Pr4S3[Si2O7] and Pr3Cl3[Si2O7]: Derivatives of praseodymium disilicate modified by soft foreign anions

For synthesizing both the disilicate derivatives Pr4S3[Si2O7] and Pr3Cl3[Si 2O7], Pr, Pr6O11 and SiO2 are brought to reaction with S and PrCl3, respect ively, in suitable molar ratios (850 degrees C, 7 d) in evacuated silica tu bes. By using NaCl as a flux, Pr4S3[Si2O7] crystallizes as pale green, tran sparent single crystals (tetragonal, I4(1)/amd, a = 1201.6(1), c = 1412.0(2 ) pm, Z = 8) with the appearance of slightly compressed octahedra. On the o ther hand, Pr3Cl3[Si2O7] emerges as pale green, transparent platelets and c rystallizes monoclinically (space group: P2(1), a = 530.96(6), b = 1200.2(1 ), c = 783.11(8) pm, beta = 109.07(1)degrees, Z = 2). In both crystal struc tures ecliptically conformed [Si2O7](6-) units of two corner-linked [SiO4] tetrahedra with Si-O-Si bridging angles of 131 degrees in the sulfide and 1 48 degrees in the chloride disilicate are present. In Pr4S3[Si2O7] both cry stallographically independent Pr3+ cations show coordination numbers of 8 1 (5S(2-) and 3 + 1O(2-)) and 9 (3S(2-) and 6O(2-)). For Pr1, Pr2 and Pr3 in Pr3Cl3[Si2O7] coordination numbers of 10 (5Cl(-) and 5O(2-)) and 9 (2x; 4Cl(-) and 5O(2-) or 3Cl(-) and 6O(2-), respectively) occur.