Certain crystalline materials, namely, Rochelle salt, quartz and tourmaline exhibit the piezoelectric effect i.e., when we apply an a.c. voltage across them, they vibrate at the frequence of the applied voltage.
Again, when they're compressed or placed under mechanical strain to vibrate, they produce an a.c. voltage.
Similar crystal which exhibit piezoelectric effect are call piezoelectric crystal. Of the various piezoelectric crystal, quartz is most generally use because it's affordable and readily available in nature.
Quartz chargers are generally use in crystal oscillators because of their great mechanical strength and simplicity of manufacture. The natural shape of quartz crystal is hexagonal as shown in Fig-1.
The three axes are show-
The z- axis is call the optic axis, the x-axis is call the electrical axis and y- axis is call the mechanical axis. Quartz crystal can be cut in different ways.
Crystal cut vertical to the x-axis is call x-cut crystal. Whereas that cut vertical to y- axis is call y- cut crystal.
The piezoelectric parcels of a crystal depend upon its cut.
Frequency of crystal
Each crystal has a natural frequency like pendulum. The natural frequency f of a crystal is given by
f = K/ t
Where K is a constant that depends upon the cut and t is the thickness of the crystal. It's clear that frequency is inversely proportional to crystal thickness. The thinner the demitasse, the lesser is its natural frequency and vice-versa. Still, extremely thin crystal may break because of climate. This puts a limit to the frequency vibrations. In practice, frequency between 25 kHz to 5 MHz have been obtain with crystals.