polar crystal
/ POH-ler KRIS-tul /
Most crystals are perfectly even-handed: their atoms are arranged so symmetrically that 'up' is no different from 'down' and there is no special direction. A polar crystal breaks that even-handedness. Its atoms are stacked so that one direction is genuinely different from the opposite one, leaving the crystal with a built-in electric arrow — a permanent direction of charge separation.
This comes down to symmetry. In a polar crystal the repeating arrangement of atoms has no center that flips the structure onto itself, and no symmetry that would reverse the special axis. As a result the centers of positive and negative charge in each unit don't coincide, and those tiny dipoles all point the same way, giving the whole crystal a net dipole direction even with no field applied.
This matters because being polar is the gateway property: only polar crystals can be pyroelectric, and the ferroelectrics — whose polarization you can switch — are a special subset of polar crystals. The common confusion is between 'polar' and 'charged'; a polar crystal is overall electrically neutral, like a water molecule. It simply has a direction baked in, the way a magnet has a north and south even though it carries no net charge.
Zinc oxide is a polar crystal: along one axis the layers alternate zinc and oxygen so that the two ends of a crystal are genuinely different, one face tending positive and the other negative. That built-in direction is why zinc oxide is both pyroelectric and piezoelectric.
In zinc oxide, alternating layers make the two ends of the crystal genuinely different — a polar crystal.
A polar crystal is electrically neutral overall; 'polar' refers to a built-in direction of charge separation, not to carrying a net charge.