A Polyselenide with a Novel Se₇^8– Unit: the Structure of Sr_19–xPb_xGe₁₁Se₄₄ with x = 5.0 and 6.4

Abstract

An unprecedented, planar Se 7 8unit with a threefold rotation axis was discovered in the new polyselenide Sr 19-x Pb x Ge 11 Se 44 . Sr 12.6 Pb 6.4 Ge 11 Se 44 crystallizes in the noncentrosymmetric hexagonal space group P6 3 with unit cell dimensions of a = b = 19.9867(2) Å, c = 12.0546(1) Å, V = 4170.28(7) Å 3 , with two formula units per cell. Its structure contains isolated [GeSe 4 ] tetrahedra, [Ge 2 Se 7 ] dimers, and a novel, planar propeller-shaped Se 7 8unit. The latter is unprecedented, but reminiscent of the isoelectronic pyramidal Se 7 8and I 7 units. It consists of a central Se atom, to which three Se 2 pairs [a]

Crystal Structure and Physical Properties of the Lanthanum Chalcoantimonate TlLa2Sb3Se9

“…A similar scenario was also observed in the cases of Pb 6 Sb 6 S 17 and Pb 6 Sb 6 Se 17 . Two of the Se atoms, Se4 and Se7, form an Se 2 dumbbell, with an Se−Se bond of 2.48 Å, a bit longer than in most cases but similar to those found in the polyselenides BaLaSb 2 Se 6 (2.45 Å) and Sr 19‐x Pb x Ge 11 Se 44 (2.45 Å–2.46 Å) [17] . Each dumbbell is connected to two others via contacts of 3.10 Å between two Se7 atoms, resulting in an infinite ribbon of four Se atoms running along the a axis.…”

Effect of Pb Substitution in Sr2-xPbxGeSe4 on Crystal Structures and Nonlinear Optical Properties Predicted by DFT Calculations

“…Generally Pb 2+ is slightly smaller than Sr 2+ , as a comparison of SrSe 41 with isostructural PbSe 42 shows (Sr−Se distances: 3.12 Å; Pb−Se: 3.06 Å). Correspondingly, the unit cell volume of Sr 11-x Pb x Ge 11 Se 44 decreases with increasing Pb content x 9 and the volume per formula unit from 200.9 Å 3 in γ-Sr 2 GeSe 4 to 198.9 Å 3 in Sr 1.3 Pb 0.7 GeSe 4 . In addition, the lone pair of Pb 2+ often leads to a more distorted coordination sphere, with Pb 2+ preferring the more distorted sites in Sr 11-x Pb x Ge 11 Se 44 .…”