Abstract
A series of in situ synchrotron X-ray diffraction (XRD) measurements were carried out, combined with first-principles calculations, to study structural phase transitions of selenium at high pressures and room temperature. Several phase transitions were observed, among which an isostructural phase transition was found at around 120 GPa for the first time. Evolved from the rhombohedral (space group R 3 m) structure (Se-V), the new phase (Se-V') exhibited an interesting increase of lattice parameter a at pressures from 120 to 148 GPa, known as negative linear compressibility (NLC). The discovery of NLC behavior observed in this work is mainly attributed to the accuracy and fine steps controlled by the membrane system for in situ XRD data collected with an exposure time of 0.5 s. After 140 GPa, a body-centered cubic (b.c.c.) structure Se-VI (space group Im 3 m) was formed, which remains stable up to 210 GPa, the highest pressure achieved in this study. The bulk moduli of phases Se-V, Se-V' and Se-VI were estimated to be 83 ± 2, 321 ± 2 and 266 ± 7 GPa, respectively, according to the P-V curve fit by the third-order Birch-Murnaghan equation of state. The Se-V' phase shows a bulk modulus almost 4 times larger than that of the Se-V phase, which is mainly due to the effect of its NLC. NLC in a higher pressure range is always more significant in terms of fundamental mechanism and new materials discovery, yet it has barely been reported at pressures above 100 GPa. This will hopefully inspire future studies on potential NLC behaviors in other materials at ultra-high pressure.
Highlights
Selenium, as well as other elements in the main group VIA, has been extensively studied in the field of materials science owing to its unique properties such as high photoconductivity, nonlinear optical response, large piezoelectricity and thermoelectric effect (Jafar et al, 2016; Saleh et al, 2017; Dieulesaint & Royer, 1982; Henkels & Maczuk, 1953)
An isostructural phase transition from Se-V to Se-V0 was observed for the first time around 120 GPa, and the newly discovered Se-V0 phase was estimated to have the largest bulk modulus of 321 Æ 2 GPa, compared with that of Se-V (83 Æ 2 GPa) and Se-VI (266 Æ 7 GPa)
The largest bulk modulus of Se-V0 indicates its lowest compressibility among the three phases, which can be attributed to the occurrence of negative linear compressibility (NLC) during compression
Summary
As well as other elements in the main group VIA, has been extensively studied in the field of materials science owing to its unique properties such as high photoconductivity, nonlinear optical response, large piezoelectricity and thermoelectric effect (Jafar et al, 2016; Saleh et al, 2017; Dieulesaint & Royer, 1982; Henkels & Maczuk, 1953). Its intriguing inner structural behavior under high pressure which leads to a rich sequence of pressure-induced phases attracts numerous studies focusing on selenium under high pressure (Akahama et al, 2021, 1993, 1992a; Brazhkin et al, 1992; Kruger & Holzapfel, 1992; Nishikawa et al, 1996, 1993; Pal et al, 2015; Parthasarathy & Holzapfel, 1988; Hejny & McMahon, 2004; Bandyopadhyay & Ming, 1996; Dai et al, 2011). Various pressure-induced phase-transition sequences have been reported on trigonal Se. Parthasarathy & Holzapfel (1988) reported the Se of the trigonal structure directly transformed to the monoclinic phase at 14 GPa, to the tetragonal phase at 28 GPa. Akahama et al (1993) reported that compressed trigonal Se experienced an intermediate phase from 14 to 23 GPa, and transferred to a base-centered monoclinic phase at 23 GPa, i.e. trigonal ! Metallic base-centered monoclinic phase (Se-III, space group C2/m, 23 GPa) ! Phase Se-VI was reported to be stable up to 317 GPa, the highest recorded experimental pressure value for Se to the best of our knowledge (Akahama et al, 2021)
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