Pyroelectricity

Pyroelectricity is the development of an electrical voltage in a crystal when its temperature changes. The mechanism is essentially the same as that of piezoelectricity: a change in temperature distorts the crystal lattice in a way that displaces positive and negative charge centres relative to each other, generating a separation of charge and therefore a voltage. The two phenomena differ only in the trigger - mechanical deformation for piezoelectricity, heat for pyroelectricity - and this difference in trigger has a profound consequence for which crystal symmetries are capable of the effect. ^[1]

Why a Unique Polar Direction Is Required

The critical constraint for pyroelectricity is that the crystal must possess a single, unique polar direction. The reason is the non-directional nature of heat. When a crystal is compressed mechanically to induce piezoelectricity, the pressure can be applied deliberately along one specific direction - for instance, along one of the a-axes of quartz, which are 2-fold axes. Because the force has a defined direction, it picks out only the piezoelectric response along that axis. ^[1]

Heat, by contrast, acts in all directions simultaneously and equally. Applying heat to quartz is therefore equivalent to applying uniform pressure in all directions at once. Quartz has multiple equivalent a-axes related by its higher symmetry, so the piezoelectric voltages generated along each axis are equal and point in symmetrically equivalent but mutually cancelling directions. The net result is zero voltage. For the same reason, quartz is piezoelectric but not pyroelectric. ^[1]

A mineral becomes pyroelectric only when there is a single unique polar direction that has no crystallographically equivalent counterpart. In that case, uniform heating produces a lattice distortion associated with that unique direction, and the resulting charge displacement cannot be cancelled. The 10 pyroelectric crystal classes are a subset of the 20 piezoelectric classes - they are those with no symmetry, a single mirror plane, or a single rotation axis (with or without parallel mirrors). These minimal symmetry conditions are the ones that permit a single unique polar direction to exist.

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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