Phase diagrams of the quasi-binary systems Cu2S-SiS2 and Cu2SiS3-PbS and the crystal structure of the new quaternary compound Cu2PbSiS4

X-ray phase analysis was used to investigated nine alloys with the general formula Cu2PbBIVX4 where BIV - Si, Ge, Sn; X - S, Se, Te. Only one compound of such composition was found, namely Cu2PbSiS4. Using differential thermal and X-ray phase analysis results, the phase diagrams of the Cu2S-SiS2 (in 0-60 mol% SiS2 range) and Cu2SiS3-PbS systems were constructed. A congruent type of formation of Cu8SiS6 and Cu2SiS3 at 1459 and 1201 K, respectively, was confirmed in the Cu2S-SiS2 system. The Cu2SiS3-PbS system is quasi-binary with the formation of two quaternary intermediate phases, ∼Cu4PbSi2S7 and Cu2PbSiS4. The ∼Cu4PbSi2S7 compound forms incongruently at 1024 K, and has a polymorphous transition at 933 K. Cu2PbSiS4 melts in peritectically at 963 K and is also dimorphous. The phase transition temperature increases with PbS content in the 883-915 K range. The crystal structure of the low-temperature (LT) Cu2PbSiS4 modification was determined using powder method. It has trigonal symmetry (space group P3221, a = 0.60565(2), c = 1.51841(5) nm, RI = 0.0641).