Structure Of GeO2 Research Articles

Densification mechanism and structural transformation in glass: molecular dynamic simulation

The structure of GeO2 glass is investigated via molecular dynamic simulation. The influence of pressure to the structure of GeO2 glass is due to the change of short-range order, network structure, domain structure and free volume regions. The obtained results show that the structure of GeO2 was formed by a continuous random network of basic structural units linking to each other via with corner-sharing, edge-sharing, face-sharing bond. At low pressure, the basic structural units are mainly linked via one bridge oxygen (the corner sharing bonds). As pressure increases, the fraction of edge sharing bonds (two bridge oxygen) increases meanwhile the fraction of corner sharing bonds significantly decrease. There is a slight increase with the fraction of face- sharing bonds (three bridge oxygen). The Ge-O bond distance is almost not dependent on pressure, meanwhile the Ge-Ge and O-O bond distances are significantly dependent on pressure. Under compression, the average coordination number of Ge tends to increase from fourfold (at ambient pressure) to six-fold (at high pressure). Moreover, structure of GeO2 glass has two-domain near 15 and 25 GPa. And at ambient pressure or high pressures, the struture of GeO2 glass has single domain. The dependence of the distribution of free volumes on pressure in GeO2 glass has been examined. The distribution of void radius has the Gaussian form and the position of the peak tend to shift to the left under compression.

X-ray Investigations of Sol–Gel-Derived GeO2 Nanoparticles

Germanium dioxide (GeO2) is a versatile material with several different crystalline polymorphs and interesting applications in, e.g., optics, microelectronics, and Li-ion batteries. In particular, many of the material’s properties depend on the size of the prepared crystallites, and thus, nanocrystalline GeO2 is of special interest. Here, GeO2 nanoparticles are prepared via sol–gel processes by the hydrolysis of Ge isopropoxide (Ge(OCH(CH3)2)4). The precipitated powders are dried at room temperature and annealed in ambient air using temperatures between 500 °C and 1000 °C from 3 to 24 h. The samples were characterized by X-ray diffraction, X-ray absorption fine structure spectroscopy, and scanning electron microscopy, providing the crystalline structures, the phase composition, as well as the morphology and crystallite size of the formed particles and their changes upon heating. According to the structural analysis, the samples are crystalline with a dominant β- (low temperature) quartz phase without any heat treatment directly after drying and increasing contributions of α- (high-temperature modification) quartz and quartz-like GeO2 structures with increasing temperature and annealing time were found. According to electron microscopy and the X-ray analysis, the particle size ranges from about 40 to 50 nm for the pristine particles and to about 100 nm and more for the annealed materials.

Correlation between microstructure and density of Germania melts: insight from molecular dynamics simulation

Molecular dynamics simulation of Germania (GeO2) melt at different densities has been carried out to investigate the microstructural transformation, polyamorphism and structural heterogeneity. The structure of GeO2 is studied by GeOx basic units and their cluster. The simulation results show that the structure of GeO2 is formed by GeOx structural units linking to each other via with corner-sharing, edge-sharing, face-sharing bond. The polyamorphism and structural heterogeneity in GeO2 depend on fraction of GeOx units and distribution of GeOx cluster. The relationship between structure and density is analyzed via the change of pair radial distribution function (PRDF). The edge- and face-sharing bonds are responsible for the first peak splitting of Ge–Ge PRDF. The location of the two first sub-peaks of Ge–Ge PRDF corresponds to the length of edge- and corner-sharing bonds.

The first peak splitting of the Ge[sbnd]Ge pair RDF in the correlation to network structure of GeO2 under compression

The network structure of GeO2 at 3500K and under pressure of 0–100GPa is investigated in terms of the short range order (SRO) and intermediate range order (IRO) by molecular dynamics simulation. The results show that the structure of GeO2 consists of GeO4 tetrahedra that link to each other, forming a tetrahedral network. Under compression, there is a gradual transition from tetrahedral network to octahedral network (GeO6-network) via GeO5 polyhedra. At a certain pressure, the structure of GeO2 comprises three kinds of basic structural polyhedra: GeO4, GeO5 and GeO6. The spatial distribution of the basic structural polyhedra is not uniform, but they form clusters of GeO4-, GeO5- and GeO6-polyhedra. The size of the GeO5-cluster reaches the maximum at pressure of approximately 15–20GPa (at density of approximately 4.95–5.25g/cm3). The GeO5 cluster exists as an immediate configuration in the structural transition. At low pressure, most GeOx polyhedra (x=4, 5, 6) link to each other by one common oxygen (corner-sharing bond). At high pressure, GeOx polyhedra link to each other by a corner-sharing, edge-sharing (two common oxygens) or/and face-sharing bond (three common oxygens). The GeGe bond length in a corner-sharing bond is much longer than that in edge-sharing and face-sharing bonds, and this is the origin of the first peak splitting of the GeGe pair RDF (Radial Distribution Function) under compression. At high pressure, the GeO5- and GeO6-polyhedra are dominant and tend to link each other through edge-sharing and face-sharing bonds, forming edge-sharing and face-sharing clusters. The size of edge-sharing and face-sharing clusters increases with increasing pressure, and this can be seen via the degree of the first peak splitting of the GeGe pair RDF.

