Piezoelectric glass‐ceramics: Crystal chemistry, orientation mechanisms, and emerging applications

Piezoelectric materials have coupled mechanical and electrical energies and have long been used in devices for actuators, sensors, energy harvesters, frequency filters, and various additional applications. Piezoelectricity requires a non‐centrosymmetric crystal structure and is therefore confined to materials that possess a periodic crystalline structure. Due to the non‐crystalline nature of glass, piezoelectricity is fundamentally forbidden. However, one way to exploit piezoelectric properties in a glassy matrix is by developing glass‐ceramics that possess controlled growth of a crystalline phase. Growth and orientation of piezoelectric crystals in a glassy matrix is a non‐trivial process that has long been explored to combine the formability of glass with the thermal and mechanical resilience of glass‐ceramics. While extensive work has been done in the field of functional glass‐ceramics, the results are presented in isolated articles and a comprehensive review pertaining to symmetry breaking methods to exploit anisotropic properties in glass‐ceramics has been absent from the literature. Here, we present a global review of the fundamental symmetry requirements for piezoelectricity, the development of polar, piezoelectric glass‐ceramic compositions (specifically those with LiNbO 3 and fresnoite‐based crystal phases), and various crystal growth and orientation mechanisms, including relevant kinetic and thermodynamic driving forces. Lastly, we discuss the challenges associated with implementing gradients to drive oriented crystal growth to develop non‐centrosymmetry, and the need for future modeling work to produce adequate time‐temperature‐transformation (TTT) diagrams that take into account kinetic and thermodynamic driving forces for oriented crystal growth. Going beyond technical challenges, we conclude with an examination of current and potential applications for piezoelectric glass‐ceramics that combine the formability of glass with the symmetry‐dependent properties of ceramics.