Metamict Transformation of Silica
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A5.18.1
Metamict Transformation of Silica Ju-Yin Cheng1, M. M. J. Treacy2 and P. J. Keblinski1 Department of Material Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 2 NEC Research Institute, Inc., Princeton, New Jersey 08540 1
ABSTRACT Alpha quartz can be transformed to a metamict phase in a physical reaction such as irradiation with fast ions, neutrons and electrons. As truly inspired by this fact, we design a simple experiment to produce disordered silica and at the same time watch the transformation using a transmission electron microscope. This work simply presents electron-damaged silica in dark field at Bragg reflection. Since the transformation is not complete, disordered silica (at the sample edge) and retained quartz can be compared in each image. Not surprisingly, more speckles are found in the quartz than in the disordered structure. But it is too early to say no medium-range ordering in amorphous silica because the experiment only shows one Bragg diffraction. Systematic work to complete and measure the disordering again is being pursued. INTRODUCTION Silica is a zeolite abundant in Earth. Its polymorphs (from low to high atmospheric temperatures) are quartz, tridymite and cristobalite. When heated up to 1723 ºC, the melting point of cristobalite, the zeolite starts to melt. The liquid then transforms to a glassy phase by passing over crystallization under cooling. The glassy phase is called vitreous silica. In very early X-ray diffraction studies [1], vitreous silica showed a sharp peak around 10–30 nm-1. This first sharp diffraction peak (FSDP) was interpreted as the evidence of intermediate-range structure in the glass. Later on, the microcrystallite theory was redefined in modeling the structure of vitreous silica [2]. On the other hand, silica has another glassy phase. This glassy phase is produced from irradiation of crystalline silica, so called “metamict” silica. Similar to vitreous silica, metamict silica shows both the glassy and crystalline characters in electron diffraction [3]. However, metamict and vitreous silicas presumably have different structures, since the former is caused by irradiation but the latter is due to melting. According to microcrystalline models [4, 5], the structure of vitreous silica is not built upon a low-temperature polymorph such as quartz. This is because in most cases vitreous silica is transformed from cristobalite, and the reverse is true (e.g. the product of devitrification is cristobalite alone) [6]. But quartz can be considered in the construction of structure models for metamict silica. In our work, metamict silica is produced from alpha quartz crystal by electron irradiation. As the irradiation proceeds, we measure the diffracted intensity (at the first Bragg reflection) of the sample. Disordering is initiated at the surface. The intensity in dark field from disordered regions is much lower than that from retained quartz. Although the intermediate-range structure of the metamict phase is not largely seen in the
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