The document discusses various crystal structures including fluorite (CaF2), rutile (TiO2), and cadmium iodide (CdI2), detailing their ionic radii, coordination numbers, and unit cell configurations. Examples of similar compounds and applications in jewelry and other areas are mentioned, along with historical context. It also includes references for further reading on solid-state and inorganic chemistry.

1. AX2 TYPE OF CRYSTAL
STRUCTURE
D. Sudha
21PGC29
1

2. FLUORITE (CaF2)
2
Ca2+ : ccp/fcc Array
F- : All Td voids
Calcium ion Fluoride ion
 Ionic radius of Ca2+ – 1.26 A
Ionic radius of F- – 1.17 A
Radius ratio = 0.26/1.17
= 0.929

3. 3
Coordination number of Ca2+ = 8
Coordination number of F- = 4
Ratio = 8 : 4
Coordination of Ca2+ Coordination of F-
Calcium ion
Fluoride ion

4. 4
 No of atoms per unit cell
Ca2+ = Face + corner
= 6  1/2 + 8 1/8
= 3 + 1
= 4
F- = All Td Voids
= 8
 Other Examples:
SrF2 MgF2 , BaF2 , ZrO2 , HfO2 etc.,

5. 5
 Also known as Fluorspar
 used in jewelry
 occur in different colors
 The blue to deep blue to black color seen in some
fluorites is derived from a color center, the F-center. The
crystal Blue John found in Derbyshire, England.

6. 6
RUTILE (TiO2)
 O2- = hcp array
Ti4+= Octahedral holes
Oxide ion
Titanium ion
 Ionic radius of Cation – 0.745 A
Ionic radius of anion – 1.26 A
Radius ratio = 0.745/1.26
= 0.591

7. 7
 Unit cell of rutile is based on Ti not O.
Filled by Ti
Oxygen atom
Empty Octahedral holes
 Unit Cell – Tetragonal (a=bc)
a=b= 4.594 A , c = 2.958 A
 Distorted hcp array with half of the
Octahedral holes are occupied.

8. 8
 Ti – two per unit cell
one at corner (0,0,0)
one at Body centre (1/2,1/2,1/2)
 O – Four per unit cell
(x, x, 0) - (0.3, 0.3, 0)
(1-x, 1-x, 0) - ( 0.7, 0.7, 0)
( ½+x, ½-x, ½ ) – (0.8, 0.2, 0.5)
( ½-x, ½+x, ½ ) - (0.2, 0.8, 0.5)

9. 9
Coordination number of Ti4+ = 6
Coordination number of O2- = 3
Ratio = 6 : 3
Ti is coordinated to six O
Two O at z = 0
Two O at z = 1
Two O at z = ½ - Collinear
with Ti
Coplanar
with Ti
O at z = 0 coordinated with Ti at
Corner, Body centre, and Body
centre of new cell

10. 10
 German geologist Abraham Gottlob Werner (1800)
 Latin rutilus - "reddish."
 Other name - "red schorl”
 Colour: Blood red, brownish yellow, brown-red, yellow,
grayish-black, black, brown, bluish or violet.
 Crystal System: Tetragonal

11. 11
Cadmium Iodide CdI2
 Similar to Rutile, Hcp anion array with half of the
Octahedral sites are ocuupied.
 Manner of occupancy of Octahedral sites are different.
 Coordination Number – 6:3
 Transition metal iodides, bromides, chlorides and
hydroxides have similar structure like this.
Cadmium ion iodide ion

12. 12
Alternate layer are empty
and occupied.
CdI2 Structure – sandwich
structure, here Cd2+ ions are
sandwiched between layers of I-
ion

13. 13
Cadmium Iodide CdCl2
 Similar to CdI2, differ in nature of anion packing.
Anion (Cl ) - ccp array
 Hexagonal unit cell in which c axis is three times longer than
c axis in CdI2
 Coordination Number – 6:3
Three layer repeat for Cd (CAB)
Six layer repeat for Cl (BACBAC)
Cd
Cl

14. 14
References:
 Solid State chemistry – Anthony R. West
 Principles of Inorganic Chemistry – Puri & Sharma
 https://www.chemtube3d.com/

15. 15