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

Abstract

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 Δ_max = 1.1112. The Raman spectrum of the new compound is similar to that of synthetic alamosite. By the calculated E^hull value it is possible to conclude that the decomposition product of Pb₁₂[Ge₁₂O₃₆]-II is its rhombohedral polymorph Pb₆[Ge₆O₁₈] (space group \(R\bar{3}\)) 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.