A New Polymorph of Lead Metagermanate Pb12[Ge12O36]-II

Crystals of new polymorph Pb12[Ge12O36]-II are obtained by the flux method at T = 550 °C in a system containing KI, PbO, PbI2, and GeO2 in equimolar amounts. The crystal structure is characterized by unit cell parameters a = 22.9394(3) Å, b = 14.4672(2) Å, c = 13.2344(2) Å, β = 119.4335(15)°, V = 3825.18(11) Å3 (space group C2/c; Z = 4) and is topologically similar to the crystal structure of Pb12[Ge12O36]-I (space group P2/c; Z = 1) studied previously by neutron diffraction and to that of synthetic alamosite Pb12[Si12O36] (space group P2/c; Z = 1). These crystal structures are based on parallel long-periodic tetrahedral chains [T12O36]24– (T = Si4+, Ge4+). The adjacent chains are linked by Pb2+ cations having a steroactive 6s2 lone pair, which explains the specific wavy bend of tetrahedral chains. The distinguishing feature of the crystal structure of new polymorph Pb12[Ge12O36]-II is the difference in orientations of the [GeO4] tetrahedra in the adjacent [Ge12O36] chains, which four times increases the unit cell volume as compared to the initial one in the Pb12[Ge12O36]-I structure. The analysis of the crystal structure of Pb12[Ge12O36]-II reveals its high degree of pseudosymmetry Δmax = 1.1112. The Raman spectrum of the new compound is similar to that of synthetic alamosite. By the calculated Ehull value it is possible to conclude that the decomposition product of Pb12[Ge12O36]-II is its rhombohedral polymorph Pb6[Ge6O18] (space group R3̄ ) considered as a stable compound. With regard to a high structural complexity, the new polymorph can be assumed to be either a low-temperature or metastable modification of lead metagermanate.