Looking at unit cells
To specify the complete structure of a crystalline solid it is only necessary to show one unit cell, but interpreting these
pictures requires practice. Figure 1 shows some views of the cesium chloride structure (CsCl, depicted as MX).
(a) Figure 1a is a perspective view (more correctly known as a clinographic projection), which is the most
common way of showing a unit cell.
(b) Figure 1b shows a projection down one axis of the cell. The position of an atom on the hidden axis is
given by specifying a fractional coordinate (e.g. 0.5 for the central atom showing it is halfway up). No
coordinate is given for atoms at the base of the cell.
(c) Figure 1c shows the atoms shifted relative to the unit cell, and emphasizes the fact that what is important
about a unit cell is its size and shape; its origin is arbitrary because of the way in which it is repeated to fill
space.
(d) In Figure. 1d the drawing has been extended to show some repeated