Structure of Solids
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While the atoms or molecules in a liquid are free to move around, those in solids are limited to vibrating about a fixed point. Research 17 September Open Access. Quasicrystals differ from traditional incommensurate structures because they have non-crystallographic rotational symmetries. Here the authors introduce a scheme to produce metallic-mean quasicrystals in two dimensions with 6-fold rotational symmetry that can be seen as approximant to periodic tilings. Research 11 September Open Access.
Stable periodic structures can be difficult to obtain in a liquid crystal compared to a solid due to the energetic instability of the former. Here the authors present a technique to fabricate quasicrystalline structures of graphene oxide liquid crystals which have high stability. Research 09 September Research 06 September Open Access.
Research 04 September Open Access. Research 03 September Open Access. Research Highlights 21 May Ultrafast electron diffraction and ultrafast scanning transmission electron microscopy with nanoscale spatial resolution were demonstrated using unique high-brightness high-repetition rate electron scattering source.
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Research Highlights 15 February Crystalline solids are those in which the atoms, ions, or molecules that make up the solid exist in a regular, well-defined arrangement. The smallest repeating pattern of crystalline solids is known as the unit cell , and unit cells are like bricks in a wall—they are all identical and repeating.
The other main type of solids are called the amorphous solids. Amorphous solids do not have much order in their structures. Though their molecules are close together and have little freedom to move, they are not arranged in a regular order as are those in crystalline solids. Common examples of this type of solid are glass and plastics. Ionic solids— Made up of positive and negative ions and held together by electrostatic attractions.
An example of an ionic solid is table salt, NaCl. Molecular solids— Made up of atoms or molecules held together by London dispersion forces, dipole-dipole forces, or hydrogen bonds. Characterized by low melting points and flexibility and are poor conductors. An example of a molecular solid is sucrose.
Covalent-network also called atomic solids— Made up of atoms connected by covalent bonds; the intermolecular forces are covalent bonds as well. Characterized as being very hard with very high melting points and being poor conductors. Examples of this type of solid are diamond and graphite, and the fullerenes.
As you can see below, graphite has only 2-D hexagonal structure and therefore is not hard like diamond. The sheets of graphite are held together by only weak London forces! Metallic solids— Made up of metal atoms that are held together by metallic bonds.
Characterized by high melting points, can range from soft and malleable to very hard, and are good conductors of electricity. These atoms, ions, or molecules are called lattice points and are typically visualized as round spheres. See Below. Figure 1: Two possible arrangements for identical atoms in a 2-D structure.
Stacking the two dimensional layers on top of each other creates a three dimensional lattice point arrangement represented by a unit cell. A unit cell is the smallest collectionof lattice points that can be repeated to create the crystalline solid. The solid can be envisioned as the result of the stacking a great number of unit cells together.
The Unit Cell
Primitive Simple Cubic Structure Placing a second square array layer directly over a first square array layer forms a "simple cubic" structure. This packing arrangement is often symbolized as "AA The coordination number of each lattice point is six. The faces of crystals always meet at some fixed angles. For any particular substances the angle between corresponding pair of faces is alway the same in all crystals.
Different crystals of the same substance may sometimes appear to be different from outside, either due to different rate of growth by different faces or due to some damage to the corners or edges but the interfacial angle is always the same. Two crystals of a single substance with the same lattice but different shapes are said to be of different habit.
On habit modification the relative areas of different faces change but the angles between such faces remain costant. Crystalline solids exibit anisotropy in many of their properties. It means all those properties wich depend upon direction or angular orientation of crystals. These show different behaviour in non-parallel directions.
Structure of solids and liquids - Latest research and news | Nature
One such consequence of anisotropy is the phenomenon of cleavage. In crystals the splitting is easier in some directions than others. For example, in a crystal of sodium chloride cleavage can only be achieved along planes parallel to cube faces. Any attempt to cleave such a crystal along any arbitrary plane will shatter it. The transition from the solid to liquid i.
An amorphous substance, on the other hand, has no sharp melting point. The transition fron solid to liquid in an amorphous solid does not take place at a define point but extends over a long range.