Mechanism of the Chain-Ring Transformation in BaGeO₃

The structural transformation from the [GeO2 & Oslash;(2-)(2)](n) (& Oslash; = bridging oxygen) chain to the [Ge3O9](6-) ring is technologically and geologically important; however, a clear understanding of this transformation is lacking. By using in situ high-temperature Raman spectroscopy and density functional theory (DFT) computation, we investigated the phase transition of BaGeO3 and the mechanism of the structural transformation from the [GeO2 & Oslash;(2-)(2)](n) chain to the [Ge3O9](6-) ring. The Raman experimental results reveal that BaGeO3 undergoes the alpha-to-beta phase transition at around 1373 K. The 480 cm(-1) (computational value, corresponding to the experimental value of 440 cm(-1)) band of beta-BaGeO3 has a significantly larger width. By studying its structural origin, we propose a mechanism to account for the chain-ring transformation. At the phase transition temperature, the bridging oxygen atom of the chain attacks the opposite Ge atom in the neighboring GeO2 & Oslash;(2-)(2) motif and substitutes another bridging oxygen atom bonded to the Ge atom. With the formation of new Ge-& Oslash; bonds and the breakage of old Ge-& Oslash; bonds, the [GeO2 & Oslash;(2-)(2)](n) chain transforms into the [Ge3O9](6-) ring. The results of transition state searching also support the mechanism and provide more structural details about the chain-ring transformation.