图 面心立方 (a) (b) FCC Face-centered cubic 4R②a 2a=4R (c) 材料科学与工程学院 7 School of Materials Science and Engineering材料科学与工程学院 School of Materials Science and Engineering 面心立方 7 (a) (b) (c) a 4R 2a 2a = 4R FCC Face-centered cubic