In Situ Raman Spectroscopy and DFT Studies of the Li2GeO3 Melt Structure

Threefold coordinated germanium in a GeO2 melt

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.

Quantitative Studies on Local Structure of Molten Binary Potassium Germanates

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 Q_n 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.

A Thorough Investigation of Electronic, Optical, Mechanical, and Thermodynamic Properties of Stable Glasslike Sodium Germanate under Compressive Hydrostatic Pressure: Ab Initio Study

In this paper, we have tried to elucidate the variation of structural, electronic, and thermodynamic properties of glasslike Na₂GeO₃ under compressive isotropic pressure within a framework of density functional theory (DFT). The result shows stable structural (orthorhombic → tetragonal) and electronic (indirect → direct) phase transitions at P ∼ 20 GPa. The electronic band gap transition plays a key role in the enhancement of optical properties. The results of the thermodynamic properties have shown that Na₂GeO₃ follows Debye's low-temperature specific heat law and the classical thermodynamic of the Dulong-Petit law at high temperature. The pressure sensitivity of the electronic properties led us to compute the piezoelectric tensor (both in relaxed and clamped ions). We have observed significant electric responses in the form of a piezoelectric coefficient under applied pressure. This property suggested that Na₂GeO₃ could be a potential material for energy harvest in future energy-efficient devices. As expected, Na₂GeO₃ becomes harder and harder under compressive pressure up to the phase transition pressure (∼20 GPa) which can be read from Pugh's ratio (k_H) > 1.75, however, at pressures above 20 GPa k_H < 1.75, which may be due to the formation of fractures at high pressure.

Synthesis and Growth of Rare Earth Borates NaSrR(BO3)2 (R = Ho-Lu, Y, Sc)

NaSrR(BO₃)₂ (R = Ho-Lu, Y, Sc) compounds were obtained for the first time. Their structures exhibit disordered positions of Sr²⁺ and Na⁺ atoms while RO₆ polyhedra are connected through the BO₃ groups. Large distances between R atoms and high transparency in the range of 250-900 nm make them promising for phosphor applications. A pathway to obtain single crystals was shown by growing NaSrY(BO₃)₂ and NaSrYb(BO₃)₂ by the top seeded solution growth method with Na₂O-B₂O₃-NaF flux.

γ-BaB2O4: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO4 Tetrahedra

The α- and β-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB₂O₄ has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB₂O₄, crystallizes in a centrosymmetrical group of monoclinic syngony (P2₁/n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, β = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB₂O₄ structure is the presence of edge-sharing tetrahedra [B₂O₆] which form infinite double chains _∞[B₄O₄O_8/2] stretching along the a axis. The number of known structural types with the [B₂O₆] group is limited. Phase γ-BaB₂O₄ has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B₂O₆] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB₂O₄ modification were studied. The width of the band gap, calculated using the HSE06 functional, is 7.045 eV which implies transparency in the deep-UV region. Experimental and numerical methods which demonstrate a good match were used to the study the Raman spectra of γ-BaB₂O₄ and β-BaB₂O₄ modifications. In the Raman spectra of γ-BaB₂O₄, the most intense band at a frequency of 853 cm^-1 was found to correspond to the symmetric bending mode of the B-O-B-O ring in edge-sharing tetrahedra.

Oxygen Vacancies Boosting Lithium-Ion Diffusion Kinetics of Lithium Germanate for High-Performance Lithium Storage

Oxygen vacancies play a positive role in optimizing the physical and chemical properties of metal oxides. In this work, we demonstrated oxygen vacancy-promoted enhancement of Li-ion diffusion kinetics in Li₂GeO₃ nanoparticle-encapsulated carbon nanofibers (denoted as Li₂GeO_3-x/C) and accordingly boosted lithium storage. The introduction of the oxygen vacancies in Li₂GeO_3-x/C can enhance electronic conductivity and evidently decrease activation energy of Li-ion transport, thus resulting in evidently accelerated Li-ion diffusion kinetics during the lithiation/delithiation process. Thus, the Li₂GeO_3-x/C nanofibers exhibit an exceptionally large discharge capacity of 1460.5 mA h g^-1 at 0.1 A g^-1, high initial Coulombic efficiency of 81.3%, and excellent rate capability. This facile and efficient strategy could provide a reference for injecting the oxygen vacancies into other metal oxides for high-performance anode materials.

The structure-Raman spectra relationships of Mg3(PO4)2 polymorphs: A comprehensive experimental and DFT study

Three Mg₃(PO₄)₂ polymorphs (Mg₃(PO₄)₂-I, II, III) were synthesized at high-pressure and high-temperature conditions. The structures and vibrational properties of Mg₃(PO₄)₂ polymorphs were studied by X-ray diffraction (XRD), Raman spectroscopy, and density functional theory (DFT) calculations. The obvious different PO stretching vibrational modes were experimentally observed for Mg₃(PO₄)₂-I, II, III. The calculated vibrational frequencies were in good agreement with measurements. All the observed vibrational modes for Mg₃(PO₄)₂-I, II, III were well assigned based on the calculations, which provided a support for investigating and comparing vibrational properties of three Mg₃(PO₄)₂ polymorphs.

Structural investigations on two typical lithium germanate melts by in situ Raman spectroscopy and density functional theory calculations

In situ Raman spectroscopy, together with density functional theory calculations, was used to monitor the structural changes of polycrystalline Li₄GeO₄ and Li₆Ge₂O₇ from room temperature to their melting temperatures.