Pressure induced lattice expansion and phonon softening in layered ReS2

Abstract

We report high pressure x-ray diffraction and systematic Raman measurements on a ReS 2 sample, which is mechanically exfoliated from a single crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a structural transition from distorted 1T to distorted 1T 0 in a triclinic structure. The same is corroborated by the appearance of new Raman modes in the same pressure range. Softening of the Raman modes corresponding to Re atom vibrations is observed in the distorted 1T 0 phase in the pressure range of 15-25 GPa. In the same pressure range, the anomalous change in the volume is found to be induced by the lattice expansion. The volume expansion is related to the sliding of layers leading to octahedral distortion and an increase in octahedral volume. The sample is found to be very incompressible above 25 GPa with respect to below 15 GPa data. The same is also reflected in the Raman mode shifts with pressure.

Structural and High-Pressure Properties of Rheniite (ReS2) and (Re,Mo)S2

“…A fit with a 3rd-order BM EoS (BM-3) yields a zero-pressure bulk modulus B 0 = 44(2) GPa, with a first derivative equal to B' = 7.7 (5). The value for B 0 is in excellent agreement with recent experimental results for this compound [23] and, as will be explained below, also with our own experimental determinations. However, a careful analysis of the normalized pressure (F) vs. strain curves (f ) that result from the theoretical P-V dependence obtained with PBE+D3 (see Figure S2a,b) suggests that a 4th-order BM EoS (BM-4) provides the best fit along the whole pressure range.…”

“…If, instead, a BM-3 EoS is employed together with the B' = 7.7 value obtained from the DFT calculations, which is still found to reasonably match the F-f dependence ( Figure S2d), the following results are obtained: V 0 = 217.9(2) Å 3 and B 0 = 46(2) GPa. These values are in excellent agreement with the PBE+D3 calculations (BM-3 fit, see Table 1) and also with the recent work by Saha et al [23], who reported a similar experimental bulk modulus (B 0 = 44(2) GPa), although with a much lower first derivative (B' = 1.6(3)). In contrast, other previous works reported much lower B 0 values of 23 GPa [21] and 35.6 GPa [22].…”

“…Natural TMDCs include rheniite (ReS 2 ), molybdenite (MoS 2 ), tungstenite (WS 2 ), or drysdallite (Mo(Se,S) 2 ). Their synthetic counterparts are currently around 6-8 GPa [23], in agreement with the HP-XRD studies, which report values around 6-11.3 GPa [21][22][23]. However, it is worth noting that the previous Raman-scattering studies only considered high-frequency modes related to intralayer vibrations rather than low-frequency interlayer modes, which should be more sensitive to the layer stacking configuration.…”

“…[26], where the structural phase transition was found around 2.5 GPa by means of Raman spectroscopy. In contrast, most experimental works have reported much larger phase transitions, typically above 5 GPa and up to 15 GPa [21][22][23]27]. The present results allow us to conclude that the onset of the phase transition for 1T-ReS 2 is as low as~2 GPa.…”

“…DFT calculations have been widely used to understand, among others, the optical [27], electrical [19] and structural [22] properties of ReS 2 . DFT studies published so far have shown to be highly dependent on the particular functional used, predicting a phase transition onset between 0.1 GPa and 3 GPa, [19,22,24] far from most experimental values, around 6-11.3 GPa [21][22][23]. The poor agreement between experimental and theoretical results highlights the importance of additional experimental and theoretical works to better constrain the pressure behavior of ReS 2 .…”