368 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



spectrum of the diamond lattice. He predicted an absorption band at 

 8/i and although the basis of his calculation was not correct, neverthe- 

 less an absorption band at 8/x was fomid by some observers. Others 

 failed to find the band. This disagreement in observation proved to 

 be a consequence of the fact that diamonds are of two types : one of 

 these does indeed have an absorption band at this wavelength, while 

 the other does not. The origin of the absorption is far from settled, 

 but it is almost certainly due to causes other than lattice vibrations. 



What has been said so far is intended to give a picture of the be- 

 havior and arrangement of the carbon atoms in the diamond. To 

 understand why they are arranged in this way and to explain many 

 of the electrical properties of the diamond, it is necessary to look a 

 little more closely at the carbon atom itself and inquire into its elec- 

 tronic structure. 



The element carbon has an atomic number 6. This means that it 

 has six miits of positive charge on the nucleus and has six electrons 

 moving in various orbits around the nucleus. It lies in the fourth 

 column of the elements in the periodic table and has similar charac- 

 teristics to silicon, germanium, tin, and lead, all of wliich belong to 

 the same group. This seemingly diverse group of elements, which 

 comprise two metals (tin and lead), two semiconductors (germanium 

 and silicon), and diamond or graphite have one important attribute 

 in common. They all have the same distribution of electrons in the 

 outermost shells of the atom. In the case of carbon there are two 

 electrons in the inner ^-shell wliich take no part in any interactions 

 with other atoms. Farther away from the nucleus, the four remain- 

 ing electrons are fairly loosely held in the outermost Z-shell. This 

 outer shell is permitted by quantum conditions to accommodate eight 

 electrons in all, so that the atom behaves as an element with four 

 valence electrons and at the same time requires four more to complete 

 the octet which fills the Z-shell. It is well known that atoms which 

 have incomplete outer shells combine readily with other elements 

 which have electrons that can be detached and fill the vacancies. The 

 chemical combination of carbon with four hydrogen atoms, each of 

 which release an electron, or of carbon and two atoms of oxygen, each 

 of which releases two electrons to fill the outer shell, are very well 

 known. All the elements of group 4 behave chemically in much the 

 same way. 



Before we examine what happens to the electrons when we bring a 

 number of carbon atoms together to form a crystal, it is necessary to 

 consider the energy of the electrons in the free atom. The energy of 

 an electron will depend in which orbit or shell it moves. If the elec- 

 tron spends most of its time near the nucleus, then its energy is lower 

 than if it is in the outer orbit. The distribution of electrons and their 



