Alkali Germanate Research Articles

Investigation of coordination number in silicate and germanate glasses using O K-edge X-ray absorption spectroscopy

A series of titanium silicate and alkali germanate glasses have been examined using oxygen K-edge X-ray absorption near edge structure (XANES) spectroscopy. Peaks in the O K-edge spectra due to O 1 s to O 2 p σ* states hybridized with cation s, p and d states can be used to determine the coordination of the Ti or Ge within the glass network. The O K-edge XANES data for the titanium silicate glasses is consistent with earlier Ti L-edge findings concerning the Ti environment in these glasses. The O K-edge XANES data of the alkali germanate glasses confirm that there is no formation of sixfold Ge within the glass before the germanate anomaly maximum. Fivefold Ge is observed in the alkali germanate glasses but only in compositions at the germanate anomaly maximum and beyond.

The effects of alkali cation mass and radii on the density of alkali germanate and alkali germano-phosphate glasses

A quantitative density model for alkali germanate and alkali germano-phosphate glasses has been developed based on Raman spectra and density data. The model demonstrates that for lighter/smaller cations such as lithium and sodium, the transition from 4- to 3-membered GeO 4 rings is the prominent structural change responsible for changes in the density trends with increasing alkali oxide concentrations. For the heavier/larger cations such as potassium and rubidium, the 4- to 3-membered ring transition remains important but other factors must be considered. These include increases in density due to additional mass of the alkali oxides and decreases in density due to depolymerization of the glass network. The model shows that these combined factors can account for the ‘germanate anomaly’ noted in these glasses.

Vibrational spectra and structure of alkali germanate glasses

Numerous structural studies have been carried out so far to elucidate the nature of the so-called germanate anomaly, but the correct structural model is still unknown. The objective of the present paper is to shed further light on the structure of alkali germanate glasses, namely those containing sodium and cesium oxides. Two series of glass samples were prepared, having molar compositions (100− x)GeO 2· xM 2O (M=Na and 0⩽ x⩽35, or M=Cs and 0⩽ x⩽25). Polarized Raman and infrared absorption spectra were recorded for these glasses, as a function of the alkali content; infrared reflectivity measurements were also carried out and Kramers–Kronig analysis of the near-normal incidence spectra were compared with the polarized Raman spectra of the glasses. An overall structural picture has emerged, which is compatible with the presence of a fraction of higher coordinated Ge atoms, either five-fold or six-fold.

Density of alkali germanate glasses related to structure

The structural origin of the `germanate anomaly' effect remains a matter of controversy. We employ in this study a quantitative description of the germanate network structure to simulate the experimental density of potassium germanate glasses with K 2O contents up to 40 mol%. The results show that the short range order structure, including the change of germanium co-ordination number from four to six, cannot reproduce the maximum in density. The `germanate anomaly' effect seems to involve both short and medium range order structural changes as metal oxide is added to GeO 2.

Paramagnetic sites in alkali germanate glasses

Electron paramagnetic resonance (EPR) spectra of sodium–germanate glasses ( xGeO 2·(1− x)Na 2O, 0.65< x<1) prepared by the conventional melting technique are analyzed as a function of the Na 2O content, before and after X-ray irradiation. Signals due to three-fold coordinated Ge sites (Ge E ′ centers) and non-bridging oxygen hole centers (NBOHCs) are observed, whereas no signal due to paramagnetic centers related to GeO 6 sites is detected. At high Na 2O concentration, a broad signal at g=2.21 is also observed. The EPR results suggest three main mechanisms for the Na + ion insertion: interstitial; network modifier; a source of medium range structural rearrangements of GeO 4 tetrahedra.

A study of selected physical properties of alkali germanate glasses over wide ranges of composition

The compositional ranges for various alkali germanate glasses have been explored, with some reaching as high as J=3 (or 75 molar percent alkali oxide), where J is the molar ratio of alkali oxide to germanium oxide. The T g (glass transition temperature) and carbon dioxide retention were found for each sample, while the density was found for the more stable samples. These data were then compared to the previous work that had been done on the alkali silicate and alkali borate families by this lab. It was found that the T g trend in the alkali germanates was similar to that found in the alkali borates due to a change in coordination in each system. Also, the density and T g could be related and explained with an understanding of the coordination of the units present in the glasses. Volumes of the structural units were obtained in the alkali germanate systems and compared with the alkali borates and silicates. In addition, packing fractions of the units were obtained in all three systems. It was found that the silicate units did not have an anomonously large packing fraction of any of its structural units at low alkali concentrations, whereas the germanate octahedral units (N 6) and the borate tetrahedral units (f 2) did. At high alkali contents the packing fractions were similar in all systems indicating a common effect due to both the formation of non-bridging oxygens and the alkali ions filling space in a random manner.

