Zinc Oxide - Wurtzite Structure, Hexagonal Lattice Constants 3.25å X 5.2å , High Piezoelectric And Pyroelectric Properties

Zinc oxide crystallizes in two main forms, hexagonal wurtzite[14] and cubic zincblende. The wurtzite structure is most stable at ambient conditions and thus most common. In addition to the wurtzite and zincblende polymorphs, ZnO can be crystallized in the rocksalt motif at relatively high pressures about 10 GPa.

Hexagonal and zincblende polymorphs have no inversion symmetry (reflection of a crystal relative to any given point does not transform it into itself). This and other lattice symmetry properties result in piezoelectricity of the hexagonal and zincblende ZnO, and pyroelectricity of hexagonal ZnO.

The hexagonal structure has a point group 6 mm (Hermann-Mauguin notation) or C6v (Schoenflies notation), and the space group is P63mc or C6v4. The lattice constants are a = 3.25 A and c = 5.2 A ; their ratio c/a ~ 1.60 is close to the ideal value for hexagonal cell c/a = 1.633.[16] As in most group II-VI materials, the bonding in ZnO is largely ionic (Zn2+a O2a ) with the corresponding radii of 0.074 nm for Zn2+ and 0.140 nm for O2a . This property accounts for the preferential formation of wurtzite rather than zinc blende structure,[17] as well as the strong piezoelectricity of ZnO. Because of the polar Zn-O bonds, zinc and oxygen planes are electrically charged. To maintain