The crystal structure of sodium selenochromite, NaCrSe₂, and preliminary investigations on related compounds

Attempts have been made to prepare new model-substances for magneto-chemical investigations. Sodium selenochromite, NaCrSe₂, rubidium selenochromite, RbCrSe₂, and rubidium thiochromite, RbCrS₂, of high purity have been prepared by reaction of the pure elements, and a potassium selenochromite, K_0.5CrSe₂ or KCrSe₂.CrSe₂, by fusion of KCN with Cr₂O₃ and selenium. The crystal structures of the alkali selenochromites and rubidium thiochromite have been investigated with the following results: NaCrSe₂: hexagonal rhombohedric; D_3d⁵-Rm; I_a = 3.708 A., I_c = 20.29 A. ± 0.05 A.; parameter z of Se: 95.3° ±  0.3°; density obs. = 4.49₇ g.cm.^-3; density calc. = 4.77 g.cm.^-3; the hexagonal unit cell contains three, the rhombohedral cell one molecule of NaCrSe₂. K_0.5CrSe₂: hexagonal rhombohedric; C_3v⁵-R3m; I_a = 3.44 A., I_c = 24.2 A.; parameter z of Se: 141.5°; parameter z of K: 60-62°. RbCrSe₂: hexagonal rhombohedric; space group not yet determined; I_a = 3.34 A., I_c = 26.9 A.; density obs. = 5.02 g.cm.^-3, density calc. = 5.26 g.cm.^-3. RbCrS₂: hexagonal rhombohedric; space group not yet determined; I_a = 3.39 A., I_c = 16.20 A. The radius of the Se^2- ions decreases from 1.85 A. in NaCrSe₂ to 1.71₅ A. in RbCrSe₂. Preliminary magnetic measurements on NaCrSe₂ show the compound to obey the Curie-Weiss law with a [theta] value of about + 70° C.