Thermodynamics of internal strain in perfect crystals. III. Piezoelectric and related properties

For pt. II see ibid., vol.7, no.5, 841 (1974). Previous generalizations of the thermodynamics of solids, in which internal strains are treated as independent variables, are extended to include electrical effects. Piezoelectric and pyroelectric coefficients and permittivities are defined in the general regime and in the four restricted regimes, one of which corresponds to ordinary macroscopic thermodynamics. Relations are derived between such quantities in different regimes and between thermoelastic properties under different electrical conditions. In the general regime all crystals are piezoelectric, but in crystals which are not macroscopically piezoelectric the generalized coefficients are not independent. It is shown that the previous generalizations apply even to highly polar non-piezoelectric crystals. The treatment is illustrated for two simple static-lattice models: a non-piezoelectric lattice of rigid ions and a pyroelectric chain of rigid dipoles. The present theory simplifies such calculations while clarifying the physical origins of piezoelectricity.