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The synthesis and structure of Ultramarine Pigments

The synthesis and structure of Ultramarine Pigments
The synthesis and structure of Ultramarine Pigments

The crystal structures of the aluminosilicate pigments ultramarine blue Na6.8[AlSiO4]6(S3-)0.8, ultramarine green Na6.7[AlSiO4]6(S2-)0.7 and their potassium exchanged derivatives have been investigated using a range of techniques including sulphur K-edge EXAFS, TOF PND, PXD and UV-visible spectroscopy. Structural characterisation shows that the space group is I4̅;3m (217) and ultramarine blue was found to possess two sodium sites, one corresponding to β-cages unoccupied by chromophore molecules, the other representing the related site in empty cages. The radical anion S3- was statically disordered and was located by Fourier mapping techniques on low temperature PND data. The polysulphide species had two S-S distances of 2.078(1) and 3.354(1) Å which were modelled by partial occupancy of two positions close to the centre of the b-cage. The S-S-S bond angle was calculated to be 109.4(3)°. Similar PND experiments located the S2- radical anion in the structure of ultramarine green which has an S-S distance of 2.050(2) Å. This was modelled by partial occupancy of an 8-fold position near the centre of the sodalite cage. Ion exchange materials were found to host the large potassium ion in empty β-cages due to steric limitations in the system.

The structure of ultramarine pink Na6[AlSiO4]6(S4)0.6 has been studied using Sulphur K- edge EXAFS, TOF PND and PXD. The material was refined in I4̅;3m (217) a=0.0972(1) and was found to possess one sodium site at x = -0.2196(1) the positions of the polysulphide groups responsible for the pink coloration were modelled by partial occupancy of a general position (0.1546, 0.1085, 0) this describes molecules which are of D2h symmetry which are statistically disordered throughout the lattice. The structure of ultramarine violet was also refined using PND techniques the structure was closely related to that of ultramarine pink I 4̅;3m (217) a = 9.0923(1) but with the addition of a second sodium position due to the presence of a small proportion of S3- molecules.

The synthesis of ultramarine blue was investigated using a variety of techniques including PXD, Colour Measurement, UV-visible and Infrared spectroscopy. Hot mixing of materials prior to fumacing was found to increase material density and reduce brick strength. The addition of potassium carbonate to formulations produced 'red-shaded' materials whereas the addition of further sodium carbonate produced green shaded materials.

The synthesis of ultramarine violet was investigated using TGA, UV-visible spectroscopy and PXD. Sodium was found to be extracted from the parent material and NaCl formed as a side product. The optimum proportion of NH4CI was found to be 6%, excess reagent caused framework degradation. Ultramarine Violet could not be prepared using nonchlorine containing reagents.

University of Southampton
Booth, Douglas Geoffrey
Booth, Douglas Geoffrey

Booth, Douglas Geoffrey (2002) The synthesis and structure of Ultramarine Pigments. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The crystal structures of the aluminosilicate pigments ultramarine blue Na6.8[AlSiO4]6(S3-)0.8, ultramarine green Na6.7[AlSiO4]6(S2-)0.7 and their potassium exchanged derivatives have been investigated using a range of techniques including sulphur K-edge EXAFS, TOF PND, PXD and UV-visible spectroscopy. Structural characterisation shows that the space group is I4̅;3m (217) and ultramarine blue was found to possess two sodium sites, one corresponding to β-cages unoccupied by chromophore molecules, the other representing the related site in empty cages. The radical anion S3- was statically disordered and was located by Fourier mapping techniques on low temperature PND data. The polysulphide species had two S-S distances of 2.078(1) and 3.354(1) Å which were modelled by partial occupancy of two positions close to the centre of the b-cage. The S-S-S bond angle was calculated to be 109.4(3)°. Similar PND experiments located the S2- radical anion in the structure of ultramarine green which has an S-S distance of 2.050(2) Å. This was modelled by partial occupancy of an 8-fold position near the centre of the sodalite cage. Ion exchange materials were found to host the large potassium ion in empty β-cages due to steric limitations in the system.

The structure of ultramarine pink Na6[AlSiO4]6(S4)0.6 has been studied using Sulphur K- edge EXAFS, TOF PND and PXD. The material was refined in I4̅;3m (217) a=0.0972(1) and was found to possess one sodium site at x = -0.2196(1) the positions of the polysulphide groups responsible for the pink coloration were modelled by partial occupancy of a general position (0.1546, 0.1085, 0) this describes molecules which are of D2h symmetry which are statistically disordered throughout the lattice. The structure of ultramarine violet was also refined using PND techniques the structure was closely related to that of ultramarine pink I 4̅;3m (217) a = 9.0923(1) but with the addition of a second sodium position due to the presence of a small proportion of S3- molecules.

The synthesis of ultramarine blue was investigated using a variety of techniques including PXD, Colour Measurement, UV-visible and Infrared spectroscopy. Hot mixing of materials prior to fumacing was found to increase material density and reduce brick strength. The addition of potassium carbonate to formulations produced 'red-shaded' materials whereas the addition of further sodium carbonate produced green shaded materials.

The synthesis of ultramarine violet was investigated using TGA, UV-visible spectroscopy and PXD. Sodium was found to be extracted from the parent material and NaCl formed as a side product. The optimum proportion of NH4CI was found to be 6%, excess reagent caused framework degradation. Ultramarine Violet could not be prepared using nonchlorine containing reagents.

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Published date: 2002

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Local EPrints ID: 464760
URI: http://eprints.soton.ac.uk/id/eprint/464760
PURE UUID: 07e0f297-5507-4af0-9b38-f1cadc0bac99

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Date deposited: 05 Jul 2022 00:00
Last modified: 05 Jul 2022 03:05

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Author: Douglas Geoffrey Booth

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