Flux-Grown Piezoelectric Materials: Application to α-Quartz Analogues

Using the slow-cooling method in selected MoO3-based fluxes, single-crystals of GeO2 and GaPO4 materials with an α-quartz-like structure were grown at high temperatures (T ≥ 950 °C). These piezoelectric materials were obtained in millimeter-size as well-faceted, visually colorless and transparent crystals. Compared to crystals grown by hydrothermal methods, infrared and Raman measurements revealed flux-grown samples without significant hydroxyl group contamination and thermal analyses demonstrated a total reversibility of the α-quartz ↔ β-cristobalite phase transition for GaPO4 and an absence of phase transition before melting for α-GeO2. The elastic constants CIJ (with I, J indices from 1 to 6) of these flux-grown piezoelectric crystals were experimentally determined at room and high temperatures. The ambient results for as-grown α-GaPO4 were in good agreement with those obtained from hydrothermally-grown samples and the two longitudinal elastic constants measured versus temperature up to 850 °C showed a monotonous evolution. The extraction of the ambient piezoelectric stress contribution e11 from the CD11 to CE11 difference gave for the piezoelectric strain coefficient d11 of flux-grown α-GeO2 crystal a value of 5.7(2) pC/N, which is more than twice that of α-quartz. As the α-quartz structure of GeO2 remained stable up to melting, a piezoelectric activity was observed up to 1000 °C.

STRUCTURES AND ELECTRONIC PROPERTIES OF FOUR CRYSTAL GeO2 AND TWO RARE-EARTH ELEMENT OXIDES La2O3 AND CeO2: FIRST PRINCIPLES CALCULATION

This paper presents a first principles calculation of the structure and electronic properties of four crystal GeO2 structures and two rare-earth element oxides CeO2 and La2O3 . A GGA was used to optimize structures and calculate band structure and density of states (DOS). It is found that La2O3 has the largest band gap (4.19 eV) among all the six structures, which also means it is the best insulator among them. When it comes to four crystal GeO2 structures, which were calculated to make a comparison with two insulators CeO2 and La2O3 , we found the q- GeO2 and b- GeO2 are more likely to work as the dielectrics used in MOS devices than the other two crystalline forms. Three of the four GeO2 forms have larger band gap than that of CeO2 (2.09 eV), which indicates CeO2 is not a wise choice when deposited directly on the surface of Ge substrate.

Synthesis, Structural, and Optical Properties of Electrochemically Deposited GeO[sub 2] on Porous Silicon

We present a method to synthesize submicrometer germanium dioxide (GeO2) on porous silicon (PS) by electrochemical deposition. The PS was electrochemically prepared in HF based electrolyte. GeCl4 was directly hydrolyzed by hydrogen peroxide to produce pure GeO2 and then electrochemically deposited on PS. The scanning electron microscopy results showed that the GeO2 structures are uniform in shape with diameter similar to 500 nm. The photoluminescence spectrum showed a prominent peak related to GeO2 at about 400 nm. The results indicated potential applications of GeO2 on the silicon based substrate for future optoelectronic nanodevices in the visible region using a simple fabrication method.

The structure of GeO2–SiO2 glasses and melts: A Raman spectroscopy study

We have investigated a series of glasses and melts along the GeO 2 –SiO 2 join using in situ Raman spectroscopy. The results for both the glasses and melts are consistent with a continuous random network in which there are ‘regions’ that are SiO 2 -like, GeO 2 -like and mixed GeO 2 –SiO 2 -like. Incorporation of GeO 2 into the SiO 2 network is initially accommodated via the 3- and 4-membered SiO 4 rings which are lost as they convert to larger mixed Ge/Si rings. The LO–TO mode behavior is also consistent with a network that is composed of different ‘regions’ and is similar to that expected from the Bruggeman effective media model. At the highest temperatures there are indications that the mixed Ge/Si rings convert back to small 3-membered GeO 4 rings and large SiO 4 rings; the small 3- and 4-membered SiO 4 rings are not reformed.

