The first rubidium rare-earth(III) thiophosphates: Rb3M3[PS4]4 (M=Pr, Er)

The two non-isotypical rubidium rare-earth(III) thiophosphates Rb3M3[PS4]4 of praseodymium and erbium can easily be obtained by the stoichiometric reaction of the respective rare-earth metal, red phosphorus and sulfur with an excess of rubidium bromide (RbBr) as flux and rubidium source at 950oC for 14 days in evacuated silica tubes. The pale green platelet-shaped single crystals of Rb3Pr3[PS4]4 as well as the pink rods of Rb3Er3[PS4]4 are moisture sensitive. Rb3Pr3[PS4]4 crystallizes triclinically in the space group P1-bar (a=926.79(5)pm, b=1050.83(5)pm, c=1453.28(7)pm, α=84.329(4)o, β=88.008(4)o, γ=80.704(4)o; Z=2), Rb3Er3[PS4]4 monoclinically in the space group P21/n (a=915.97(5)pm, b=1575.86(7)pm, c=1843.32(9)pm, β=95.601(6)o; Z=4). In both structures, there are three crystallographically different rare-earth cations present. (M1)3+ is eightfold coordinated in the shape of a square antiprism, (M2)3+ and (M3)3+ are both surrounded by eight sulfur atoms as bicapped trigonal prisms each with a coordination number of eight as well as for the praseodymium, but better described as CN=7+1 in the case of the erbium compound. These [MS8]13- polyhedra form a layer according to ∞2([M3[PS4]4]3-) by sharing edges with the isolated [PS4]3- tetrahedra (d(P-S)=200-209pm, -bar (S-P-S)=102-116o). These layers are stacked with a repetition period of three in the case of the praseodymium compound, but of only two for the erbium analog. The rubidium cation (Rb1)+ is located in cavities of these layers and tenfold coordinated in the shape of a tetracapped trigonal antiprism. The also tenfold but more irregularly coordinated rubidium cations (Rb2)+ and (Rb3)+ reside between the layers