Two-photon absorption and non-linear refraction in binary alkali germanate glasses at 355 nm

We report on the measurements of degenerate two-photon absorption coefficient, β, and non-linear index of refraction, γ, at 355 nm in binary alkali germanate glasses. By applying the Z-scan technique, the β was evaluated to be 10.1±2.0 cm/GW for both 30Na 2O:70GeO 2 and 30K 2O:70GeO 2 glasses, whereas γ was 6.42±1.3×10 −5 and 4.17±0.8×10 −5 cm 2 /GW , respectively. The observations provide evidence for the two-photon absorption activity of GeO 2 as a component of oxide glasses.

Evidence for site memory effects in the ionic relaxation of (Li 2O) x(Na 2O) y(GeO 2) 1−x−y glasses

We report measurements of the dielectric response due to ionic relaxation in alkali germanate glasses (Li 2O) x (Na 2O) y (GeO 2) 1− x − y . The relaxation is investigated in both electric modulus and ac conductivity formalisms, between which important discrepancies are observed. In single alkali glasses, the width of the electric modulus decreases with decreasing alkali concentration. However, the power law dispersion of the ac conductivity scales regardless of the alkali concentration. This scaling property of the ac conductivity implies that inter-ionic interactions are not responsible for the non-Debye relaxation. In mixed alkali glasses, the exponent of the power law dispersion of the ac conductivity decreases and the dielectric relaxation strength increases relative to the single alkali glasses. With increasing total alkali concentration in the mixed alkali glasses, the frequency of maximum dielectric loss decreases, approaching frequencies comparable to that of the mixed alkali mechanical loss peak. These findings indicate a transition of the ionic relaxation toward a slower polarization process which is consistent with site memory mechanisms recently proposed for the mixed alkali effect.

The local structure of mixed alkali germanate glasses by EXAFS

Interatomic distance, coordination number and degree of disorder around mobile (K, Rb) as well as network forming (Ge) cations have been determined using extended X-ray absorption fine structure (EXAFS) analysis for a 0.2[ yK · (1 − y)Rb] 2O · 0.8GeO 2 mixed alkali glass series. The glasses were prepared by casting the melts (at 1200–1400°C) in a metal mold at ambient temperature. Quenching stresses were relieved by annealing the glass at ∼450°C and then furnace cooling to room temperature. The X-ray absorption spectra were measured near liquid helium temperature to eliminate thermal disorder. The larger structural disorder around K and Rb indicates that the modified random network structure model is not applicable to germanate glasses. The replacement of K by Rb has little effect on the various interatomic distances, which indicates that the local static structure is not important for the mixed alkali effect. The monotonic change in molar volume or excess volume does not correlate with the variation of the interatomic distances, indicating that there exists an intermediate range spatial heterogeneity of the ‘unoccupied’ or ‘excess’ volume in the glass structure.

Behavior of the packing densities of alkali germanate glasses

Packing densities are more suitable than the mass densities for structural studies. A comparison of the corresponding data on alkali germanate glasses with those of alkali borate glasses corroborates the suggestion of the formation of GeO 5 and/or GeO 6 units in systems (Me 2O) x (GeO 2) 1− x in that range of composition where the maximum packing density occurs. The packing densities of both the alkali germanate and borate glasses strongly obey a common behavior while the data of the related crystals are widely spread. Simple structural rules like those used for the borate glasses are suggested for an explanation of the germanate anomaly. However, a specific characteristic of some germanate crystals is the occurrence of three-fold coordinated O sites accompanied by clusters of GeO 6 units. There is no evidence for a significant role of such clusters in the glass structures though minor indications for such features were detected by Raman spectroscopy.

The change of the Ge–O coordination number in potassium germanate glasses probed by neutron diffraction with high real-space resolution

Neutron diffraction experiments of high real-space resolution are used for studying the Ge–O coordination numbers and the Ge–O bond lengths in two K 2O–GeO 2 glasses (11 and 20 mol% K 2O). On the long-distance side of the first-neighbour Ge-O peak lying at 175 pm, which is likewise found for vitreous GeO 2, appears a clear shoulder. This feature indicates the existence of GeO n units larger than GeO 4 tetrahedra. The shoulder is not resolved as a separate peak which could be attributed to Ge–O bonds of GeO 6 octahedra unambiguously. Hence, it cannot be differentiated between GeO 5 or GeO 6 polyhedra being formed in addition to the GeO 4 tetrahedra. The increase of the Ge–O coordination number complies with the concept of a lack of non-bridging oxygen atoms in alkali germanate glasses up to concentrations of about 20 mol% K 2O where the additional oxygen atoms are incorporated in Ge–O–Ge bridges.

