Volume compression of period 4 elements: Zn, Ge, As, and Se above 200 GPa: Ordering of atomic volume by atomic number

Abstract

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.

Negative linear compressibility in Se at ultra-high pressure above 120 GPa

“…More P-V data, especially at a higher-pressure range between 210 and 317 GPa would achieve a better estimation of the bulk modulus. For easy comparison, the derivative of the bulk modulus B 0 was fixed at 3.2 during the fitting in our case, the same as the value of the Se-VI phase obtained in the recent report by Akahama et al (2021). However, due to the flattening in the P-V curve near 300 GPa, their reported bulk modulus of Se-VI is slightly higher than the value from the current report.…”

“…All the other fitting parameters are shown in Table 1. Recently, Akahama et al (2021) reported the bulk modulus of Se-VI as 290 GPa using the diffraction data up to 317 GPa. Generally, more compression data covering a wider pressure region could yield a more precise bulk modulus.…”

“…In the higher-pressure range, the phase transition from an incommensurately modulated monoclinic structure to a -Po type structure at 40-60 GPa was observed, whereas McMahon et al (2004) suggested this transition occurs at 82 GPa by extrapolating the monoclinic structural parameters, and Degtyareva et al (2005a) experimentally observed this phase transition at 80 GPa. So far, about seven selenium allotropes related to high-pressure and high-temperature conditions have been reported experimentally (Degtyareva et al, 2005b(Degtyareva et al, , 2007Akahama et al, 2021). Therefore, the pressure-induced phase transition sequence of trigonal Se should be as follows: trigonal phase (Se-I, space group P3 1 21) !…”

“…Among the inconsistencies with pressure-induced phasetransition sequences, the specific phase transition from the Se-V phase to the Se-VI phase has drawn much theoretical and experimental attention. Though Akahama et al (2021Akahama et al ( , 1993Akahama et al ( , 1992b reported the Se-V to Se-VI transitional pressure is around 140 GPa in experiment, many calculations indicate there might be a phase transition around 120, 110 GPa or even as low as 95 GPa. Rudin et al (2001) predicted the phase transition occurrs at 120 GPa based on density-functional theory (DFT).…”

Negative linear compressibility at extreme pressure

“…Below 60 GPa selenium exists in four various crystal phases. According to Akahama et al (2021), the rhombohedral phase named Se-V (space group R3m, -Po structure) occurs between 60 GPa and 140 GPa, where it transforms to the Se-VI b.c.c. phase, stable up to at least 317 GPa.…”