Raman scattering under pressure and the phase transition in ε‐GaSe

Abstract

Raman scattering in ε-GaSe has been investigated at room temperature in a diamond anvil cell by using helium gas as a pressure transmitting medium. The first observation of the solid − solid phase transition under pressure from a hexagonal structure at ambient pressure to a NaCl-type at around 29.2 GPa is reported. A photoinduced change in the pressurised crystal was observed. The transition under pressure was found to be irreversible at room temperature. It was shown that the pressurised form of ε-GaSe at ambient conditions most probably belongs to the γ-type as characterised by an increased intensity of the Raman modes at 234 cm −1 and 250 cmGallium selenide is a layer semiconductor. Four distinct modifications, the β-, the ε-, the γ-and the δ-types were reported for GaSe, differing by the stacking order of the two-dimensional Se−Ga−Ga−Se sheet [1]. The differences between these types arise from the stacking of layers as well as from the number of layers per unit cell. The ε-modification (D 3h 1 space group (SG), containing two layers per unit cell) is the most common and best studied. Up to now the existence of the β-modification (D 6h 4 SG, two layers per unit cell) has not been confirmed. The γ-type has three layers (C 3v 5 SG), and the δ-type contains four layers per unit cell (C 6v 4 SG). Normally the crystals grown by the Bridgman method are a mixture of ε-and γ-types, with a predominant content of the former.Most common and well investigated among the different polytypes is ε-GaSe, which is one of the promising materials for far-infrared conversion applications [2]. The properties of ε-GaSe at ambient conditions (room temperature, atmospheric pressure) are reasonably well known. Several Raman scattering experiments under pressure have already been performed on GaSe [3−6]. The pressure dependence and the mode Grüneisen parameters for Raman-active phonons of ε-GaSe have been measured up to 1 GPa [3, 4], 3.5 GPa [5], and 8 GPa [6]. Recently [7,8] high-pressure X-ray powder diffraction studies have been made on ε-GaSe. It was shown that at pressures of 29 GPa [7] and of around 25 GPa [8] the crystal undergoes a phase transition (PT) to the NaCl-type structure. The reason for this discrepancy is

Effect of pressure on the structural properties and electronic band structure of GaSe

“…That phase is almost fully developed at 27.8 GPa. The pressure-induced phase transition is not reversible on releasing the pressure, in accordance with results of, e.g., HP Raman studies [10].…”

“…Different pressures have been reported in the literature for the ε-phase to cubic transition of GaSe, ranging from 16 to 29 GPa [3,4,10,12]. It is believed that the large pressure span in part arise from the stress conditions in a particular experimental run, i.e.…”

“…Recent HP experiments performed on the most common ε -polytype of GaSe have addressed the structural properties [3,4], the dielectric response [5], band gap changes including a band crossing from direct to indirect [6][7][8], the photoluminescence [9], the Raman scattering [10], and the electric transport behavior [11]. Furthermore, the transition of GaSe at around 20 GPa to a metallic HP phase [12] has been characterized by X-ray diffraction [13,14], X-ray absorption [3], and Raman scattering [10].…”

Synthesis and Characterization of Novel Glycerol‐Derived Polycarbonates with Pendant Hydroxyl Groups

“…Raman spectrum of bulk GaSe consists of three intense bands and one very weak band, which are characteristics of the GaSe single crystal. Raman spectrum of bulk GaSe crystal consists intense peaks for the A 1 1g mode (133 cm −1 ), E 1 2g mode (211.9 cm −1 ), and A 2 1g (306.8 cm −1 ), in addition to one weak peak for the mode E 2 1g (250 cm −1 ) 31–34. In an ultrathin layer of GaSe the Raman scattering signals from the A 1 1g mode and E 1 2g mode disappeared, the E 2 1g (256 cm −1 ) mode become more intense and the mode A 2 1g (301 cm −1 ) was observed to be shifted towards the lower wavelength side.…”

Boron monosulfide: Equation of state and pressure-induced phase transition

“…According to the literature data, phase transitions in the 10-30 GPa pressure range are rather typical for A III B VI layered compounds [1,3,36,[39][40][41]. Nevertheless, the crystal structures of the new high-pressure A III B VI phases have not been experimentally refined so far.…”