Tunnel-structured willemite Zn2SiO4: Electronic structure, elastic, and thermal properties

Abstract

Willemite Zn2SiO4 crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO4 and SiO4 tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO4 tetrahedron connects eight ZnO4 tetrahedra, and one ZnO4 tetrahedron links four ZnO4 tetrahedra and four SiO4 tetrahedra. The unique crystallographic configuration gives rise to parallel tunnels with a diameter of 5.7 Å along the c-axis direction. The tunnel structure of Zn2SiO4 definitely correlates with its interesting elastic and thermal properties. On the one hand, the elastic modulus, coefficient of thermal expansion (CTE), and thermal conductivity are low. Zn2SiO4 has low Vickers hardness of 6.6 GPa at 10 N and low thermal conductivity of 2.34 W/(m·K) at 1073 K. On the other hand, the elastic modulus and CTE along the c-axis are significantly larger than those along the a- and b-axes, showing obvious elastic and thermal expansion anisotropy. Specifically, the Young’s modulus along the z direction (Ez = 179 GPa) is almost twice those in the x and y directions (Ex = Ey = 93 GPa). The high thermal expansion anisotropy is ascribed to the empty tunnels along the c-axis, which are capable of more accommodating the thermal expansion along the a- and b-axes. The striking properties of Zn2SiO4 in elastic modulus, hardness, CTE, and thermal conductivity make it much useful in various fields of ceramics, such as low thermal expansion, thermal insulation, and machining tools.