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

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 (E_z = 179 GPa) is almost twice those in the x and y directions (E_x = E_y = 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.