M4X3[SI2O7]-TYPE LANTHANIDE CHALCOGENIDE DISILICATES (M = CE-ER X = S, SE)

Attempts to produce single crystals of MSe2 (or MSe2-x) by vapour phase transport with iodine or the oxidation of MCl2 (or MClH(x)) with sulfur in the presence of NaCl in sealed evacuated quartz containers often yielded well-grown single crystals with the composition M4X3[Si2O7] (M = Pr, Sm, Gd, X = Se, and M = Nd, Er, X = S) as by-products. The crystal structures (tetragonal, I4(1)/amd (no. 141), Z = 8, M/X = Pr/Se: a = 1 215.4(1), c = 1 445.5(3) pm, k = 0.067, R(w) 0.034, M/X = Sm/Se: a = 1 201.3(1), c = 1 421.1(2) pm, R = 0.045, R(w) = 0.032, M/X = Gd/Se: a = 1194.0(1), c = 1410.5(2) pm, R = 0.046, R(w) = 0.030, M/X = Nd/S: a = 1 198.6(1), c = 1 406.4(2) pm, R = 0.026, R(w) = 0.023, M/X=Er/S: a = 1164.6(1), c = 1347.3(2)pm, R = 0.023, R(w) = 0.021) contain two crystallographically independent M3+ cations that are interconnected by chalcogenide (X2-) and disilicate anions ([Si2O7]6-). (M1)3+ is surrounded by eight (five X2 and three terminal O2- of the disilicate group), (M2)3+ by nine (three X2- and six terminal O2- of the [Si2O7]6 anion) chalcogenide anions. The disilicate anion itself exhibits the eclipsed conformation with non-linear Si-O-Si bridges (angles: 128 - 133-degrees).