Color of Alumina

Without impurities alumina is colorless. However, addition of transition metal ions to alumina leads to spectacular colors, gem stones, and practical applications such as ruby lasers. Many aspects of color are discussed in detail in [51].

With the addition of about 1% of Cr2O3 to Al2O3 (replacement of one out of one hundred of the aluminum ions with chromium ions), alumina acquires a beautiful red color and is known as ruby, one of the most prized gem stones. The red color results from transitions of electrons between energy levels in the ruby, as described in [51], p. 8Iff. Ruby also shows a bright red fluorescence when it is illuminated with ultra­violet light (energy of 4-5 eV). Ruby also shows pleochoism (multicolors); in polar­ized light the color changes as the ruby crystal is rotated [51].

Table 23 Refractive index, n, and absorption index, k, of the ordinary ray for sapphire in the ultra-violet spectral range, at 25°C, from [49, 50]

Energy (eV)

Wavelength, l, (pm)

n

k

142.000

0.008731

0.975

0.0130

120.000

0.010330

0.972

0.0177

100.206

0.01237

0.892

0.088

79.996

0.01550

0.929

0.130

60.001

0.02066

0.911

0.136

42.000

0.02952

0.859

0.098

30

0.04133

0.841

0.367

27.013

0.04590

0.605

0.317

24.113

0.05142

0.524

0.542

19.992

0.06202

0.800

0.983

18.008

0.06885

1.028

1.093

15.475

0.08012

1.142

1.125

13.582

0.09129

1.375

1.434

12.972

0.09558

2.581

1.573

12.361

0.1003

1.981

1.476

11.263

0.1101

2.400

1.131

10.292

0.1205

2.441

0.831

9.523

0.1302

2.559

0.686

8.852

0.1401

2.912

0.125

8.791

0.1410

2.841

0.037

8.760

0.1415

2.797

0.011

8.730

0.1420

2.753

0

Table 24 Refractive index, n, and absorption index, k, of the ordinary ray for sapphire in the infrared spectral range, at 25°C, from [49]

Wavelength, l, (pm)

n

k

4.000

1.675

1.30 (10)-6

5.000

1.624

3.76 (10)-5

7.143

1.459

3.60 (10)-3

10.00

0.88

0.053

10.99

0.27

0.224

12.05

0.08

1.04

12.99

0.07

1.57

14.93

0.10

2.96

16.13

0.47

3.56

16.95

0.75

6.30

17.24

1.69

11.28

17.86

9.10

1.53

19.23

3.82

0.115

20.00

2.64

0.106

22.73

10.08

14.01

25.00

4.64

0.156

30.30

3.95

0.0145

33.33

3.69

0.00668

50.00

3.26

0.0117

60.13

3.19

0.0130

82.71

3.13

0.0085

100.0

3.12

0.0070

200.0

3.08

0.0035

333.3

3.07

0.0027

600.0

3.05

0.0015

The deep blue color or sapphire gems results from the addition of a few hundredths of one percent of iron and titanium impurities to alumina. The Fe2+ and Ti4+ ions sub­stitute for aluminum in the sapphire, and when light of energy of 2.11 eV is shone on the sapphire, it is absorbed by the charge transfer reaction:

Fe2+ + Ti4+ = Fe3+ + Ti3+ (22)

See [51], p. 140ff for a complete description of this process.

A variety of other colors are found in natural and synthetic alumina crystals [2, 51]. For example, an orange-brown color is produced by Cr4+ (padparadscha sapphire) [51]; different transition metal ions in different concentrations and oxidation states produce many colors.