晶体Crystal

晶体结构可以看成由一个最小重复单位——晶胞——在三维空间平移堆砌而成，晶胞的排列方式用点阵描述。长程有序使晶体具有确定的熔点（熔化时晶格瓦解）、对 X 射线产生衍射、解理面平整、物理性质（导热、折射、硬度）随方向而变。按微粒间作用力分为离子晶体、共价晶体、分子晶体和金属晶体。

A crystal structure can be viewed as a smallest repeating unit, the unit cell, translated throughout space, and the pattern is described by a lattice. Long-range order gives a sharp melting point (where the lattice collapses), X-ray diffraction, flat cleavage planes and properties such as thermal conductivity, refraction and hardness that depend on direction. By the forces between particles, crystals are ionic, covalent network, molecular or metallic.

1669 年斯泰诺发现同种晶体相应晶面间的夹角恒定（面角守恒定律）。1784 年阿维提出晶体由微小的“基本分子”堆砌而成。1848 年布拉维证明三维空间只有 14 种点阵。1891 年费多罗夫和舍恩夫利斯独立推导出 230 种空间群。1912 年劳厄发现晶体的 X 射线衍射，证实了晶体内部的周期性结构。

In 1669 Steno found that the angles between corresponding faces of a crystal species are constant (the law of constancy of interfacial angles). In 1784 Haüy proposed that crystals are built of tiny 'integrant molecules'. In 1848 Bravais showed there are only 14 lattices in three dimensions, and in 1891 Fedorov and Schoenflies independently derived the 230 space groups. In 1912 Laue discovered X-ray diffraction by crystals, confirming their internal periodicity.

1982 年谢赫特曼在铝锰合金中发现具有五次对称、长程有序但不具有周期性的准晶，打破了“晶体只能有 1、2、3、4、6 次旋转对称”的传统认识，获 2011 年诺贝尔化学奖，国际晶体学联合会随后修改了晶体的定义。单晶硅、蓝宝石、石英振子、激光晶体和药物晶型控制都是晶体科学的重要应用。

In 1982 Shechtman found quasicrystals in aluminium–manganese alloys, with fivefold symmetry and long-range order but no periodicity, overturning the rule that crystals can have only 1-, 2-, 3-, 4- or 6-fold rotational symmetry; he received the 2011 Nobel Prize in Chemistry, and the International Union of Crystallography revised its definition of a crystal. Single-crystal silicon, sapphire, quartz oscillators, laser crystals and control of drug polymorphs are key applications.