The Structural Anomaly of Zinc: Evolution of Lattice Constants and Parameters of Thermal Motion in the Temperature Range of 40 to 500 K

The crystal structure of elemental zinc deviates from the ideal hexagonal close packing by a significantly increased c / a ratio. We determined the evolution of the lattice constants and thermal displacement parameters in the temperature range of 40 to 500 K by accurate single‐crystal X‐ray diffraction. The c axis varies linearly within the whole temperature range, whereas the a lattice constant stays constant below ≈︁ 160 K, which indicates that at lower temperatures the Zn–Zn distances are no longer thermally contractible in the ab plane. As a further significant observation, the thermal expansion coefficient along the c direction exceeds the one in a direction by a factor of ~10, and consequently the c / a ratio slightly reduces, but stays unusually high ( c / a = 1.826, extrapolated for T = 0 K). The thermal motion of zinc shows a striking anisotropy as expressed by a ratio U 33 / U 11 of ≈︁ 2.6 at 500 K. This ratio decreases significantly towards lower temperatures and reaches almost 1.6 at 40 K. The thermal behavior as observed is thus reflecting the structural and bonding anisotropy of zinc at ambient conditions, but thermal motion turns isotropic at cooling, in contrast to the atomic equilibrium structure.