Isothermal section of the Cu2Se-HgSe-GeSe2 system at 670Â K and crystal structures of the compounds Cu2HgGeSe4 and HT-modification of Cu2HgGeS4

The isothermal section of quasi-ternary Cu2Se-HgSe-GeSe2 system at 670 K was constructed. The crystal structures of the compounds Cu2HgGeSe4 (Cu2FeSnS4 structure type, space group I4¯2m, a = 0.57456(2) nm, c = 1.10834(4) nm, RI = 0.0468) and HT-modification of Cu2HgGeS4 (Cu2CdGeS4 structure type, space group Pmn21, a = 0.76882(3) nm, b = 0.65561(2) nm, c = 0.63144(3) nm, RI = 0.0835) were determined using X-ray powder diffraction. The Se atoms in the structure of the Cu2HgGeSe4 compound are stacked in a close-packed arrangement with layers in the sequence ABC. Half of the tetrahedral interstices are occupied by Cu, Hg and Ge atoms. At the same time the S atoms in the structure of the HT-modification of Cu2HgGeS4 are stacked in a close-packed arrangement with layers in the sequence AB. Half of the tetrahedral interstices are occupied by cations.