Washing soda (natron), Na2CO3 · 10H2O, revised: Crystal structures at low and ambient temperatures
- Author(s)
- Eugen Libowitzky
- Abstract
Crystals of natron, Na2CO3 ž 10H2O, 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 R1 = 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°, b = 8.816(1)/8.955(1)/9.001(2) A°, c = 12.571(1)/12.593(1 /12.590(2) A°, ß = 115.97(1)/115.89(1)/ 115.83(1)° at 110/270/295 K. The structure is composed of pairs of edge-sharing Na(H2O)6 octahedra forming Na2(H2O)10 units, and CO3 molecules. These units are connected by hydrogen bonds (dO...O>2.73 A°) in a NaCl-like arrangement. The structure is pseudo-centric, only the carbonate groups and four H atoms violating a centric symmetry. The CO3 molecules are perfectly ordered at 110 K and show almost isotropic ADPs of the carbonate O atoms. The CO3 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 CO3 position is occupied to only ~10%. A definite phase transition from ordered to partially disordered CO3 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.
- Organisation(s)
- Department of Mineralogy and Crystallography
- Journal
- Mineralogy and Petrology
- Volume
- 77
- Pages
- 177-195
- No. of pages
- 19
- ISSN
- 0930-0708
- DOI
- https://doi.org/10.1007/s00710-002-0215-2
- Publication date
- 2003
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/90996c3d-4962-49e3-a16b-36440a330993