Polar point group
In geometry, a polar point group is a point group in which every symmetry operation leaves more than one point unmoved. Therefore, a point group with more than one axis of rotation or a mirror plane perpendicular to the axis of rotation cannot be polar.
A straight line joining unmoved points defines a unique axis of rotation, unless symmetry operations do not allow any rotation at all, such as mirror symmetry, in which case, the polar direction must be parallel to any mirror planes.
|Crystal system||Polar point groups|
When materials having a polar point group crystal structure are heated or cooled, they may temporarily generate a voltage called pyroelectricity.