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Structural changes of synthetic opal by heat treatment

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Abstract

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO4 tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO4 tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.

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Acknowledgments

This work was supported JSPS KAKENHI [Grant-in-Aid for Challenging Exploratory Research] Grant Number 24654172. This work also was supported by Grant-in-Aid for Joint research between Kanazawa University and JAIST in 2011–2012. A part of this work was conducted in Kyoto-Advanced Nanotechnology Network, supported by “Nanotechnology Network” of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Correspondence to Akane Arasuna.

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Arasuna, A., Okuno, M., Okudera, H. et al. Structural changes of synthetic opal by heat treatment. Phys Chem Minerals 40, 747–755 (2013). https://doi.org/10.1007/s00269-013-0609-1

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