Vibrational properties of CdGa2S4 at high pressure

Abstract

Raman scattering measurements have been performed in cadmium digallium sulphide (CdGa2S4) with defect chalcopyrite structure up to 25 GPa in order to study its pressureinduced phase transitions. These measurements have been complemented and compared with lattice-dynamics ab initio calculations including the TO-LO splitting at high pressures in order to provide a better assignment of experimental Raman modes. In addition, experimental and theoretical Grüneisen parameters have been reported in order to calculate the molar heat capacity and thermal expansion coefficient of CdGa2S4. Our measurements provide evidence that CdGa2S4 undergoes an irreversible phase transition above 15 GPa to a Raman-inactive phase, likely with disordered rocksalt structure. Moreover, the Raman spectrum observed on downstroke from 25 GPa to 2 GPa has been attributed to a new phase, tentatively identified as a disordered zincblende structure, that undergoes a reversible phase transition to the Raman-inactive phase above 10 GPa.

Combined Experimental and Theoretical Studies: Lattice-Dynamical Studies at High Pressures with the Help of Ab Initio Calculations

“…In general, there is a good agreement between the experimental and theoretical results on structural, vibrational, and optical properties as far as pressure-induced disorder is not crucial in the LP range. The excellent agreement of experimental and theoretical results allowed us to discuss the lattice dynamical properties of these semiconductors [8][9][10]12,13] in analogy with zincblende-type semiconductors (see Figure 2). The lack of agreement between the experimental and theoretical pressure dependence of the direct bandgap energy above certain pressure has allowed us to discuss the ranges of the pressure-induced disorder process that OVCs undergo prior to their transformation to the disorder rocksalt structure at HP (in analogy with zincblende-type semiconductors).…”

“…We have published some joint experimental and theoretical studies on structural, vibrational, and optical properties of adamantine OVCs under compression [7][8][9][10][11][12][13][14]. In general, there is a good agreement between the experimental and theoretical results on structural, vibrational, and optical properties as far as pressure-induced disorder is not crucial in the LP range.…”

High-Pressure Synthesis of β- and α-In2Se3-Like Structures in Ga2S3

“…This mode is similar to the A′(6) of the α′-Ga 2 S 3 , where S atoms vibrate toward the center of the channels. Instead, such strongest peaks in the RS spectra of these DZ phases correspond to an optical mode related to the vacancies and it has no counterpart in binary zincblende AX compounds (with no vacancies in the structure), as was discussed in refs and . Therefore, on the light of the similitudes commented on here, we conclude that decompressed Ga 2 S 3 likely has the same DZ structure as γ-Ga 2 S 3 .…”

“…In this context, further HP experiments must be conducted in these compounds and especially in DZ-Ga 2 S 3 to see if it will undergo a PT to β′-Ga 2 S 3 or to the DR phase on compression. We initially expect that DZ-Ga 2 S 3 undergoes a reversible PT to the DR phase at HP, as it occurs in Ga 2 Se 3 , and also in CdGa 2 S 4 and CdGa 2 Se 4 …”

“…Finally, we will propose that the second phase observed on decompression shows a DZ structure and therefore is identified with the γ-Ga 2 S 3 phase observed at HT. Since the DZ-like phase has been already observed on the downstroke in ternary ordered-vacancy compounds (OVCs), like A Ga 2 X 4 compounds ( A = Zn,Cd,Hg; X = S,Se), with original defective chalcopyrite (DC) and defect stannite (DS), , we have reformulated the PT sequence of these ternary OVCs at HP . In this way, we propose a new PT diagram of OVCs at HP that include the family of binary Ga 2 X 3 compounds and establish that the presence of vacancies and their pressure-induced disorder connects both the LP and HP phases in both families of compounds.…”