CHEC 501 Structure and Energetics solids Ionic Solidselibrary.vssdcollege.ac.in/web/data/books-com-sc/bsc-1/chemistry... · Ionic Solids: AX 2 type Fluorite Structure: CaF 2 ! Ca2+

© Dr. Rajesh Chakrabarty, RGU 1

CHEC 501 Structure and Energetics in Metallic and Ionic solids

Rajesh Chakrabarty Department of Chemistry

Rajiv Gandhi University Rono Hills, Doimukh

Ionic Solids

n  Ions of opposite charge surround each other

n  Larger (usually anion) forms the unit cell and the smaller ions goes into the ‘holes’

Ionic Solids


Ionic Crystal Structure Types

Formula Type and fraction of sites occupied

ccp hcp

AX All octahedral Rock Salt (NaCl) Nickel Arsenide (NiAs)

Half tetrahedral (T+ or T-) Zinc Blende (ZnS) Wurtzite (ZnS)

AX2

All Tetrahedral Fluorite (CaF2) Anti-Fluorite (Na2O)

Not known

Half octahedral (ordered framework)

Anatase (TiO2) Rutile (TiO2)

Half octahedral (Alternate layers full/ empty)

Cadmium Chloride (CdCl2)

Cadmium iodide (CdI2)

AX3 One-third octahedral YCl3 BiI3

A3X All octahedral & All Tetrahedral Li3Bi Not known

Ionic Solids : AX type


Cesium Chloride Structure: CsCl

•  Chloride ions occupy the corners of a cube, with a cesium ion in the center (called cubic hole) or vice versa.

•  Cl- = 0.96Cs+ size (0.73r in center is ideal) •  Both ions have a coordination number of 8, with the two ions

fairly similar in size •  Rare structure, need big cation (Cs, Tl only cations known with

this structure)


Coordination: 8:8 (cubic)

Unit cell of CsCl = Cs+ (174 pm) = Cl- (181 pm)



Number Cs atoms per unit cell:

1 = 1 atom per unit cell Central atom

Number Cl atoms per unit cell:

(8 × 1/8) = 1 atom per unit cell Corner atoms

Total number CsCl molecules per unit cell (Z) = 1

Rock Salt Structure: NaCl

n  NaCl structure can be viewed as a face-centered cubic (fcc) array of the anions, with the cations in all of the octahedral holes (or vice versa)

n  Large size difference of ions facilitate this structure (ionic radii: Na+ = 116 pm; Cl- = 167 pm)

n  Both ions have a coordination number of 6, each Cl- has six nearest Na+ neighbors and vice versa

n  Most alkali halides have this structure


= Na+ (116 pm) = Cl- (167 pm)


= Na+ = Cl-



= Na+ = Cl-


= Na+ = Cl-



= Na+ = Cl-


= Na+ = Cl-



= Na+ = Cl-

Na+ Cl- Unit cell of NaCl


= Na+ = Cl-

Can be viewed as a face-centered cubic (fcc) array of the anions, with the cations in all of the octahedral holes, or



= Na+ = Cl-

As a face-centered cubic (fcc) array of the cations with anions in all of the octahedral holes

Coordination: 6:6 (octahedral)


Na+ Cl-

Number Na atoms per unit cell:

1 + (12 × 1/4) = 4 atoms per unit cell central edge atom atoms

Number Cl atoms per unit cell:

(6 × 1/2) + (8 × 1/8) = 4 atoms per unit cell Face corner atom atoms

Total number NaCl molecules per unit cell (Z) = 4



Zinc Blende or Spalerite Structure: ZnS

n  The anions (S2-) ions are in a face-centered cubic (fcc) arrangement, with cations (Zn2+) in ½ of the tetrahedral holes

n  Alternate: Zn and S each in (fcc) lattices combined so each ion is in a Td hole of the other lattice

n  Stoichiometry: only ½ of the Td holes are occupied and ½ are vacant



All Td hole occupied Half of the Td hole occupied Zinc Blende (ZnS) Structure


Coordination: 4:4 (tetrahedral)

Ionic radii: Zn2+ = 74 pm; S2- = 170 pm



Number Zn atoms per unit cell:

4 = 4 atoms per unit cell central atom

Number S atoms per unit cell:


Total number ZnS molecules per unit cell (Z) = 4


Wurtzite Structure: ZnS

n  Rarer than Zinc Blende structure for ZnS; formed at higher temperatures

n  Zn and S each in (hcp) lattices combined so each ion is in a Td hole of the other lattice

n  Stoichiometry: ½ of the Td holes are vacant other ½ are vacant


Total number ZnS molecules per unit cell (Z) = 4

Wurtzite Structure: ZnS

Spalerite vs. Wurtzite Structure


Ionic Solids: AX2 type

Fluorite Structure: CaF2

n  Ca2+ in (ccp) lattice with 8 F- surrounding each and occupying all Td holes

n  Alternate: F- in simple cubic lattice with Ca2+ in alternate body centers

n  Nearly perfect radius fits for this structure n  Coordination: Ca2+ 8 (cubic) : F- 4 (tetrahedral)


Antifluorite Structure: Na2O

n  In the related Anti-Fluorite structure cation and anion positions are reversed

n  Every Td hole in the anion lattice is occupied by a cation



Fluorite (CaF2) Structure

= F- (167 pm) = Ca2+ (126 pm)

Coordination: Ca2+ 8 (cubic) : F- 4 (tetrahedral)


Number Ca atoms per unit cell:

8 = 8 atoms per unit cell central atom

Number F atoms per unit cell:


Total number CaF2 molecules per unit cell (Z) = 4

Coordination: Ca2+ 8 (cubic) : F- 4 (tetrahedral)



Layered Structures

Nickel Arsenide: NiAs

n  Arsenic atoms forms hexagonal close packed (hcp) lattice layers exactly above each other

n  Ni atoms occupy all the Oh holes between all the layers of As atoms

n  Coordination: Ni 6 (octahedral) : As 6 (trigonal prismatic) n  Usual for MX compounds where X = Sn, As, Bi, S, Se, Te n  Example: NiS, FeS, PtSn, CoS n  These are better regarded as intermetallic phases rather than

true compounds



There are 6 Ni (orange) arranged in a trigonal prism around the central

As (green)

There are 6 As (green) arranged octahedrally around the central Ni

(brown)


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CHEC 501 Structure and Energetics solids Ionic Solidselibrary.vssdcollege.ac.in/web/data/books-com-sc/bsc-1/chemistry... · Ionic Solids: AX 2 type Fluorite Structure: CaF 2 ! Ca2+