周 体心立方 (a) (b) BCC 尽 Body-centered cubic a 2a 材料科学与工程学院 6 3a=4R (c) School of Materials Science and Engineering材料科学与工程学院 School of Materials Science and Engineering 体心立方 6 4R a 3a 2a 3a =4R (c) (a) (b) BCC Body-centered cubic