semipolarity

Semipolarity is a concept in materials science referring to a crystal structure that possesses a net dipole moment along a specific crystallographic direction, but lacks the full symmetry required for true ferroelectricity. In a polar crystal, all atomic positions are inherently asymmetric, leading to a spontaneous electric polarization that can be reversed by an external electric field. Semipolar crystals, however, exhibit a dipole moment due to their specific atomic arrangement but do not possess the domain structures characteristic of ferroelectric materials. This means their polarization cannot be easily switched or reversed by external fields.

The distinction between polar, semipolar, and nonpolar materials lies in their point group symmetry. Nonpolar materials have centrosymmetric point groups, meaning they possess a center of inversion, and thus no net dipole moment. Polar materials belong to non-centrosymmetric point groups that allow for a spontaneous electric polarization. Semipolar materials fall into a category of non-centrosymmetric point groups that permit a dipole moment but lack the specific symmetry operations (like a 180-degree rotation or reflection through a specific plane) that would enable the reversal of this polarization in the manner of ferroelectrics.

Semipolar materials can still exhibit interesting piezoelectric and pyroelectric properties. Piezoelectricity is the ability of a material to generate an electric charge in response to applied mechanical stress, and pyroelectricity is the generation of an electric charge in response to a change in temperature. While the absence of polarization reversal limits their application in some traditional ferroelectric devices, semipolar materials are explored for their potential in sensors, actuators, and other electromechanical devices where a fixed or temperature-dependent polarization is beneficial. Examples of semipolar materials include certain wurtzite-structured semiconductors like GaN when grown on specific substrates, leading to variations in their properties depending on crystallographic orientation.