A State Between Liquid and Crystal: Locally Crystalline but with the Structure Factor of a Liquid
نویسندگان
چکیده
We are familiar with large highly-ordered single crystals, such as those used in the semiconductor industry. There the crystalline ordering in the silicon crystals extends for billions and more lattice spacings. As a result, X-ray diffraction (XRD) patterns of these crystals have sharp Bragg peaks. We are also familiar with liquids, where the molecules have a local structure that is very different to that in a crystal. In addition, in liquids there is only ordering over a few molecular diameters, and so they do not have Bragg peaks. But there are materials, where the situation is much less clear. Many of these materials are referred to as amorphous. For example, the structure of amorphous silicon is controversial. There is debate1–3 over whether it possesses some crystalline ordering of the sort variously referred to as paracrystalline order, nanocrystalline order or medium-range crystalline order, or whether it has no crystalline ordering. We do not know if locally, silicon atoms are arranged in the structure of a liquid, or whether they are in tiny crystals only a few silicon atoms across. There are other systems at the borderline between liquid and crystal. The mineral ferrihydrite can be classified as crystalline (this is called six-line ferrihydrite), or as amorphous (this is called two-line ferrihydrite).4–6 The structures of Amorphous Calcium Carbonate (ACC),7–16 Amorphous Calcium Phosphate (ACP),17 and Amorphous Calcium Sulphate (ACS)18–20 are also poorly understood. The properties of ACC clearly depend on how it is prepared,9,21–24 and Gebauer et al.23 refer to ‘protovaterite’ and ‘proto-calcite’ forms of ACC, which
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