Structure of GeO2–P2O5 glasses studied by x-ray and neutron diffraction

The structures of three xGeO2–(1−x)P2O5 glasses, where x = 0.98,0.88, and 0.81, have been studied by neutron and x-ray diffraction experiments that yieldwell resolved P–O and Ge–O bond distances. The Ge–O coordination number(NGeO) increasedfrom 4.0 ± 0.2 to4.5 ± 0.2 with the decrease inx from 0.98 to 0.81.The increase in NGeO is consistent with a structural model that assumes that all oxygen form Ge–O–Geand P–O–Ge linkages between Ge polyhedra and P tetrahedra and that newGeO5 orGeO6 polyhedra areformed with isolated PO4 units when P2O5 is added to GeO2. The bond valencies in the P–O bonds of thePO4 tetrahedra are greater than unity and are balanced in P–O–Ge bridges with underbonded Ge–O links inthe GeO5 or GeO6 polyhedra. Mixed site connections are expected for theGeO5 (orGeO6) andPO4 units in glasseswith relatively low (<20 mol%) P2O5 content due to the overwhelming fraction ofGeO4 tetrahedra. The structural changes are compared with those reported for alkali germanateglasses. Several features indicate different characteristics for the compositional dependence ofNGeO forthe GeO2–P2O5 and alkali germanate glasses. However, the distributions of thefirst-neighbour Ge–O distances are found to be nearly identical for theGeO2–P2O5 and K2O–GeO2 glasses ofequimolar K2O and P2O5 content.

Transformation of Germanium Dioxide to Microporous Germanate 4-Connected Nets

Extensive research efforts worldwide continue to aim for new compositions and framework topologies of zeolites and related microporous crystalline materials due to their current widespread applications in catalysis and separation technologies1-3 and the advantages they present as platforms for the assembly of spatially organized molecular recognition systems.4,5 Although, 4-connected nets of zeolitic silicates and aluminosilicates represent a vast, well-established, and useful class of materials,6 the analogous germanates remain conspicuously absent and largely unexplored. We have employed a synthetic route similar to that employed for zeolites in the conversion of GeO2 to previously unknown, microporous germanate 4-connected nets. Here, two crystalline materials of GeO2 composition will be reported, namely, [GeO2]10‚ (DMA)(H2O), ASU-7, (DMA ) dimethylamine), and [GeO2]10‚ DABCO)(H2O), ASU-9, (DABCO ) 1,4-diazabicyclo[2,2,2]octane), with the first possessing a remarkable new zeolite-type net containing 1-D channels and the second adopting an octadecasil net with large cages. At the outset of this study, germanate framework structures have been limited to those constructed from combinations of tetrahedral GeO4, trigonal bipyramidal GeO5, and/or octahedral GeO6 centers7-10 with no 4-connected microporous structures based solely on the GeO4 tetrahedron reported yet. GeO2 (150 mg, 1.434 mmol) was dissolved in a 40% aqueous DMA (1.26 mL, 10.04 mmol) followed by the addition of pyridine (3.20 mL, 39.78 mmol) and 48-51% aqueous HF (0.02 mL, 0.572 mmol). The solution (pH ) 10.6) was heated in a Teflon-lined vessel to 165 °C for 4 days and then cooled to room temperature to give 90 mg (56% yield based on GeO2) of ASU-7 as rodlike crystals (shown in Figure 1 for a selected sample). Octahedrally shaped crystals of ASU-9 (Figure 1) were obtained by a similar procedure. Here, GeO2 (50 mg, 0.478 mmol) and DABCO (250 mg, 2.232 mmol) were dissolved in water (0.45 mL) followed by the addition of pyridine (1.60 mL, 19.89 mmol) and 48-51% aqueous HF (0.02 mL, 0.572 mmol) and then the solution (pH ) 8.8) was heated to 160 °C for 2 days to give 6 mg product (10% unoptimized yield based on GeO2). Elemental microanalysis11 and single-crystal X-ray studies12 on the two crystals confirmed that these conditions are ideal for transforming the original condensed structure of GeO2 into 4-connected microporous nets having cubes and tetrahedra as building blocks (shown below). In both structures, the cubes are bridged by tetrahedral units that connect each of the eight corners, resulting in larger building units with distinct stereochemistry having the cubes either rotated by 45° or identically oriented with respect to other cubes along the same direction. The first arrangement belonging to ASU-7 yields a 4-connected net containing extended 1-D channels running along the crystallographic c-axis, where a single water molecule occupies the center of each cube and the DMA guest molecules reside in the channels (Figure 2). This is a new type of microporous net having the vertex symbols, 6‚6‚62‚62‚128‚128 (vertex 1) and 4‚6‚4‚6‚4‚6 (vertex 2, on cube).13a To examine the stability of this network in the absence of the DMA guests, a sample (39.9 mg) of ASU-7 was heated in a thermal gravimetric apparatus to reveal a weight loss of 5.2% at 420-540 °C corresponding to the removal of DMA and water (5.7% calculated per formula unit). Decomposition of DMA

