1. In the solid state literature ionic crystals are often called “polar” crystals. Note that in this review the word “polar” is not used in this sense.
2. G. Goodman, unpublished.
3. “Piezoelectricity.”;Cady,1946
4. Every polar crystal is piezoelectric, but not every piezoelectric crystal is polar.
5. “Piezoelectricity”;Cady,1946