Structure ofA2Ge4O9-Type Sodium Tetragermanate (Na2Ge4O9) and Comparison with Other Alkali Germanate and Silicate Mixed Tetrahedral–Octahedral Framework Structures

Long-standing uncertainty on the structure type of Na2Ge4O9has been resolved. Sodium tetragermanate has been grown by crystallization from a supercooled melt and its single-crystal X-ray structure has been determined (R=0.022). Sodium tetragermanate is trigonal witha=11.3234(12),c=9.6817(9) Å, space groupP3c1,Z=6, andDx=4.451 g cm−3. The structure is comprised of a mixed tetrahedral–octahedral framework with three-membered [Ge3O9] rings of GeO4tetrahedra interconnected by isolated GeO6octahedra via shared corners and isA2Ge4O9-type. Bond distances and angles for GeO4tetrahedra and GeO6octahedra are very similar to the corresponding values in the type structure of K2Ge4O9, the two structures differing mainly in the accommodation of the smaller (medium–large-sized) Na cation, which is now in a 5+2 coordination. The structure–composition relationships of wadeite-type,A2Ge4O9-type, and Na2Si4O9-type structures of germanates and silicates depend largely on theT–O distance and the size of the monovalent cation. We confirm that sodium tetragermanate is a metastable phase at all pressures up to 2 kbar, the stable assemblage for the Na2Ge4O9composition being sodium enneagermanate (Na4Ge9O20) plus a more sodic phase.

Alkali sites in glass

In this paper we study the nature of sites occupied by alkali metal ions in oxide and organic glasses by probing the localised metal-site vibrations which are active in the far-infrared. Analysis of the far-infrared profiles of the glasses considered in this investigation indicates the presence of one or two distributions of cation hosting sites. It is argued that the energetics of these sites are determined by both the bonding requirements of the metal cation and the versatility of the anionic glass network to fulfil these requirements. This is demonstrated by comparing earlier data on alkali borates and more recent results on alkali germanates and ionic organic glasses, as well as by studying the effect of the method of glass formation (fast melt quenching, slow cooling, sol–gel) on the far-infrared absorption envelope. The alkali metal sites inferred from far-infrared have been compared to those probed by ultraviolet spectroscopy using Tl + and Pb 2+ ions electrolysed into borate glasses. The results suggest that mobile alkali ions are mainly those occupying sites of high basicity.

Significance of intermediate range structure for electrical conduction in alkali germanate glasses

Significance of intermediate range structure for electrical conduction in alkali germanate glasses

Significance of intermediate range structure for electrical conduction in alkali germanate glasses

The dc ionic conductivity of the xM 2O:(1 - x)GeO 2 glass series has been measured as a function of temperature and composition where M is K or Rb and 0.0023 ≤ x ≤ 0.60. The structure of the same glasses have been determined by measuring X-ray absorption fine structures, densities and X-ray photoelectron, Raman and infrared absorption spectroscopies. With increasing alkali concentration the activation energy of conductivity decreases followed by an increase and then a more rapid decrease at x ≈ 0.1. By correlating this complex composition dependence of electrical conductivity with the structura information we demonstrate that the intermediate range structure of glass affects ion transport in alkali germanate glasses.

Intermediate-range order in lead metasilicate glass

The complementarity of anomalous X-ray scattering (AXS) and neutron diffraction (ND) techniques is used here to investigate the nature and origin of intermediate-range order in lead metasilicate glass. Both X-ray and neutron structure factors reveal small peaks at low wave vector which are shown to be associated with intermediate-range order of the PbO network. The combination of AXS and ND is shown to be a powerful tool to correlate contributions from the different atom pairs to such a peak. The information thus derived is compared with results from alkali germanate glasses and with structural data on corresponding crystalline compounds.

