Toward an Understanding of the Anisotropy in Hcp Zinc Metal: Total Scattering Structural Study Using Synchrotron-Based, Temperature-Dependent, X-Ray Pair Distribution Function Analysis

Abstract

Among hexagonal close packing (hcp) metals, zinc has a non-ideal hcp structure due to a significantly increased axial ratio c/a. We studied the behavior of the ratio of lattice constants c/a and the ratio of thermal displacement parameters (U_33 / U_11) in the temperature range of 100-300 K, using the X-ray total scattering atomic pair distribution function (PDF) analysis over different refinement r-ranges. The temperature-dependent PDF analysis confirmed that the crystal structure of zinc has significant static distortion along the c-direction. The measured value of atomic displacement parameters of zinc showed a notable anisotropy as expressed by the ratio U_33 / U_11 of ≈ 2.2 at 100 K. The lattice parameter ratio c/a was slightly reduced at low temperatures but remained unusually large. The extrapolated value of the ratio c/a reached 1.832 at 0 K. The PDF refinements using the crystallographic model revealed that there are some local structure features in zinc that are not captured by the crystallographic model. On the other hand, our local structure refinement indicated a presence of local bond distortion along the z-direction that averaged out over wider r-range refinements with a large U_33 value. Keywords: Hcp anisotropy, Zinc metal, Total scattering, X-ray scattering, Pair distribution function. PACs numbers: 61.46.Df, 61.10.-i, 78.66.Hf, 61.46.-w.