Origin of Properties

Material properties arise from composition, structural order, and bonds. Because all properties arise from the same basic factors, they are highly interrelated. It is unrealistic to develop a set of desired properties and then attempt to discover a material that meets them. Rather, one needs to understand the origin of the properties, and design around the available range of the interrelated properties of real materials.

Composition includes consideration of the size and mass of the atoms, the electronic structure of the atoms (including chemical valence), and stoichiometry (ratio of the constituent atoms). Structural order factors include the bonding arrangements (short-range order) and the long-range order (i.e., whether a material is crystalline or amorphous). The number of direction-dependent components in a property of a crystalline material depends on the particular crystalline structure.

The chemical bonds have great influence over properties. Three basic bond types are recognized: covalent, ionic, and metallic. Covalent bonding is moderately strong and directional. Diamond, silicon, and germanium are prototypical covalent compounds. The characteristics of these prototypical covalent materials belie the normal covalent bond; the nonfilled outer shells of these materials allow electron overlap without infrared vibrational excitations. This charge overlap creates an attractive interaction, that, combined with high coordination number and symmetry of these materials, creates a very strong binding force. In other covalent materials, the electronic overlap and coordination are reduced; hence the bonds become weaker (and also have some ionic character).

Alkali halides are representative ionic bonded compounds. Ionic...