Introduction

A crystal is a regular array of atoms or molecules. Crystals are important because most of the solids around us (and in technological applications) are crystalline phases. The crystalline nature of these materials has a significant effect on their properties, so to understand solid-phase behaviors from the atomic perspective it is necessary to understand the basic geometric features of crystals. To this end there can be no substitute for visualization of the crystalline lattice.

Physical models formed from plastic balls joined by glue or sticks offer one means to visualize lattices. Such models are tangible, and can be held in one's hands, rotated and studied from all angles. However, their substance and rigidity also limits the type of views that one can take on them. They cannot be observed from the inside, or deformed to provide alternately expanded (atoms far apart) and condensed (atoms touching) perspectives. The aim of this module is to provide this capability using computer-based graphics, and thereby complement the physical models as a means for understanding lattice structures.

Perhaps more important than the bulk crystal itself is the nature of the crystalline surface. Of course, anything that interacts with a solid phase (i.e., a crystal) must first encounter its surface, so features particular to the surface greatly affect how crystals interact with their surroundings, as well as how they grow. Crystals are anisotropic, so their surfaces---and thus their surface properties---differ markedly when taken at different angles. This module is designed also to help in visualizing and understanding issues regarding crystal surfaces. Here it is possible to view any surface of a variety of crystalline lattices. The surfaces are specified in terms of the miller index, so while examining surface features one can gain familiarity with this important notational convention.