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A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca2Al2O5 transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca2Al2O5 were synthesized in an end-loaded piston cylinder press at 2.5 GPa and 1273 K. The diffraction pattern observed at 1090 (10) K by in situ single-crystal diffraction experiments can be indexed by an I-centred orthorhombic cell and a modulation wavevector of q = 0.595 (1)c*. A (3 + 1)-dimensional model in superspace group Imma(00γ)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO6] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO4] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca2Al2O5 is isotypic to the modulated high-temperature modification of Ca2Fe2O5. A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075 (10) K; on cooling, satellite reflections can be observed down to 975 (10) K. The characterization of Ca2Al2O5 is completed by Raman spectroscopy, including a partial interpretation of the spectra.
Supporting information
Data collection: STOE X-AREA; cell refinement: STOE X-AREA; data reduction: STOE X-RED; program(s) used to refine structure: Jana2000 (Petricek, Dusek & Palatinus, 2000); software used to prepare material for publication: Jana2000 (Petricek, Dusek & Palatinus, 2000).
Crystal data top
Ca2Al2O5 | F(000) = 424 |
Mr = 214.12 | Dx = 3.388 Mg m−3 |
Orthorhombic, Imma(00γ)s00† | Mo Kα radiation, λ = 0.71073 Å |
q = 0.59520c* | Cell parameters from 3428 reflections |
a = 5.2699 (5) Å | θ = 2.8–30.8° |
b = 14.6984 (15) Å | µ = 3.04 mm−1 |
c = 5.4196 (5) Å | T = 1090 K |
V = 419.80 (7) Å3 | Fragment, colourless |
Z = 4 | 0.14 × 0.12 × 0.1 mm |
† Symmetry operations: (1) x1, x2, x3, x4; (2) −x1, x2, x3, 1/2+x4; (3) x1, 1/2−x2, x3, x4; (4) −x1, 1/2−x2, x3, 1/2+x4; (5) −x1, −x2, −x3, −x4; (6) x1, −x2, −x3, 1/2−x4; (7) −x1, 1/2+x2, −x3, −x4; (8) x1, 1/2+x2, −x3, 1/2−x4; (9) 1/2+x1, 1/2+x2, 1/2+x3, x4; (10) 1/2−x1, 1/2+x2, 1/2+x3, 1/2+x4; (11) 1/2+x1, −x2, 1/2+x3, x4; (12) 1/2−x1, −x2, 1/2+x3, 1/2+x4; (13) 1/2−x1, 1/2−x2, 1/2−x3, −x4; (14) 1/2+x1, 1/2−x2, 1/2−x3, 1/2−x4; (15) 1/2−x1, x2, 1/2−x3, −x4; (16) 1/2+x1, x2, 1/2−x3, 1/2−x4.
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Data collection top
STOE IPDS 2 diffractometer | 848 independent reflections |
Plane graphite monochromator | 680 reflections with I > 3σ(I) |
Detector resolution: 6.67 pixels mm-1 | Rint = 0.053 |
rotation method scans | θmax = 29.1°, θmin = 1.8° |
Absorption correction: integration | h = −7→7 |
Tmin = 0.794, Tmax = 0.834 | k = −20→20 |
5551 measured reflections | l = −7→7 |
Refinement top
Refinement on F | 43 parameters |
R[F2 > 2σ(F2)] = 0.055 | Weighting scheme based on measured s.u.'s w = 1/(σ2(F) + 0.0001F2) |
wR(F2) = 0.068 | (Δ/σ)max = 0.0001 |
S = 3.52 | Δρmax = 1.16 e Å−3 |
848 reflections | Δρmin = −1.04 e Å−3 |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
Al1 | 0 | 0 | 0.5 | 0.0267 (4) | |
Al2 | 0.9527 (3) | 0.25 | 0.5744 (3) | 0.0224 (4) | 0.5 |
Ca1 | 0 | 0.10837 (5) | 0.02219 (13) | 0.0276 (2) | |
O1 | 0.75 | 0.00922 (14) | 0.75 | 0.0224 (6) | |
O2 | 0 | 0.14308 (17) | 0.4457 (6) | 0.0325 (8) | |
O3 | 0.3830 (6) | 0.25 | 0.6372 (8) | 0.0274 (11) | 0.5 |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Al1 | 0.0159 (6) | 0.0471 (9) | 0.0173 (7) | 0 | 0 | −0.0008 (5) |
Al2 | 0.0278 (7) | 0.0191 (7) | 0.0204 (7) | 0 | 0.0015 (5) | 0 |
Ca1 | 0.0325 (4) | 0.0230 (4) | 0.0273 (4) | 0 | 0 | 0.0010 (2) |
O1 | 0.0185 (9) | 0.0286 (10) | 0.0201 (11) | 0 | 0.0031 (7) | 0 |
O2 | 0.0379 (14) | 0.0262 (13) | 0.0333 (14) | 0 | 0 | −0.0081 (10) |
O3 | 0.0252 (16) | 0.0299 (19) | 0.027 (2) | 0 | −0.0045 (13) | 0 |
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