Joint experimental and theoretical study of PbGa2S4 under compression

The effect of pressure on the structural, vibrational, and optical properties of lead thiogallate, PbGa2S4, crystallizing under room conditions in the orthorhombic EuGa2S4-type structure (space group Fddd), is investigated. The results from X-ray diffraction, Raman scattering, and optical-absorption measurements at a high pressure beyond 20 GPa are reported and compared not only to ab initio calculations, but also to the related compounds α′-Ga2S3, CdGa2S4, and HgGa2S4. Evidence of a partially reversible pressure-induced decomposition of PbGa2S4 into a mixture of Pb6Ga10S21 and Ga2S3 above 15 GPa is reported. Thus, our measurements and calculations show a route for the high-pressure synthesis of Pb6Ga10S21, which is isostructural to the stable Pb6In10S21 compound at room pressure.

Experimental.Single orange plate crystals of PbGa2S4 were used.A suitable crystal with dimensions 0.12 × 0.11 × 0.04 mm 3 was selected and mounted on a SuperNova, Dualflex, EosS2 diffractometer.The crystal was kept at a steady T = 293(2) K during data collection.The diffraction data was collected with the CrysAlis Pro software [S1] and the structure was solved with the ShelXT 2018/2 [S2,S3] solution program using dual methods and by using Olex2 [S4] as the graphical interface.The model was refined with ShelXL 2018/3 using full matrix least squares minimisation on F 2 .
The structure was solved and the space group Fddd (# 70) determined by the ShelXT 2018/2 [S2,S3] structure solution program using using dual methods and refined by full matrix least squares minimisation on F 2 using version 2018/3 of ShelXL 2018/3 [S2,S3].All non-hydrogen atoms were refined anisotropically.Sulfur atom positions were calculated geometrically and refined using the riding model.
Figure S1.Detail of the crystal structure of PbGa2S4 along the b and c axes.Pb, Ga, and S atoms are big gray, small gray, and orange circles, respectively.

Figure S2 .Figure S3 .
Figure S2.Detail of the crystal structure of PbGa2S4 in a perspective that highlights its layered characteristic.GaS4 tetrahedra are shown in green and PbS8 square antiprismatic polyhedra is black.Sulfur atoms are shown in yellow.

Figure S4 .
Figure S4.Normalized polarized Raman spectra of PbGa2S4 at high pressure up to 24 GPa.

Figure S5 .
Figure S5.Pressure dependence of the experimental (symbols) and theoretical (lines) Raman frequencies of Pb6Ga10S21 during compression.The red (blue) lines correspond to the theoretical Ag (Bg) modes.

Figure S6 .
Figure S6.(Top) Zoom of the total and partial electronic density of states of PbGa2S4 at 0 GPa.(Bottom) Zoom of the total and partial electronic density of states of Pb6Ga10S21 at 15.3 GPa

Table S4
Raman-active frequencies of , 1, 2 and 3 modes in PbGa2S4 at room pressure obtained from polarized RS measurements.Data from Ref. Kamenshchikov et al. (2007) ( n o t e d w i t h s u b i n d e x k ) a r e p r o v i d e d f o r c o m p a r i s o n .