short- and long-range order

short-range order: a few atoms (like unit cells)
long-range order: (like crystals) "lattice periodicity implied long range order"  知道一個就可以知道全部 ==> transnational symmetry

rotational symmetry???
Crystallographic restriction theorem
https://en.wikipedia.org/wiki/Crystallographic_restriction_theorem
旋轉對稱和平移對稱要相容
通常只會看到二重, 三重, 四重, 六重對稱

lattice structure ==> structure order
beside structure order
we have
charge ordering
spin ordering
magnetic ordering