The chlorides Na3MCl6 (M  Eu?Lu,Y, Sc): Synthesis,crystal structures,and thermal behaviour

Single crystals of Na₃ErCl₆ were obtained via the metallothermic reduction of ErCl₃ with Na. The crystal structure is that of the mineral cryolite with a = 684.54(4), b = 725.18(4), c = 1012.39(6) pm, β = 90.768(5)°, Z = 2, space group P2₁/n. Two applicable synthetic routes to pure powder samples of the chlorides Na₃MCl₆ (M ? Eu? Lu, Y, Sc) are described. With M ? Dy? Lu, Y, Sc, these are isotypic with Na₃ErCl₆ while those with M ? Eu, Gd, Tb adopt a ?stuffed”? LiSbF₆-type structure. The dimorphism of Na₃GdCl₆ and dependence of the lattice constants and the molar volume upon temperature has been investigated: At 205°C, a first-order phase transition from the stuffed LiSbF₆-type Na₃GdCl₆-I to the cryolite-type Na₃GdCl₆-II occurs exhibiting a 3.71% negative molar volume discontinuity.