Chapter 9 Structural chemistry of ionic compounds9.1 lonic radius ratio & coordination polyhedra of ionsIn ionic crystals the positions in the crystal lattice areoccupiedbycationsandanions·In most cases, anions have much larger radii than cations,andformaclose-packedstructure·Consequently cations occupy the interstices of the close-packedstructureformedbyanionsLocal motifaround a cation:coordination polyhedronformed by cation and its surroundinganions
Chapter 9 Structural chemistry of ionic compounds 9.1 Ionic radius ratio & coordination polyhedra of ions • In most cases, anions have much larger radii than cations, and form a close-packed structure. • Consequently cations occupy the interstices of the closepacked structure formed by anions. Local motif around a cation: coordination polyhedron formed by cation and its surrounding anions. In ionic crystals the positions in the crystal lattice are occupied by cations and anions
Interstices in fcc close-packed structure(Ifformed by anions with a radius ofR)octahedralholestetrahedralholes4Ra=V2=2V2Ra/3aR=0.414RR(hole)R= 0.225RR(hole)24Hole size is 0.414R: CN = 6Holesizeis0.225R:CN=4
octahedral holes • Hole size is 0.414R; CN = 6 tetrahedral holes • Hole size is 0.225R; CN=4 Interstices in fcc close-packed structure (If formed by anions with a radius of R) R R a 2 2 2 4 R R a R hole 0.414 2 ( ) R R a R hole 0.225 4 3 ( )
C.N.Thole /RInterstice type30.155Triangular0.2254Tetrahedral60.414Octahedral80.732CubicRadiusof anion121.000Cuboctahedral(rnole: the radius of interstice formed by close-packing of anions)triangularCubicCuboctahedral
Triangular 3 0.155 Tetrahedral 4 0.225 Octahedral 6 0.414 Cubic 8 0.732 Cuboctahedral 12 1.000 Interstice type C.N. rhole /R Cubic Cuboctahedral triangular (rhole: the radius of interstice formed by close-packing of anions) Radius of anion
The rule for the packing of ionsAnions directlyAnions directlyAnions directlycontact with eachcontact with cationcontact with eachwithout anion-anionother with no cationother as well as thecentral cation!anion contact.contact.(R rhole)(R, = rhole)Strong C-A attraction!SStrong C-A attraction!weak C-Aattraction!Weak A-Arepulsion!Strong A-A repulsion!Strong A-A repulsion!Condition of coordination polyhedron: R, ≥ rhole
The rule for the packing of ions Anions directly contact with cation without anion-anion contact. Anions directly contact with each other as well as the central cation! Anions directly contact with each other with no cationanion contact. Strong C-A attraction! Weak A-A repulsion! Strong C-A attraction! Strong A-A repulsion! weak C-A attraction! Strong A-A repulsion! Condition of coordination polyhedron: R+ rhole (R+ > rhole) (R+ = rhole) (R+ < rhole)
Table 1.The limiting R./R_ratio ofsome coordination polyhedraC.N.Coordination polyhedronminimumR/R≥ 0.1553Triangular≥ 0.225Tetrahedral6≥ 0.414Octahedral8Cubic≥ 0.73212≥1.000Cuboctahedral
Coordination polyhedron C.N. minimum R+ /R Triangular 3 0.155 Tetrahedral 4 0.225 Octahedral 6 0.414 Cubic 8 0.732 Cuboctahedral 12 1.000 Table 1. The limiting R+ /R ratio of some coordination polyhedra
