Pyroelectricity is the generation of an electrical potential across a crystal in response to a uniform change in temperature. The effect occurs only in non-centrosymmetric crystals belonging to one of ten polar point groups, in which the crystal structure has a permanent net dipole moment along a unique polar axis. As the crystal warms or cools, the magnitude of this internal dipole shifts, producing surface charges of opposite sign at opposite ends of the polar axis. Pyroelectricity is most familiar in gemmology as a property of tourmaline, where it has been recognised since the early eighteenth century and remains a useful diagnostic test.

Mechanism

The internal dipole moment of a polar crystal arises from the asymmetric arrangement of positive and negative charges within the unit cell. In a centrosymmetric crystal, every charge has a counterpart symmetrically placed across the centre of inversion, and the net dipole moment is zero. In a polar crystal — one of the ten polar point groups: 1, 2, m, mm2, 3, 3m, 4, 4mm, 6, and 6mm — no such symmetry exists along the polar axis, and the internal charge distribution sums to a finite dipole. This dipole is partly compensated under steady-state conditions by adsorbed ions and atmospheric charges, so the macroscopic polarisation is not normally observed. A change in temperature alters the unit-cell dimensions and atomic positions, shifting the dipole moment and producing transient surface charges that can be detected before they neutralise.

The strength of the pyroelectric response is characterised by the pyroelectric coefficient, the change in surface charge per unit temperature change per unit area, with units of coulombs per square metre per kelvin. Tourmaline has a pyroelectric coefficient of approximately 4 microcoulombs per square metre per kelvin along its c-axis, comparable to many ferroelectric materials. The effect is necessarily reversible: cooling produces charges of opposite sign to those produced by heating.

Historical observation

The pyroelectric properties of tourmaline were noted by Dutch traders in the early eighteenth century, who observed that crystals heated in the embers of a fire attracted ash and dust. The mineral was known in Sri Lanka and the Netherlands at the time as aschentrekker, the ash-puller, and the property was sufficiently striking to attract the attention of European naturalists. Carl Linnaeus included tourmaline in his early classifications partly on the basis of its electrical behaviour, and the systematic study of pyroelectricity emerged in the nineteenth century alongside work on piezoelectricity by the Curie brothers.

Relationship to piezoelectricity

Pyroelectricity is closely related to piezoelectricity, the generation of electrical charge by mechanical stress. All pyroelectric crystals are necessarily piezoelectric, but not all piezoelectric crystals are pyroelectric — the pyroelectric subset is the ten polar point groups out of the twenty piezoelectric point groups. Quartz, for example, is piezoelectric but not pyroelectric, because its trigonal crystal class lacks the polar axis necessary for the pyroelectric effect. Tourmaline is both, and its dual electrical response has made it useful in pressure transducers, infrared sensors, and a range of specialised technical applications beyond its gemmological role.

A subset of pyroelectric materials are also ferroelectric, meaning the spontaneous polarisation can be reversed by an applied electric field. Tourmaline is not ferroelectric, but barium titanate, lithium niobate, and several synthetic crystals are. Ferroelectric crystals find use in non-volatile memory and electro-optical modulators, applications outside the scope of gemmology but reflecting the same underlying polar symmetry.

Diagnostic use

The pyroelectric test for tourmaline is a long-standing field and laboratory diagnostic. A clean tourmaline crystal heated to about 100 degrees Celsius and allowed to cool will attract small particles such as ash, paper fragments, or sulphur powder to its terminations. The test is rarely used in modern laboratory practice because better methods are available, but it remains a useful demonstration in teaching collections and an occasional field test where instrumentation is unavailable. The effect is most easily observed on long prismatic crystals where the polar axis is well-defined and the terminations are accessible.

Further reading

• GIA — tourmaline reference
• International Gem Society — tourmaline and electrical properties
• GIA Gems & Gemology — crystal physics articles