Past exam of the materials science course of the University of Cambridge / 2018 / Paper 1 / 2 / b

For a conventional initially random, untextured piezoceramic, orientational averaging suppresses the macroscopic linear response. Electrical poling of ferroelectric domains is the usual way to generate a lasting net response. Under that conventional preparation assumption, a ferroelectric constituent is the expected route.

However, the word polycrystalline alone does not logically require ferroelectricity. A deliberately textured aggregate of nonswitchable piezoelectric grains can have a nonzero effective tensor if their orientations and polar directions do not cancel. Ferroelectricity means electrically switchable spontaneous polarization, whereas piezoelectricity means linear stress–charge coupling. Thus the unconditional statement that every polycrystalline piezoelectric must be ferroelectric is too strong.

No. A piezoelectric single crystal need not be ferroelectric. A noncentrosymmetric crystal can couple strain to polarization without having a spontaneous polarization that an electric field can switch. Quartz is a standard example.

A single crystal already has an oriented crystallographic tensor, so the random-grain cancellation issue does not arise. The relevant tests are crystal symmetry and the nonzero piezoelectric coefficients; ferroelectric switching is a separate property.