Structure and thermal expansion of sulfuric acid octahydrate

Synchrotron X-ray powder diffraction has been used to structurally characterize crystallization products from 37.8 and 40.5 wt% aqueous sulfuric acid solutions. It is confirmed that, despite speculation in the literature, the structure that predominately crystallized from these solutions is sulfuric acid octahydrate (SAO). The existence of an uncharacterized phase is also noted. It was found that existing models proposed for the crystal structure of SAO did not satisfactorily fit to the data acquired here, and hence a new structure solution was sought. It is reported here that the structure of SAO is contained within a unit cell with I2 symmetry with a = 7.44247 (11), b = 7.4450 (1), c = 26.1168 (3) Å, β = 125.0428 (7)°, V = 1184.78 (3) ų at 80 K. Data were collected at temperatures between 80 and 198 K, which enabled determination of the thermal expansion of SAO.

Computing details

Cell refinement: Topas4.1; program(s) used to solve structure: FOX1.19; program(s) used to refine structure: Topas4.1.

(I)

Crystal data

H18O12S	γ = 90°
Mr = 242.07	V = 1184.78 (3) Å^3
I2	Z = 4
a = 7.44247 (11) Å	Melting point: 210 K
b = 7.4450 (1) Å	Synchrotron radiation, λ = 0.989536 Å
c = 26.1168 (3) Å	T = 80 K
α = 90°	clear
β = 125.0428 (7)°

Data collection

Australian Synchrotron diffractometer	Data collection mode: transmission
Si(111) monochromator	Scan method: continuous
Specimen mounting: borosilicate capiliary	2θmin = 2.188°, 2θmax = 82.168°, 2θstep = 0.002°

Refinement

Rwp = 9.811	Profile function: TCHZ
Rexp = 1.085	40 parameters
RBragg = 7.462	7 constraints
20380 data points	H-atom parameters not defined?

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

	x	y	z	Uiso*/Ueq
S1	0.99437	0.16332	0.87662	1.0018
O11	0.76869	0.07669	0.83251	1.0018
O12	1.17008	0.01700	0.90767	1.0018
O13	1.06049	0.26060	0.83822	1.0018
O14	0.97483	0.32196	0.91037	1.0018
O1	0.1119 (4)	0.613 (11)	0.8538 (2)	1.0018
O2	0.1840 (4)	0.726 (11)	0.1421 (2)	1.0018
O3	0.2108 (11)	0.850 (14)	0.0085 (3)	1.0018
O4	0.3424 (9)	0.479 (13)	0.0092 (3)	1.0018
O5	0.0907 (9)	0.814 (13)	0.3974 (3)	1.0018
O6	0.9831 (11)	0.505 (13)	0.3961 (3)	1.0018
O7	0.074 (1)	0.108 (13)	0.7447 (3)	1.0018
O8	0.3807 (9)	0.239 (13)	0.7433 (3)	1.0018

Atomic displacement parameters (Ų)

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