Voronoi polyhedra and Delaunay simplexes in the structural analysis of molecular-dynamics-simulated materials

Voronoi and Delaunay tessellations are applied to pattern recognition of atomic environments and to investigation of the nonlocal order in molecular-dynamics ~MD!-simulated materials. The method is applicable also to materials generated using other computer techniques such as Monte Carlo. The pattern recognition is based on an analysis of the shapes of the Voronoi polyhedron ~VP!. A procedure for contraction of short edges and small faces of the polyhedron is presented. It involves contraction to vertices of all edges shorter than a certain fraction x of the average edge length, with concomitant contraction of the associated faces. Thus, effects of fluctuations are eliminated, providing ‘‘true’’ values of the geometric coordination numbers f , both local and averaged over the material. Nonlocal order analysis involves geometric relations between Delaunay simplexes. The methods proposed are used to analyze the structure of MD-simulated solid lead @J. Rybicki, W. Alda, S. Feliziani, and W. Sandowski, in Proceedings of the Conference on Intermolecular Interactions in Matter , edited by K. Sangwal, E. Jartych, and J. M. Olchowik ~Technical University of Lublin, Lublin, 1995! ,p . 57; J. Rybicki, R. Laskowski, and S. Feliziani, Comput. Phys. Commun. 97, 185 ~1997!# and germianium dioxide @T. Nanba, T. Miyaji, T. Takada, A. Osaka, Y. Minura, and I. Yosui, J. Non-Cryst. Solids 177, 131 ~1994!#. For Pb the contraction results are independent of x. For the open structure of GeO2 there is an x dependence of the contracted structure, so that using several values of x is preferable. In addition to removing effects of thermal perturbation, in open structures the procedure also cleans the resulting VP from faces contributed by the second neighbors. The analysis can be combined with that in terms of the radial distribution g(R), making possible comparison of geometric coordination numbers with structural ones @W. Brostow, Chem. Phys. Lett. 49, 285 ~1977!#. @S0163-1829~98!05721-X#

Photoemission and electron energy loss spectroscopy of GeO2 and SiO2

The electronic structure of GeO2 and SiO2 has been studied by ultraviolet photoemission spectroscopy (UPS) and electron energy loss spectroscopy (ELS) using thermally grown films prepared in ultrahigh vacuum (p ∼ 10−10 Torr). A consistent energy level model of both filled and empty states is constructed from the UPS and ELS data. The filled states correspond to localized bonding or nonbonding O(2p) molecular orbitals. The empty states observed in ELS correspond to excitons formed from either localized antibonding states or more extended ``conduction'' band states.

Solution top-seeding: Growth of GeO2 polymorphs

Single crystals of both tetragonal (rutile) GeO2 and hexagonal (quartz) GeO2 have been grown by top-seeding of appropriate molten salt solutions. Tetragonal GeO2 is grown from a Na2O solution (92.3 molar% GeO2), and hexagonal crystals are prepared using a Li2O · 2WO3 solution (26.2 molar% GeO2). The top-seeded method permits one to control solution growth so that only one crystal grows and the major crystal growth direction can be selected. Also, the growth process can be observed to some extent. Details of the apparatus and procedures for growing each of the GeO2 structures are given. Variations of thermal geometry in the solution, stirring, and seed orientation are shown to markedly influence the top-seeded growth process. The crystals are characterized by X-ray, optical absorption, and physical property studies. Spectrographic analysis shows that impurity levels in the GeO2 polymorphs are low.