RESUMEN
The local structure around germanium is a fundamental issue in material science and geochemistry. In the prevailing viewpoint, germanium in GeO2 melt is coordinated by at least four oxygen atoms. However, the viewpoint has been debated for decades due to several unexplained bands present in the GeO2 melt Raman spectra. Using in situ Raman spectroscopy and density functional theory (DFT) computation, we have found a [GeOØ2]n (Ø = bridging oxygen) chain structure in a GeO2 melt. In this structure, the germanium atom is coordinated by three oxygen atoms and interacts weakly with two neighbouring non-bridging oxygen atoms. The bonding nature of the chain has been analyzed on the basis of the computational electronic structure. The results may settle down the longstanding debate on the GeO2 melt structure and modify our view on germanate chemistry.
RESUMEN
In situ high temperature Raman spectra of xK2O-(100-x)GeO2, samples containing 0, 5, 11.11, 20, 25, 33.3, 40, and 50 %mol K2O, were measured. The structure units and a series of model clusters have been designed, optimized, and calculated by quantum chemistry ab initio calculations. The computational simulation in conjunction with the experiments put forward a novel method to correct the experimental Raman spectra of the melts. Deconvolution of the stretching vibrational bands of nonbridging oxygen of [GeO4] tetrahedra of Raman spectra by Gaussian functions was carried out, and the quantitative distribution of different Qn species in molten binary potassium germanates was obtained. The result on all molten samples show that four-fold coordinated germanium atoms occupy a dominant position in the melt and only four-fold coordinated exists in the melt when the K2O content exceeds a certain amount. For melts with high GeO2 content, with the increasing K2O content, the structure of [GeO4] tetrahedra gradually changes from a three-dimensional network consisting of both six-membered and three-membered rings to a three-dimensional network that presents all three-membered rings.
RESUMEN
In situ high-temperature Raman spectra of polycrystalline KBi(MoO4)2 were recorded from room temperature to 1073 K. Thermal stability of the monoclinic KBi(MoO4)2 was examined by temperature-dependent XRD. The monoclinic phase transformed into the scheelite tetragonal structure at 833 K, and then to the monoclinic phase at 773 K. Quantum chemistry ab initio calculation was performed to simulate the Raman spectra of the structure of KBi(MoO4)2 high-temperature melt. The experimental Raman band at 1023 K was deconvoluted into seven Gaussian peaks, and the calculated results were in good agreement with the experimental data. Therefore, the vibrational modes of Raman peaks of molten KBi(MoO4)2 were assigned. It was confirmed that the isolated structure of [Bi(MoO4)2]- monomer, consisting of Mo6+ centers and Bi3+ sub-centers connected by edge-sharing, mainly exists in the melt of KBi(MoO4)2.
RESUMEN
Knowledge of the molecular-level structure of the Li2GeO3 melt is essential to understand its basic physicochemical properties. In this work, in situ Raman spectroscopy, factor group analysis, and density functional theory (DFT) calculations were applied to investigate the Li2GeO3 crystal Raman spectrum and its transformation during the crystal melting process. Finally, the Li2GeO3 melt structure was determined. The Li2GeO3 lattice phonons were fully analyzed by the factor group. The DFT calculations confirmed the analysis results and assigned all of the experimental Raman bands. There are two characteristic Raman bands in the experimental spectrum. The 495 cm-1 band (mid-frequency band) is attributed to the symmetric bending vibration of the Ge-O-Ge bond, and the 814 cm-1 band (high-frequency band) arises from the symmetric stretching vibration of the O-Ge-O bond. The mid-frequency band anomalously shifted to a higher frequency and the high-frequency band normally shifted to a lower frequency when the crystal melted. The DFT method was employed to investigate two possible Li2GeO3 melt structures, one consisting of the [GeO2Ø2] n (Ø = bridging oxygen) chain and the other consisting of the [Ge3O9] ring. The chain-type structure was demonstrated to provide a better description of the Li2GeO3 melt than the ring-type structure. The anomalous shift of the mid-frequency band is related to the shrinkage of the [GeO2Ø2] n chain. On the basis of the chain-type structure, the high viscosity of the Li2GeO3 melt and the growth phenomena of the Li2GeO3 crystal were explained.
