How Can Surface-Crystallized Glass-Ceramics Be Piezoelectric?

The surface crystallization of non-ferroelectric, polar crystals from a glass often leads to piezoelectric surface layers. The surface layer not only shows a crystallographic orientation preference (texture) but also prefers one polarity during crystal growth. This cannot be explained by the common periodic bond chain theory or Bravais–Friedel–Donnay–Harker law. For the most frequently studied polar crystals in glass-ceramics, fresnoites of the general composition (Ba/Sr)2Ti(Si/Ge)2O7 or lithium disilicate (Li2Si2O7), the dipoles are, respectively, aligned antiparallel or parallel to the dominant growth direction after crystallization. Hence, the polarity of the crystals cannot be the decisive factor for texture development. Taking the structure of the respective unit cells into account shows that these crystals grow with the corners of coordination polyhedra oriented toward the bulk. This is in agreement with the coordination polyhedra rule, according to which crystal growth along the direction of polyhedral corners should be preferred while the edges show the second-highest growth rate. Furthermore, the planes parallel to the face of the coordination polyhedron undergo the slowest growth.