Structural study of Zn and Cd to ultrahigh pressures

High-pressure powder x-ray diffraction experiments on Zn and Cd have been carried out at room temperature to above 100 GPa with high precision. Both metals remain in the hcp structure up to the highest pressures investigated (126 GPa for Zn and 174 GPa for Cd). The c/a axial ratios continuously decrease with pressure down to a value of 1.59. We have observed a clear change in the slope of the volume dependence of the axial ratio at c/a=√3 for both metals. The anomaly may be related to the electronic topological transition observed in the M\"ossbauer study on Zn under pressure at low temperature, but the special value (√3) of the axial ratio is difficult to explain simply by the electronic topological transition. The hcp structure with the axial ratio of √3 has unique symmetry both in real and reciprocal spaces. This suggests a universal change in the bonding properties of the hcp structure at c/a=√3. The bulk modulus and its pressure derivative of Zn and Cd have been determined over a large pressure range.