Incommensurate structure of Ca₂Al₂O₅ at high temperatures – structure investigation and Raman spectroscopy

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.

Computing details

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).

(I)

Crystal data

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) Å^3	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.

Data collection

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

Refinement on F	43 parameters
R[F^2 > 2σ(F^2)] = 0.055	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
wR(F^2) = 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 (Ų)

	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 (Ų)

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