清华材料科学基础(英文)skja.pptVIP

材料科学基础 材料科学基础Fundamental of Materials §3.4 Some Rules Governing the Ionic Compounds Pauling’s three rules governing the structure of ionic compounds. 1. Anion Coordination polyhedron rule Each cation is surrounded by an anion polyhedron, the distance between anion and cation is ro＝r+＋r-, while the CN depends on r+/r-. ro＞r+＋r－ 2. Valence rule There is a definite relation between valence and CN. Let S be the strength of electrostatic bonding between a pair of anion and cation §3.5 Crystal Structure of Ceramic Materials AB AB2 A2B3 ABO3 AB2O4 2. CsCl r+/r－＝0.91 ∴ hexahedron ＝ CN+＝8 Z－＝1＝CN－(Z+/CN+)＝CN－×1/8 ∴ CN－＝8 3. Zinc blende ( ZnS ) 4. Wurtzite ( ZnS, ZnO ) Ⅱ. AB2 type 1. Fluorspar (CaF2) Li2O Na2O 2. Rutile (TiO2) Ⅲ. A2B3 type ( ) Ⅳ. ABO3 type ( BaTiO3, CaTiO3 ) Ⅴ. AB2O4 type (spinel, MgAl2O4 ) Examples and Discussions Exercise * * Lesson nine ro＝r+＋r－ stable ionic compound ro＜r+＋r－ there is some covalent bounding stable unstable 0.155 3 trigonal 0.225 4 tetrahedron 0.414 6 octahedron 0.732 8 hexahedron ( )min CN＋ anion polyhedron if S1＝S2＝……＝S, Then Z ＝(CN－)×S＝（CN－）×（Z+/CN+） 3. The anion polyhedron prefer to share apex instead of sharing edges or faces: Ⅰ. AB type 1. NaCl r+/r－＝0.54 ∴ octahedron ＝ CN+＝6 Z－＝1＝CN－(Z+/CN+)＝CN－×1/6 ∴ CN－＝6 CN－ ＝ 4 CN+ ＝ 4 S(O)＝1/2＋1/2＋1 ＝2 Zn＝1/4×4＋1 ＝2 CN+ ＝ 8 CN- ＝ 4 Mg: 4＋4＝8 Al: 4×4＝16 8(MgAl2O4) O: 4×8＝32