9.2 The crystal structures of sometypical ionic compoundsAnswerthe following questions:. The packing style of anions ?The position of cations ?Crystal System (or morepreciselyBravaisLattice)The CN's of anions and cations?Number of atoms in a unit cell ?Structural formula ?Fractionalcoordinatesofanions/cations
9.2 The crystal structures of some typical ionic compounds Answer the following questions: • The packing style of anions ? • The position of cations ? • Crystal System ( or more precisely Bravais Lattice) ? • The CN’s of anions and cations ? • Number of atoms in a unit cell ? • Structural formula ? • Fractional coordinates of anions/cations
9.2.1Binaryioniccompounds(AB)Rocksaltstructure,NaCI typeR./R_~ 0.732-0.414fcc/ccpPacking of anions:Position of cations::OctahedralholesCubicFBravais Lattice:6:6C.N. ofA- & B+:Na:Np in a unit cell: 4:4NaCIStructural motif:0, 1/2, 1/21/2, 1/2, 0; 1/2, 0, 1/2;A: 0, 0, 0;0, 0, 1/2B: 1/2, 1/2, 1/2;1/2, 0, 0;0, 1/2, 0;(0.732-0.414) KCl, KBr, SrS, Rbl, BaTe, SrSe, CaS, KI, SrTeMgO,LiF,CaSe,NaBr,CaTe,MgS,Nal,LiCl,(>0.732)KFSrO,BaO,Rb,RbCl,BaS,CuO,CsF,RbBr,BaSe,NaF(<0.414) MgSe,LiBr, LiF, NiO, CoO
R+ /R ~ 0.732-0.414 A: 0, 0, 0; 1/2, 1/2, 0; 1/2, 0, 1/2; 0, 1/2, 1/2 B: 1/2, 1/2, 1/2; 1/2, 0, 0; 0, 1/2, 0; 0, 0, 1/2 (0.732-0.414) KCl, KBr, SrS, RbI, BaTe, SrSe, CaS, KI, SrTe, MgO, LiF, CaSe, NaBr, CaTe, MgS, NaI, LiCl, (>0.732) KF, SrO, BaO, RbF, RbCl, BaS, CuO, CsF, RbBr, BaSe, NaF, (<0.414) MgSe, LiBr, LiF, NiO, CoO . 9.2.1 Binary ionic compounds (AB) Rocksalt structure， NaCl type • Packing of anions: fcc/ccp • Position of cations: Octahedral holes • Bravais Lattice: Cubic F • C.N. of A- & B+ : 6:6 • NA :NB in a unit cell: 4:4 • Structural motif: NaCl
Example 1: CoOO-OO-2Co+2NaCl-typestructureofCoOIts (1l1) surface are charged, having higher surface energythan othernon-polarsurfaces.However,the charged (lll)surface could be stabilized uponcoordination of counterionsfromionicliquids
Example 1: CoO • NaCl-type structure of CoO. • Its (111) surface are charged, having higher surface energy than other non-polar surfaces. • However, the charged (111) surface could be stabilized upon coordination of counterions from ionic liquids
Control the surface byintroducing electrostaticinteractions!R=1.73R=0.58M=30.00.K2Growth of CoOin the200nmabsence of ionic liquidGrowth of CoO in ionic liquid
Growth of CoO in ionic liquid Growth of CoO in the absence of ionic liquid Control the surface by introducing electrostatic interactions!
β-ZnS（Sphalerite/闪锌矿/立方硫化锌）(R./R_:0.225-0.414)fcc/ccpPacking of anions:Position of cations:Tetrahedral holes (1/2)cubic FBravais lattice:4:4C.N. of A- & B+:Na:Np in a unit cell: 4:4ZnSStructural motif:(0,1/2,1/2)A: (0,0,0);(1/2,1/2,0);(1/2,0,1/2);B: (1/4,1/4,1/4); (3/4,3/4,1/4);(1/4,3/4,3/4)(3/4,1/4,3/4);
b-ZnS (Sphalerite/闪锌矿/立方硫化锌) (R+ /R : 0.225-0.414) A: (0,0,0); (1/2,1/2,0); (1/2,0,1/2); (0,1/2,1/2) B: (1/4,1/4,1/4); (3/4,3/4,1/4); (3/4,1/4,3/4); (1/4,3/4,3/4) • Packing of anions: fcc/ccp • Position of cations: Tetrahedral holes (1/2) • Bravais lattice: cubic F • C.N. of A- & B+ : 4:4 • NA :NB in a unit cell: 4:4 • Structural motif: ZnS