 |
Rwp = 0.030 | 7700 data points |
Rexp = 0.017 | 28 parameters |
RBragg = 0.041 |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
x | y | z | Biso*/Beq | Occ. (<1) | |
Sr | 0 | 0 | 0.25 | 2.06 (2)* | 0.864 (3) |
Eu | 0 | 0 | 0.25 | 2.06* | 0.136 |
Al | 0.33333 | 0.66667 | 0.94731 | 1.49 (3)* | |
O1 | 0.4074 (6) | 0.81477 | 0.75 | 5.8 (2)* | 0.333 |
O2 | 0.36026 (6) | 0.0012 (9) | 0.0435 (4) | 2.12 (7)* | 0.500 |
Experimental details
Crystal data | |
Chemical formula | Al2Eu0.136O4Sr0.864 |
Mr | 214.33 |
Crystal system, space group | Hexagonal, P6322 |
Temperature (K) | 297 |
a, c (Å) | 5.11635 (10), 8.3673 (2) |
V (Å3) | 189.69 (1) |
Z | 2 |
Radiation type | ?, λ = 0.77571 Å |
µ (mm−1) | 4.8 |
Specimen shape, size (mm) | Cylinder, ? × ? × ? |
Data collection | |
Data collection method | SPring-8 BL02B2 |
Specimen mounting | Sample was installed into Lindeman glass capillary. |
Data collection mode | Transmission |
Scan method | Step |
2θ values (°) | 2θmin = 2.50 2θmax = 70.00 2θstep = 0.01 |
Refinement | |
R factors and goodness of fit | Rp = 0.020, Rwp = 0.030, Rexp = 0.017, RBragg = 0.041, χ2 = 3.240 |
No. of data points | 7700 |
No. of parameters | 28 |
No. of restraints | ? |
Computer programs: PowderX (Dong, 1999) developed by C. Dong, RIETAN-2000 (Izumi et al., 2000) developed by F. Izumi, VENUS (Izumi & Dilanian, 2002) developed by F. Izumi et al..