If stress is applied to certain crystalline materials such as quartz, equal and opposite charges appear on opposite faces of the sample. This phenomenon is called piezoelectricity. Conversely, if a potential difference is applied across the sample, it will change shape (as if it had compressed or stretched along a certain axis). This effect is used widely in manufacturing transducers and crystal oscillators. Devices such as gramophone pick-ups, ultrasonic transducers, accelerometers, microphones and load cells may all employ piezoelectric sensors to convert forces into electrical signals. The reverse effect is employed in the crystal control of electrical oscillator circuits that are used in clocks and precision measurement apparatus.
Piezoelectricity occurs almost exclusively in ionically bonded crystalline solids. In such materials there exists an ordered array of positive and negative ions. If an applied stress produces a strain or distortion of the crystal lattice that displaces one type of charge in a particular direction with respect to the other type an electric field or dipole moment will be produced. Simple crystals such as common salt which has a simple cubic arrangement do not exhibit this property. However, more complicated classes of crystal structure that lack a center of symmetry are piezoelectric. The simplest structure of this type is sphalerite, or zinc blende (ZnS) (Figure 1a). The diagram (Figure 1b) shows that a shearing stress may result in compression of the cubic cell along one horizontal diagonal axis and extension along the other diagonal. As the zinc atoms a, b and c move apart the two sulfur atoms d and e will tend to move upwards. Also, as the three zinc atoms p, q and r are compressed together the two sulfur atoms s an t will again move upwards. The overall effect is a total upward displacement of the negatively charged ions in the diagram. Hence, the top of the specimen will assume a negative charge and the bottom will have an equal positive charge.
Piezoelectricity is also found in all ferroelectric materials. Detailed analysis of this phenomenon is found in Kittel (1971). These are crystalline structures that possess a permanent dipole moment. This means that for a number of reasons the centers of positive charges in the unit cells of the crystal structure are permanently displaced with respect to the centers of negative charge. If all these individual crystal cell dipoles are aligned, then the entire specimen will exhibit a permanent electric field. Such a system is called an electret (c.f. magnet). In practice, an electret will accumulate free charge from its environment that will after a short time neutralize the external fields (i.e., free negative charge will accumulate on the positive "pole" face etc.) If, however, the material is compressed along an axis parallel to the internal field, thereby altering the dipole separation, the internal field will change. The field change will be observed externally for a short time before the accumulated charge on the faces can change to neutralize the new field. A fuller treatment of ferroelectricity is given by Feynman (1964).
Feynman, R. P.. Leighton R. B.,and Sands M. (1964) The Feynman Lectures on Physics, Addison Wesley.
Kittel, C. (1971) Introduction to Solid State Physics, John Wiley.