From eulytine to the ambidextrous crystal via the amorphous route: Lead & calcium bismuth germanate glasses

Lead and calcium bismuth germanate glasses containing at least 40 mole% GeO₂ were prepared. Physical properties and infrared spectra suggest that for glasses close to the PbO · Bi₂O₃ · 2GeO₂ composition: (a) packing efficiency is directly dependent upon cation size, (b) refraction is dependent upon cation type, (c) calcium can substitute for lead in nearly ideal fashion, (d) the open eulytine arrangement of a 2Bi₂O₃ · 3GeO₂ glass has collapsed, and (e) Ge₂O₇ dimers and small chains of GeO₄ tetrahedra (characteristic of Pb₅Ge₃O₁₁ and PbGeO₃ respectively) may persist. These findings offer a rationale for the limited yields of Ca₃Al₂Ge₃O₁₂ garnets obtained from such melts at high temperatures. A noncubic crystalline phase that may be Pb₃Bi₂Ge₃O₁₂ is also reported.