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 stability of indium was investigated under high pressures up to 247 GPa using synchrotron X-ray diffraction techniques. A reentrant structural transition from a face-centered tetragonal (fct-I with c/a > 1) to a face-centered orthorhombic (fco) and further to an fct-I structure was revealed. The first transition (from fct-I to fco) and the second transition (from fco to fct-I) were observed at approximately 50 GPa and 150 GPa, respectively. The second transition was of the second order despite the first transition being of the first order. The theoretically predicted fct-II (c/a < 1) phase was not observed at pressures up to 247 GPa. The equation of state of indium is discussed compared to that of other period 5 elements.