crystalline solid

rock crystal

Rock crystal from the Dauphiné region of France.

Transparent variety of the silica mineral quartz that is valued for its clarity and total lack of colour or flaws. Rock crystal formerly was used extensively as a gemstone, but it has been replaced by glass and plastic; rhinestones originally were quartz pebbles found in the Rhine River. The optical properties of rock crystal led to its use in lenses and prisms; its piezoelectric properties (see piezoelectricity) are used to control the oscillation of electrical circuits.

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Optoelectronic device used in displays for watches, calculators, notebook computers, and other electronic devices. Current passed through specific portions of the liquid crystal solution causes the crystals to align, blocking the passage of light. Doing so in a controlled and organized manner produces visual images on the display screen. The advantage of LCDs is that they are much lighter and consume less power than other display technologies (e.g., cathode-ray tubes). These characteristics make them an ideal choice for flat-panel displays, as in portable laptop and notebook computers.

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Substance that flows like a liquid but maintains some of the ordered structure characteristic of a crystal. Some organic substances do not melt directly when heated but instead turn from a crystalline solid to a liquid crystalline state. When heated further, a true liquid is formed. Liquid crystals have unique properties. The structures are easily affected by changes in mechanical stress, electromagnetic fields, temperature, and chemical environment. Seealso liquid crystal display.

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Any solid material whose atoms are arranged in a definite pattern and whose surface regularity reflects its internal symmetry. Each of a crystal's millions of individual structural units (unit cells) contains all the substance's atoms, molecules, or ions in the same proportions as in its chemical formula (see formula weight). The cells are repeated in all directions to form a geometric pattern, manifested by the number and orientation of external planes (crystal faces). Crystals are classified into seven crystallographic systems based on their symmetry: isometric, trigonal, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. Crystals are generally formed when a liquid solidifies, a vapour becomes supersaturated (see saturation), or a liquid solution can no longer retain dissolved material, which is then precipitated. Metals, alloys, minerals, and semiconductors are all crystalline, at least microscopically. (A noncrystalline solid is called amorphous.) Under special conditions, a single crystal can grow to a substantial size; examples include gemstones and some artificial crystals. Few crystals are perfect; defects affect the material's electrical behaviour and may weaken or strengthen it. Seealso liquid crystal.

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The Crystal Palace at Sydenham Hill, London. It was designed by Sir Joseph Paxton for the Great elipsis

Giant glass-and-iron exhibition hall in Hyde Park, London, that housed the Great Exhibition of 1851. It was taken down and rebuilt (1852–54) at Sydenham Hill, where it survived until its destruction by fire in 1936. Designed by the greenhouse builder Sir Joseph Paxton (1801–1865), it was a remarkable assembly of prefabricated parts. Its intricate network of slender iron rods sustaining walls of clear glass established an architectural standard for later international exhibitions, likewise housed in glass conservatories.

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An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. (Solids in which there is long-range atomic order are called crystalline solids or morphous). Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous ceramic, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids.

In principle, given a sufficiently high cooling rate, any liquid can be made into an amorphous solid. Cooling reduces molecular mobility. If the cooling rate is faster than the rate at which molecules can organize into a more thermodynamically favorable crystalline state, then an amorphous solid will be formed. Because of entropy considerations, many polymers can be made amorphous solids by cooling even at slow rates. In contrast, if molecules have sufficient time to organize into a structure with two- or three-dimensional order, then a crystalline (or semi-crystalline) solid will be formed. Water is one example. Because of its small molecular size and ability to quickly rearrange, it cannot be made amorphous without resorting to specialized hyperquenching techniques.

Amorphous materials can also be produced by additives which interfere with the ability of the primary constituent to crystallize. For example, addition of soda to silicon dioxide results in window glass, and the addition of glycols to water results in a vitrified solid.

Some materials, such as metals, are difficult to prepare in an amorphous state. Unless a material has a high melting temperature (as ceramics do) or a low crystallization energy (as polymers tend to), cooling must be done extremely rapidly. As the cooling is performed, the material changes from a supercooled liquid, with properties one would expect from a liquid state material, to a solid. The temperature at which this transition occurs is called the glass transition temperature or Tg.

Toward a strict definition

It is difficult to make a distinction between truly amorphous solids and crystalline solids in which the size of the crystals is very small. Even amorphous materials have some short-range order at the atomic length scale due the nature of chemical bonding. Furthermore, in very small crystals a large fraction of the atoms are located at or near the surface of the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.

The transition from the liquid state to the glass, at a temperature below the equilibrium melting point of the material, is called the glass transition. It was found that the topology of bonds changes at glass transition from 3D below to fractal (<3) above the glass transition. This change characterises the glass transition as a percolation transition in the system of broken bonds assotiated by a decrease of Hausdorff dimension. From a practical point of view, the glass transition temperature is approximately the temperature at which the viscosity of the liquid exceeds a certain value (about 1012 Pa·s). The transition temperature depends on cooling rate, with the glass transition occurring at higher temperatures for faster cooling rates. The precise nature of the glass transition is the subject of ongoing research. While it is clear that the glass transition is not a first-order thermodynamic transition (such as melting), there is debate as to whether it is a higher-order transition, or merely a kinetic effect.

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