Structural anomalies in hcp metals under pressure: Zn and Cd

Abstract

First-principles total-energy calculations with WIEN2k using a procedure that finds the equilibrium states of hcp structures under pressure from minima of the Gibbs free energy have found a structural anomaly in hep Zn and in hep Cd under pressure. The calculated small but definite anomaly in the pressure dependences of the structural parameters of hep Zn allows a reinterpretation of the data to show the anomaly. We find a similar but stronger anomaly in Cd than in Zn. The calculated anomaly in Zn contradicts a recent theoretical conclusion that found that the anomaly disappears at a large number of k-points in the Brillouin zone. The calculation also shows that the uncertainty in locating equilibrium found in another recent paper is absent here. Reasons are given for computational differences from previous work. Evaluation of the pressure dependence of various elastic quantities which are much more sensitive to the anomaly shows the anomalies in hcp Zn and hep Cd exist over a considerable range of pressure; several abrupt changes in the electron distribution are thereby indicated in that pressure range.