Mixed alkali effect on calcium aluminogermanate glasses

Germanate glasses containing 0–6.44 wt% Li 2O were prepared by progressive weight percent substitution of Na 2O for Li 2O, at a constant ratio of concentration of CaO, Al 2O 3 and GeO 2. The mixed alkali effect on the optical properties, chemical resistance and electrical resistivity of calcium aluminogermanate glasses have been studied. The results of FT-IR spectra show that absorption band in 800–900 cm −1 region corresponds to GeO 4 groups and the Ge remains in 4-coordination state for these germanate glasses. As 0 ≤ Na 2 O ( Li 2 O + Na 2 O) ≤ 0.5 , the refraction index of and reflectance decrease with increasing Na 2O content, during 1 ≥ Na 2 O ( Li 2 O + Na 2 O) ≥ 0.5 , then its values are increasing with increasing Na 2O content. The leaching results indicate that Haze uv values for these germanate glasses with different content of Li 2O and Na 2O are greater than those of glasses containing only Li 2O or Na 2O, but the variations of pH value when the glasses are immersed in acidic solutions are greater in single alkali germanate glasses than in mixed alkali glasses. For the electrical resistivity measured, a maximum value occurs at Na 2 O ( Li 2 O + Na 2 O) = 0.5 .

Far-infrared spectra of alkali germanate glasses and correlation with electrical conductivity.

The infrared spectra of 0.20{ital R}{sub 2}O-0.80GeO{sub 2} ({ital R}=Li,Na,K,Rb,Cs) and {ital x}Rb{sub 2}O-(1{minus}{ital x})GeO{sub 2} (0{lt}{ital x}{le}0.27) glasses were measured in the reflectance mode and analyzed by the Kramers-Kronig technique to investigate the nature and composition dependence of metal ion sites in germanate glasses. The deconvolution of the far-infrared profiles showed that in glasses of low alkali content ({ital x}{le}0.075 for {ital R}=Rb) alkali ions occupy one type of sites ({ital M}), while for higher alkali contents two types of site ({ital L} and {ital H}) were found. The ion motion frequencies in these sites are in the order {nu}{sub {ital L}}{lt}{nu}{sub {ital M}}{lt}{nu}{sub {ital H}}, and increase with increasing alkali oxide content. Factor group analysis of the alkali motion modes in analogous crystalline germanate compounds showed that the {ital H} band in glass can be assigned to ion motion in sites similar to those in the crystal. The low-frequency band ({ital L}) was attributed to ion motion in {open_quote}{open_quote}secondary{close_quote}{close_quote} energetic sites, whose coordination numbers and charge density are correspondingly larger and smaller than their optimum values. The presence of {ital L} sites is the cause of the extra absorption exhibited by glasses at low far-infrared frequencies, as comparedmore » to the crystals of similar composition. {ital M}-type sites were shown to be the precursors of {ital H} sites, but for the organization of the latter a minimum alkali oxide content is required. The comparison of activation energies for conductivity calculated on the basis of the free-ion model with observed values suggests that long-range ion movement is probably facilitated along {ital M} and {ital H}-type sites. {copyright} {ital 1996 The American Physical Society.}« less

Localized states in germanate glasses. Ultraviolet absorption tail of crystalline and glassy germanium dioxide and alkali germanate

The optical absorption of crystalline and glassy germanate were measured with a double-beam instrument. Pure GeO2, Li2GeO3 crystal, pure GeO2, sodium germanate and lanthanum germanate glasses were studied. Glass samples were prepared by melting and some by evaporation during melting. Melting conditions were normal and oxidizing. Intrinsic absorption tails of a GeO2 crystal with a structure of α-quartz and in a Li2GeO3 crystal are situated at 6 eV. Two absorption thresholds were observed in a GeO2 glass: one is situated at 6 ± 0.2 eV, the other at 4.5 ± 0.2 eV. The intensity of the latter increases in sodium germanate glasses and corresponds to the intrinsic absorption threshold of a tetragonal GeO2 crystal. This threshold is known from the literature. The absorption spectra and their thermal dependences are well approximated by the Urbach rule in two forms. The threshold at 6 eV in all the materials studied obeys the Urbach-Toyozawa rule. The threshold at 4.5 eV agrees with the so called ‘glass’ type rule.

AC Conductivity of Crystalline Materials and Glasses Ascribed to ADWPs

ABSTRACTThe behavior of the frequency dependence of the conductivity, σ(ω), of numerous crystalline materials and glasses is close to an ω1.0 dependence in the limit of low temperatures and/or high frequencies (referred to as the “nearly constant loss”, or NCL, regime). Detailed analysis of this behavior, including the frequency dependence of both ε′ and ε″, shows that it can be described phenomenologically as produced by a broad distribution of asymmetric double-well potentials (ADWPs) with low activation energies. In order to obtain an understanding of the atomic origins of such potentials, we investigate the composition dependence of this behavior in such materials as crystalline CeO2:1%Y3+ ceramics with variable [Y3+] and alkali germanate glasses with variable alkali concentration. The appearance of a discrete loss peak in CeO2: 1%Y3+ helps us understand the ADWPs as due to “off-symmetry” configurations that undergo wiggling motion between adjacent minimum-energy positions.