Vibrational spectra and structure of mixed alkali borotungstate glasses: Evidence of mixed alkali effect

Mixed alkali borotungstate glasses with xLi₂O–(30 − x)Na₂O–10WO₃–60B₂O₃ (0 ≤ x ≤ 30) composition were prepared by melt quench technique. FT-IR and Raman spectroscopic studies were employed to investigate the structure of all the prepared glasses. Acting as complementary techniques, both IR and Raman measurements revealed that the network structure of the present glasses mainly based on BO₃ and BO₄ units placed in different structural groups. Raman spectra confirm the IR results regarding the presence of tungsten ions mainly as WO₆ groups. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system using FTIR and Raman studies.     

Electrical conductivity studies in sulphate glasses and the mixed alkali effect

Electrical conductivities of alkali sulphate-zinc sulphate glasses have been measured. The variation of conductivity with compositon confirms the presence of the mixed alkali effect. The origin of mixed alkali effect has been explained on the basis of structural considerations reported earlier by us. Communication No. 160 from the Solid State and Structural Chemistry Unit.     

The mixed alkali effect in ionically conducting glasses revisited: a study by molecular dynamics simulation

When more than two kinds of mobile ions are mixed in ionic conducting glasses and crystals, there is a non-linear decrease of the transport coefficients of either type of ion. This phenomenon is known as the mixed mobile ion effect or Mixed Alkali Effect (MAE), and remains an unsolved problem. We use molecular dynamics simulation to study the complex ion dynamics in ionically conducting glasses including the MAE. In the mixed alkali lithium-potassium silicate glasses and related systems, a distinct part of the van Hove functions reveals that jumps from one kind of site to another are suppressed. Although, consensus for the existence of preferential jump paths for each kind of mobile ions seems to have been reached amongst researchers, the role of network formers and the number of unoccupied ion sites remain controversial in explaining the MAE. In principle, these factors when incorporated into a theory can generate the MAE, but in reality they are not essential for a viable explanation of the ion dynamics and the MAE. Instead, dynamical heterogeneity and "cooperativity blockage" originating from ion-ion interaction and correlation are fundamental for the observed ion dynamics and the MAE. Suppression of long range motion with increased back-correlated motions is shown to be a cause of the large decrease of the diffusivity especially in dilute foreign alkali regions. Support for our conclusion also comes from the fact that these features of ion dynamics are common to other ionic conductors, which have no glassy networks, and yet they all exhibit the MAE.     

Structural role of fluoride in aluminophosphate sol-gel glasses: high-resolution double-resonance NMR studies

The local structure of Na-Al-P-O-F glasses, prepared by a novel sol-gel route, was extensively investigated by advanced solid-state NMR techniques. 27Al{19F} rotational echo double resonance (REDOR) results indicate that the F incorporated into aluminophosphate glass is preferentially bonded to octahedral Al units and results in a significant increase in the concentration of six-coordinated aluminum. The extent of Al-F and Al-O-P connectivities are quantified consistently by analyzing 27Al{31P} and 27Al{19F} REDOR NMR data. Two distinct types of fluorine species were identified and characterized by various 19F{27Al}, 19F{23Na}, and 19F{31P} double resonance experiments, which were able to support peak assignments to bridging (Al-F-Al, -140 ppm) and terminal (Al-F, -170 ppm) units. On the basis of the detailed quantitative dipole-dipole coupling information obtained, a comprehensive structural model for these glasses is presented, detailing the structural speciation as a function of composition.     

Electrical conductivity and relaxation in mixed alkali tellurite glasses

The authors have reported the electrical conductivity and the conductivity relaxation in mixed alkali tellurite glasses of compositions of 70TeO2-xNa2O-(30-x)Li2O in the frequency range from 10 Hz to 2 MHz and in the temperature range from room temperature to just below the glass transition temperature. They have analyzed the relaxation data in the framework of different models. They have observed the mixed alkali effect in the dc and ac conductivities, the crossover frequency, and the conductivity relaxation frequency as well as in their respective activation energies in these glasses. They have also observed the mixed alkali effect in the decoupling index. The scaling property of the modulus spectra of these mixed alkali glasses shows that the conductivity relaxation in the mixed alkali tellurite glasses is independent of temperature but depends on the glass compositions.     

Cation Effects in some series of mixed alkali fluoride glasses

An investigation of electrical properties of two series of fluoride glasses containing simultaneously LiF and NaF has been undertaken. The alkali fluoride contents are respectively x_AF = 0.10 and x_AF = 0.30 : (ZrF₄)_0.58(BaF₂)_0.25(LaF₃)_0.07(LiF)_0.10-x(NaF)_x and (ZrF₄)_0.50(BaF₂)_0.15(LaF₃)_0.05(LiF)_0.30-x(NaF)_x. In both series, a mixed alkali effect revealed by the existence of a conductivity minimum is apparently observed. It is shown, however, that the minima have a quite different nature and none of them is actually compatible with the classical ?mixed alkali effect”? of mobile cations. In the series x_AF = 0.10, where charge transport is essentially assured by the F^?-ions, this effect results from the presence of non mobile alkali ions of different nature in the vicinity of the F^?-ions. In the series x_AF = 0.30, the electrical properties of the extreme compositions x_LiF = 0.30 and x_NaF = 0.30 are due respectively either to Li⁺-ions only or to mobile F^?-ions only. The variation of the ratio r = Na/(Li + Na) from 0 to 1 corresponds to progressive cross over from a cationic conducting glass to an anionic conducting one. The presence of a minimum of conductivity for 0.50 < r < 0.75 has been explained.     

