1.1 晶格
The crystal lattice: Bravais lattice (3D) A Bravias lattice is a lattice of points, defined by This reflects the translational symmetry of the lattice 反映了晶格的平移对称性
Primitive cell 原胞 • 一个晶格的周期重复单元称作点阵的晶胞（unit cell)，最小周期重复 单元称作原胞（Primitive cell ） • 以点阵基矢构成平移矢量，可以把原胞复制填满整个空间
|a1|、 |a2|、 |a3| 被称为晶格常数（lattice constant)
How to form a crystal ? • We could think: all that remains to do is to put atoms on the lattice points of the Bravais lattice. 向晶格点阵上堆积原子 • But: not all crystals can be described by a Bravais lattice (ionic, molecular, not even some crystals containing only one species of atoms.)并不是所有的晶体 都能够被晶格点阵简单描述，有时一种晶体包含有不同类型的原子。 • BUT: all crystals can be described by the combination of a Bravais lattice and a basis. This basis is what one “puts on the lattice lattice points”.所有的晶体都可以被 所有的晶体都可以被 描述为：晶格点阵+基元，基元就是往点阵上堆积的东西
晶面与晶向
Example