Washing soda (natron), Na2CO3[sdot]10H2O, revised: crystal structures at low and ambient temperatures

• Published in: Mineralogy and petrology 2003-03, Vol.77 (3-4), p.177
• Main Authors: Libowitzky, E, Giester, G
• Format: Article
• Language: English
• Subjects: Ambient temperature; Calorimetry; Crystal lattices; Crystal structure; Crystals; Evaporation; Hydrogen; Low temperature; Mineralogy; Sodium; Sodium chloride; Spectrum analysis; Temperature
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-002-0215-2

Crystals of natron, Na 2 CO 3 [sdot]10H 2 O, were grown by slow evaporation of a saturated sodium carbonate solution at room temperature. A single-crystal fragment was measured on a CCD X-ray four-circle diffractometer at 110, 270 and 295 K. Whereas the crystal structure of natron at room temperature had first been published in 1969 with R = 18%, in the present investigation the non-centrosymmetric structure was solved by direct methods and refined subsequently to R 1 = 1.9% (110 K), 2.5% (270 K), and 3.5% (295 K). In contrast to the previous structure description, even the hydrogen atoms were located and refined with isotropic displacement parameters. Space group is Cc , lattice parameters are a = 12.740(1)/12.763(1)/12.750(2) A degrees b = 8.816(1)/8.955(1)/9.001(2) A degrees c = 12.571(1)/12.593(1)/12.590(2) A degrees beta = 115.97(1)/115.89(1)/115.83(1) degrees at 110/270/295 K. The structure is composed of pairs of edge-sharing Na(H 2 O) 6 octahedra forming Na 2 (H 2 O) 10 units, and CO 3 molecules. These units are connected by hydrogen bonds ( d O...O > 2.73 A ring;) in a NaCl-like arrangement. The structure is pseudo-centric, only the carbonate groups and four H atoms violating a centric symmetry. The CO 3 molecules are perfectly ordered at 110 K and show almost isotropic ADPs of the carbonate O atoms. The CO 3 position is constrained by hydrogen bonds, involving at least three H bonds per acceptor oxygen atom. At 270/295 K the carbonate groups still hold the former low-temperature position to 60/46%, whereas 40/54% are disordered into at least two different positions with large and strongly smeared anisotropic displacement parameters of the carbonate O atoms. However, a centrosymmetric equivalent of the low-temperature CO 3 position is occupied to only 10%. A definite phase transition from ordered to partially disordered CO 3 groups was neither observed in differential scanning calorimetry, nor in temperature-dependent birefringence, IR and Raman spectroscopic measurements, as the process takes place over a wide temperature interval and without any change of the space group symmetry Cc. [PUBLICATION ABSTRACT]