This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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In general, ~90% elemental metals crystallize into three crystal structures which are BCC, FCC, and HCP.

The gap between the top of valence band and the bottom of conduction band is defined as energy gap E_g. In insulators, the energy gap is very large and no vacant conduction band is available to the valence electrons. Otherwise, the energy gap is either zero due overlapping between valence and conduction bands for metals or is very small for semi-conductors so that conduction band is easily available to valence electrons.