Observation of orientational disorder in the hexagonal stuffed tridymite Sr_0.864Eu_0.136Al₂O₄ by the maximum-entropy method

The crystal structure of a strontium europium aluminate, Sr_0.864Eu_0.136Al₂O₄, 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 R_wp = 2.99% and R_B = 4.16%. Subsequently, MEM-based pattern fitting (MPF) decreased the R factors to R_wp = 2.81% and R_B = 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 Sr_0.864Eu_0.136Al₂O₄ are orientationally disordered.

Computing details

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..

Figures

strontium europium aluminate

Crystal data

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) Å^3	cylinder, ? × ? × ? mm
Z = 2	Specimen preparation: Prepared at 1523 K
F(000) = 198

Data collection

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

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
χ^2 = 3.240	Background function: an 6th-order Legendre polynomial multiplied by a set of numerical values

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

	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

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?