X-ray diffraction measurements of period 4 elements, including Zn, Ge, As, and Se, were extended up to pressures of 251, 249, 250, and 317 GPa, respectively, and the structural phase transitions were investigated. The hcp phase of Zn was stable up to 251 GPa, while a decrease in the optical reflectance of the sample above 50 GPa suggests a change in the electronic structure, such as a semimetallic transition. Ge transformed from the Cmca phase to the hcp phase with a volume reduction (−ΔV) of approximately 1% at 180 GPa. In the stable region of the Cmca phase, the hcp phase, which has a larger atomic volume than the Cmca phase, coexisted with the Cmca phase, and both atomic volumes reversed at approximately 180 GPa. For As, the monoclinic host–guest phase transformed to the bcc phase with −ΔV = 2.6% at 126 GPa. The β-Po-type phase of Se transformed to the bcc phase with −ΔV = 1.9% at 140 GPa, and the bcc phase was stable up to 317 GPa although a bcc–fcc phase transition was theoretically predicted. The equations of state of the monatomic metallic high-pressure phases were determined. The atomic volume of the monatomic metallic phases for period 4 elements increased with increasing atomic number, and the systematicity reported for period 5 elements was observed. Moreover, in the monatomic metallic phases of typical group 12–17 elements for periods 4 and 5, the atomic volume is ordered with increasing atomic number above 200 GPa.

The structural phase transitions of tellurium (Te) are investigated at pressures of up to 330 GPa at 298 K using an X-ray powder diffraction technique. In the experiments, it was found that the high-pressure bcc phase (Te-V) transitioned to the fcc phase (Te-VI) at 99 GPa, although a double hcp phase (dhcp) coexisted with the fcc phase. As the pressure was increased and decreased, the dhcp phase vanished at 255 and 100 GPa, respectively. These results suggest that the dhcp phase is metastable at 298 K and the structure of the highest-pressure phase of Te is fcc. The present results provide important information regarding the high-pressure behavior of group-16 elements.