there are also a number that are nor birefringent naturally but in which application of a voltage induces birefringence.
this phenomenon is called the electro-optic effect.
the electro optic effect leads to ability to control light beams in a variety of ways and is the basis of a number of applications,including light-beam modulators and deflectors.
examples of crystals that exhibit the electro-optic effect are potassium dihydrogen phosphate (commonly called KDP),ammonium dihydrogen phosphate(ADP)