晶体(crystal)
原子组元按一定图形排成的固体材料，其表面的规则性反映出晶体内部的对称性。晶体由无数个原子的结构单元（称为晶胞）组成。这些晶胞在所有方向重复排列，形成几何图形，而这种几何图形又以晶体的外表平面（称为晶面）的数目和取向显示出来。晶体的一个基本性质是对称性，所有晶体都根据其主要的对称元素分类。有七个基本晶系︰立方晶系、三角晶系、六角晶系、四方晶系、正交晶系、单斜晶系和三斜晶系。晶体一般在液体凝固时或溶液过饱和时形成。对於许多简单物质，固态就包含有晶体结构，许多金属、合金、矿物以及半导体都属於这种情况。若液态物质被冷却到刚性状态而不结晶，则所形成的固体就是玻璃状的；这种固体能逐渐变成晶体，但过程极其缓慢。由於许多物质由微晶（晶体中的许多细粒，每一细粒都是完整的晶体）组成，晶体结构未必能用肉眼看出，需用X射线衍射或电子显微镜来识别。在某些极特殊的情况下，一单晶结构能长大到可以看到，例如天然宝石。人工生长的晶体显示出异常的规则性，这使科学家能获得更多的关於晶胞内部的讯息，而最先利用这一特徵的是地质学家。晶体不总是完整的，缺陷可能起因於固体的形变、高温快速冷却或高能辐射轰击，能影响晶体的力学、电学和光学性质。参阅liquid crystal。
English version：
crystal
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 or a vapor or a liquid solution can no longer retain dissolved material, which is precipitated (see precipitation). Metals, alloys, minerals, and semiconductors are all crystalline, at least microscopically. (A noncrystalline solid is “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 behavior and may weaken or strengthen it. See also liquid crystal.
