Solids Measurements of crystal structures are performed
Minimal motion using X–ray diffraction spectroscopy
Long range structural order (longerthanliquids)
See fig. B12.1 p. 447
Crystalline solids = solids with long range internal
Particles are located at specific sites within the crystal.
Amorphous solids = considerable disorder (like glass)
2011 Nobel Prize in Chemistry – Dan Shechtman
Quasicrystals – crystals with regular patterns (follow
mathematical rules) but never repeat themselves.
Crystal structure Consider the following 2D pattern
To describe the structure of crystals, we need 3D
We use three sets of parallel planes called a lattice that
will provide a 3D system for representing the position
What is the unit cell?
of particles (atoms, ions, molecules).
Several choices are possible. The usual convention is
to choose the one that indicates all of the symmetry
A unit cell is the smallest portion of the crystal that
present in the lattice (beyond the scope of this course).
when stacked together repeatedly without gaps can
reproduce the entire crystal.
Not smallest ! 5 6
There are 7 systems and 14 types of unit cells that Cubic lattice
occur in nature. All planes are equidistant
A cube has 8 corners
s eca f 6
r e t nec 1
Stacking cubes next to each other without gaps, how
many cubes does one corner share? A face, an edge or
We'll deal mainly with the cubic lattice but tetragonal
and orthorhombic with all 90 can be on exams.
For the cubic lattice, 3 types occur in nature. Simple cubic
See Silberberg fig. 12.27 (top) The centers of 8 identical particles (often atoms)
define the corners of a cube.
Coordination number = how many particles are