Quartz crystal is a representative piezoelectric material with the chemical composition SiO₂ and crystallizes in the hexagonal crystal system. Its macroscopic morphology is typically a regular hexagonal prism. The crystallographic orientation of quartz is defined by three mutually orthogonal axes: the x-axis, y-axis, and z-axis. The z-axis, referred to as the optical axis, coincides with the longitudinal axis of the crystal. The x-axis, also known as the electrical axis, is perpendicular to the optical axis and passes through two opposite edges of the hexagonal prism, while the y-axis, termed the mechanical axis, is perpendicular to the pair of opposing prismatic faces.
The piezoelectric behavior of quartz originates from its non-centrosymmetric crystal structure. Each unit cell consists of three silicon ions and six oxygen ions, arranged in an alternating configuration, with oxygen ions forming paired groups. When observed along the optical axis, the atomic arrangement can be equivalently described as a regular hexagonal lattice, resulting in charge displacement under applied mechanical stress and thereby inducing electric polarization.
Owing to its intrinsic piezoelectric properties, high mechanical stability, and excellent frequency–temperature characteristics, quartz crystal is extensively employed in precision frequency control devices, including crystal resonators, oscillators, and sensing elements, where high stability and reliability are critical.
