Page 12 - Modern physical chemistry

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Structure in Solids

1. 1 Limits on Homogeneity
TO AN OBSERVER, THE UNIVERSE APPEARS to be made up of objects and beings that
act on each other. With an appropriate divider, each object may be broken down into
smaller parts. When the object is homogeneous, each part is like any other part of the
same size and shape.
But can such subdivision be carried out indefinitely? Or does a person finally reach
units whose rupture would destroy the properties of the material making up the object?
Thinkers in ancient times considered both possibilities. However, they were not able
to arrive at any conclusion. Only in modem times has the submicroscopic nature of
objects been investigated and determined. The Simplest evidence involves how the con-
stituent material can be arranged in space, particularly at low temperatures.

1.2 States of Matter
When enough heat is removed from a given material, it becomes frozen; it tends to
maintain both a definite volume and shape. Such material is said to be a solid. Then
heating it above a certain temperature gives it the ability to flow. It assumes a shape
determined by the lower surfaces of the containing vessel. The material is then said to
be a liquid. Above a higher temperature, determined by the applied pressure, it boils.
The resulting phase then will fill any small containing vessel, regardless of the vessel's
shape or size. The material is then said to be a gas. A gas of the given material over the
corresponding liquid or solid is called a vapor.
We will study the nature of temperature, pressure, heat, and work later. The result-
ing relationships will enable us to analyze the behavior of transitions among the states.
But here our primary concern is geometric, the arrangement of materials in space at low
temperatures and what one can deduce from such simple considerations.
In the absence of polarizing forces, a gas or liquid is isotropic, with no macroscopic
property that varies with direction. An appreciable impressed force field does destroy some
of this symmetry and make the system anisotropic. The solids called glasses are also isotropic
in the macroscopic sense. Other solids exhibit some anisotropy. Such a solid exists either
as a single unit called a crystal or quasicrystal, or as a conglomerate of such units.
A single crystal, placed in a uniform supersaturated solution, grows faster in some
directions than in others. Under shear strain, slippage tends to occur along planes par-
allel to those developed at the surface and along planes intersecting these at regular
angles. Under shock, cleavage occurs along such planes. In addition, the compressibil-

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