Alpha (α)-Alumina (Sapphire)

Alpha α-alumina (also called Sapphire) crystal is the most popular form of crystalline aluminium oxide and has a corundum structure. Sapphire is the hardest of all oxide crystals, has good thermal properties, excellent electrical and dielectric properties, and is resistant to chemical attack. The Al atoms are octahedrally coordinated by six oxygen atoms. The oxygen ions nearly form a hexagonal close-packed structure with aluminium ions (Al3+) filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral.

Sapphire (α-Al2O3) as a hexagonal structure, belonging to the space group R3c, can be expressed both as a hexagonal as well as a  rhombohedral unit cell. The basic structure consists of hexagonal close-packed planes of oxygen intercalated with aluminum planes. As shown in Figure 2591, the aluminum planes have a similar hexagonal close-packed arrangement but with 1/3 of the sites vacant, resulting in an Al/O ratio of 2/3. Each Al atom is coordinated by 6 oxygen atoms, and each oxygen atom has 4 Al neighbours. The vacancies are aligned on the so-called R-planes, giving sapphire the ability to cleave along these rhombohedral planes. Due to these properties, the unit cell of sapphire is chosen to take into consideration of the position of the A1 vacancies.

crystal lattice of sapphire

 

Figure 2591.The crystal lattice of sapphire. (a) A-, M-, R-, and C-planes of the crystal; (b) Schematic illustration of the aluminum planes. Oxygen planes are intercalated with the aluminum planes (not shown).

Table 2591a shows the lattice properties of α-Al2O3.

Table 2591a. The lattice properties of α-Al2O3.

 

Crystalline properties

Parameter

Crystalline properties

Parameter

Crystal structure

Hexagonal System

Melting Point

2040 °C

Thermal Shock Resistance

790 W/m

 

 

Lattice parameter (Å) 
(wurtzite structure)

a = 4.758

Thermal Conductivity

25.12 W/m/K

c = 12.991

Thermal expansion coefficient 
(x10-6C-1)

a = 7.5

c/a = 2.730

c = 8.5

Density (g/cm3)

3.97

Specific Heat

0.418 W.s/g/K

Hardness

9 Mohs 

Refractive Index

1.83 @0.26 µm, 1.76 @0.63 µm, 1.58 @5.57µm

Table 2591b shows the lattice mismatch (%) between the substrates and epitaxial layers, and the resulting misfit dislocation separation (in Å) corresponding to complete misfit relaxation for the basal plane interfaces.

Table 2591b. The lattice mismatch (%) between the substrate and epitaxial layers, and the resulting misfit dislocation separation (in Å).

Crystalline Properties

 

6H-SiC

α-A1203

InN

AlN

GaN

Lattice mismatch with

Sapphire

11.5%

--

25.4%

12.5%

14.8%

SiC

--

-11.5%

14.0%

1.0%

3.3%

GaN

-3.3%

-14.8%

10.6%

-2.4%

--

Dislocation distance on

Sapphire

21.9

--

10.6

20.3

17.2

SiC

--

21.9

20.4

276.7

80.9

GaN

80.9

17.2

27.3

114.4

--

Table 2591c. Coordinates of ions of two molecules in the rhombohedral unit cell α-A1203, α = 5,124 Å. Origin of coordinates is in the rhombohedron vertex. 

Ion

Type

x

y

z

Ion

Type

x

y

z

1

Al

0

0

0.3750

6

Al

0

0

0.3750

2

O

−0.2457

−0.1418

0.6334

7

O

0.2457

0.1418

−0.6337

3

O

0.2457

−0.1418

0.6334

8

O

−0.2457

0.1418

0.6337

4

O

0

0.2837

0.6334

9

O

0

−0.2837

−0.6337

5

Al

0

0

0.8918

10

Al

0

0

−0.8918

Table 2591d. Interplanar distances in α-A1203. Cu-radiation.

d

I/I0

(hkl)

(hkil)

θ

d

I/I0

(hkl)

(hkil)

θ

3.479

75

012

01-12

11°32′

2.552

90

104

10–14

15°50′

2.379

40

1110

11–20

17°1′

2.165

<1

006

0006

18°45′

2.085

100

113

11–23

19°30′

1.964

2

202

20–23

20°15.5′

1.74

45

024

02–24

23°35′

1.601

80

116

11–26

25°46′

1.546

4

211

21–31

26°46′

1.514

6

122

12–32

27°22.5′

1.51

8

018

01–18

27°27′

1.404

30

124

12–34

29°43′

1.374

50

030

03–30

30°26′

1.337

2

124

12–35

31°22′