HgSe-K2Se: The phase diagram is not constructed. K2Hg3Se4 and K6HgSe4 ternary compounds are formed in this system (Sommer and Hoppe 1978, Axtell et al. 1988, Kanatzidis and Park 1990). The rst of them crystallizes in an orthorhombic structure with lattice parameters a = 1082.0 ± 0.2, b = 678.3 ± 0.1, and c = 1404.2 ± 0.3 pm at 153 K and calculation density 6.42 g cm−3 (Axtell et al. 1988, Kanatzidis and Park 1990), and the second crystallizes in a hexagonal structure with lattice parameters a = 1036 and c = 788.3 pm and calculation and experimental density 3.404 and 3.40 g cm−3, respectively (Sommer and Hoppe 1978).