Structure of calcium aluminate decahydrate (CaAl₂O₄·10D₂O) from neutron and X-ray powder diffraction data

Calcium aluminate decahydrate is hexagonal with the space group P6₃/m and Z = 6. The compound has been named CaAl₂O₄·10H₂O (CAH₁₀) for decades and is known as the product obtained by hydration of CaAl₂O₄ (CA) in the temperature region 273–288 K – one of the main components in high-alumina cements. The lattice constants depend on the water content. Several sample preparations were used in this investigation: one CAH₁₀, three CAD₁₀ and one CA(D/H)₁₀, where the latter is a zero-matrix sample showing no coherent scattering contribution from the D/H atoms in a neutron diffraction powder pattern. The crystal structure including the positions of the H/D atoms was determined from analyses of four neutron diffraction powder patterns by means of the ab initio crystal structure determination program FOX and the FULLPROF crystal structure refinement program. Additionally, eight X-ray powder diffraction patterns (Cu Kα₁ and synchrotron X-rays) were used to establish phase purity. The analyses of these combined neutron and X-ray diffraction data clearly show that the previously published positions of the O atoms in the water molecules are in error. Thermogravimetric analysis of the CAD₁₀ sample preparation used for the neutron diffraction studies gave the composition CaAl₂(OD)₈(D₂O)₂·2.42D₂O. Neutron and X-ray powder diffraction data gave the structural formula CaAl₂(OX)₈(X₂O)₂·γX₂O (X = D, H and D/H), where the γ values are sample dependent and lie between 2.3 and 3.3.

Computing details

Program(s) used to refine structure: FULLPROF.

Figures

(C3-2K)

Crystal data

CaAl2O48.42D2O	V = 1905.11 Å^3
Mr = 326.67	Z = 6
Hexagonal, P63/M	Neutron radiation, λ = 1.885400 Å
Hall symbol: -P 6c	T = 2 K
a = 16.30365 Å	white
c = 8.27597 Å	?, ? × ? × ? mm

Data collection

HRPT at SINQ diffractometer	Scan method: step
Radiation source: neutron spallation source, HRPT at SINQ, CH-5232 Villigen PSI, Switzerland	2θmin = 4.057°, 2θmax = 164.857°, 2θstep = 0.100°
Ge hkk monochromator

Refinement

Rp = 2.264	1609 data points
Rwp = 2.985	68 parameters
Rexp = 0.641	30 restraints
RBragg = 8.392	(Δ/σ)max = 0.05
χ^2 = 18.900

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Al	0.5877 (8)	0.5038 (11)	0.0712 (13)	0.01267*
Ca	0.5154 (13)	0.2625 (17)	0.2500	0.01267*
O1	0.6674 (14)	0.564 (2)	0.2500	0.01267*
D1	0.6990 (12)	0.6373 (18)	0.2500	0.02533*
O2	0.5203 (10)	0.5728 (13)	0.0833 (18)	0.01267*
D2	0.5736 (9)	0.6462 (12)	0.0917 (19)	0.02533*
O3	0.5179 (11)	0.4217 (15)	0.2500	0.01267*
D3	0.4560 (11)	0.4293 (16)	0.2500	0.02533*
O4	0.6858 (9)	0.5773 (14)	−0.0698 (14)	0.01267*
D4	0.6623 (10)	0.5853 (15)	−0.1869 (13)	0.02533*
O5	0.6184 (10)	0.4086 (13)	0.0616 (15)	0.01267*
D5	0.6442 (12)	0.3979 (14)	−0.0378 (15)	0.02533*
O9	0.3948 (8)	0.2490 (11)	0.0396 (16)	0.01267*
D91	0.3160 (8)	0.2121 (13)	0.0783 (19)	0.02533*
D92	0.4152 (10)	0.2012 (13)	0.005 (2)	0.02533*
O6	0.1890 (13)	0.3445 (13)	0.2500	0.01267*	0.422 (14)
D61	0.1106 (12)	0.3064 (18)	0.2500	0.02533*	0.422 (14)
D62	0.215 (2)	0.4226 (10)	0.2500	0.02533*	0.422 (14)
O7	−0.004 (2)	0.8967 (19)	0.091 (3)	0.01267*	0.461 (5)
D71	0.002 (2)	0.9279 (18)	0.213 (2)	0.02533*	0.461 (5)
D72	0.023 (2)	0.8483 (18)	0.086 (3)	0.02533*	0.461 (5)
O8	0.2310 (12)	0.2308 (16)	−0.040 (2)	0.01267*	0.539 (5)
D81	0.2499 (16)	0.215 (2)	0.071 (3)	0.02533*	0.539 (5)
D82	0.1551 (12)	0.1899 (18)	−0.068 (3)	0.02533*	0.539 (5)

Bond lengths (Å)

Al—Ca

3.796