Answers to Introductory Questions

The total mass of all of the atoms in the unit cell is then 

For example, the theoretical density of halite is calculated in the following way. First count the number of different atom types in the unit cell. To count the number of atoms in a unit cell, use the information  

A quick method to count the number of atoms in a unit cell is to displace the unit cell outline to 

The unit cell of the halite structure contains 4 sodium (Na) and 4 chlorine (Cl) atoms. The mass of the unit cell, m, is then given by  

The volume, V, of the cubic unit cell is given by a thus  

A point defect is a localized defect that consists of a mistake at a single atom site in a solid. The simplest point defects that can occur in pure crystals are missing atoms, called vacancies, or atoms displaced from the correct site into positions not normally occupied in the crystal, called self-interstitials. Additionally atoms of an impurity can occupy a normal atom site to form substitutional defects or can occupy a normally vacant position in the crystal structure to form an interstitial. Other point defects can be characterized in pure compounds that contain more than one atom. The best known of these are Frenkel defects, Schottky defects, and antisite defects. 

An antisite defect is an atom on an inappropriate site in a crystal, that is, a site normally occupied by a different chemical species. In a compound of formula AB the antisite defects that can occur are an A atom on a site normally occupied by a B atom, or a B atom on a site normally occupied by an A atom. Antisite defects are not very important in binary ionic compounds, as the misplacement of an ion is energetically costly, and so unfavorable. In ternary ionic compounds, however, such as spinels, AB204, the transfer of A ions to B sites and vice versa, is not 

Point defects that form in a crystal exposed to radioactivity are called  

The number of Schottky defects in a crystal of formula MX is equal to  

A single anion Frenkel defect in an ionic crystal of formula MX2 needs to be balanced by  

Steady-state difiusion differs from nonsteady-state diffusion in that the concentration of the diffusing atoms at any point, x, and hence the concentration gradient at x, in the solid, remains constant. Steady-state diffusion may be achieved when air diffuses through plastic food wrapping film. 

Under steady-state conditions, the diffusion coefficient is obtained by using Pick s first law  

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