"Cooperativity blockage" in the mixed alkali effect as revealed by molecular-dynamics simulations of alkali metasilicate glass

The relaxation dynamics of a complex interacting system can be drastically changed when mixing with another component having different dynamics. In this work, we elucidate the effect of the less mobile guest ions on the dynamics of the more mobile host ions in mixed alkali glasses by molecular-dynamics (MD) simulations. One MD simulation was carried out on lithium metasilicate glass with the guest ions created by freezing some randomly chosen lithium ions at their initial locations at 700 K. A remarkable slowing down of the dynamics of the majority mobile Li ions was observed both in the self-part of the density-density correlation function, Fs(k,t), and in the mean-squared displacements. On the other hand, there is no significant change in the structure. The motion of the Li ions in the unadulterated Li metasilicate glass is dynamically heterogeneous. In the present work, the fast and slow ions were divided into two groups. The number of fast ions, which shows faster dynamics (Levy flight) facilitated by cooperative jumps, decreases considerably when small amount of Li ions are frozen. Consequently there is a large overall reduction of the mobility of the Li ions. The result is also in accordance with the experimental finding in mixed alkali silicate glasses that the most dramatic reduction of ionic conductivity occurs in the dilute foreign alkali limit. Similar suppression of the cooperative jumps is observed in the MD simulation data of mixed alkali system, LiKSiO3. Naturally, the effect found here is appropriately described as "cooperativity blockage." Slowing down of the motion of Li ions also was observed when a small number of oxygen atoms chosen at random were frozen. The effect is smaller than the case of freezing some the Li ions, but it is not negligible. The cooperativity blockage is also implemented by confining the Li metasilicate glass inside two parallel walls formed by freezing Li ions in the same metasilicate glass. Molecular-dynamics simulations were performed on the dynamics of the Li ions in the confined glass. Slowing down of the dynamics is largest near the wall and decreases monotonically with distance away from the wall.     

Structural changes above the glass transition and crystallization in aluminophosphate glasses: an in situ high-temperature MAS NMR study

We present an in situ high-temperature nuclear magnetic resonance study on the structural changes in aluminophosphate glasses occurring in the temperature range between the glass transition temperature Tg and the crystallization temperature Tc, Tg < T < Tc. Decisive changes in the network organization between Tg and Tc in potassium aluminophosphate glasses in the compositional range 50K2O-xAl2O3-(50 - x)P2O5 with 2.5 < x < 20 could be monitored for the first time employing 1D 31P- and 27Al-MAS NMR. Accompanying ex situ NMR experiments (31P-RFDR NMR and 31P-{27Al} CP-HETCOR NMR) on devitrified samples were performed at room temperature to further characterize the phases formed during the crystallization process. The structural role of boron-which is known to inhibit the crystallization process in these aluminophosphate glasses-on short and intermediate length scales was analyzed employing 11B-MQMAS, 11B-{27Al} TRAPDOR and 11B-{31P} REDOR NMR spectroscopy.     

Structural investigation and electron paramagnetic resonance of vanadyl doped alkali niobium borate glasses

Glasses with compositions xNb₂O₅·(30 ? x)M₂O·69B₂O₃ (where M = Li, Na, K; x = 0, 4, 8 mol%) doped with 1 mol% V₂O₅ have been prepared using normal melt quench technique. The IR transmission spectra of the glasses have been studied over the range 400–4000 cm^?1. The changes caused by the addition of Nb₂O₅ on the structure of these glasses have been reported. The electron paramagnetic resonance spectra of VO²⁺ ions in these glasses have been recorded in X-band (9.14 GHz) at room temperature (300 K). The spin Hamiltonian parameters, dipolar hyperfine coupling parameter and Fermi contact interaction parameter have been calculated. It is observed that the resultant resonance spectra contain hyperfine structures (hfs) due to V⁴⁺ ions which exist as VO²⁺ ions in octahedral coordination with a tetragonal compression in the present glasses. The tetragonality of V⁴⁺O₆ complex decreases with increasing concentration of Nb₂O₅. The 3d_xy orbit contracts with increase in Nb₂O₅:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported.     

First structural evidence for multiple alkali metals between sandwich decks in a metallocene

A tetralithio salt (1) derived by treating 1,4-bis(trimethylsilyl)-cyclooctatriene with (n)BuLi serves as the first structural evidence for a multi-alkali metallocene. Single-crystal XRD confirms two Li(+) each asymmetrically bind to η(3) and η(4) between two COT' rings and two Li(+) terminally bind to η(3). Solid-state NMR studies confirm the presence of two distinct lithium ion sites while the solution NMR studies suggest the formation of an (COT' dianion) ion-pair in solution. Further treating of the tetralithio salt with NaCl leads to linear sodium polymeric chains. Therefore, simply changing the ionic radius changes the molecular structure.     

Optical absorption and photoluminescence properties of Er3+ doped mixed alkali borate glasses

An investigations of the optical absorption and fluorescence spectra of 0.2 mol% Er2O3 in mixed alkali borate glasses of the type 67.8B2O3 x xLi2O(32-x)Na2O, 67.8B2O3 x xLi2O(32-x)K2O and 67.8B2O3 x xNa2O(32-x)K2O (where x = 8, 12, 16, 20 and 24) are presented. The glasses were obtained by quenching melts consisting of H3BO3, Li2CO3, Na2CO3, K2CO3 and Er2O3 (950-1100 degrees C, 1.5-2 h) between two brass plates. Spectroscopic parameters like Racah (E1, E2 and E3), spin-orbit (xi(4f)) and configuration interaction (alpha) parameters are deduced as function of x. Using Judd-Ofelt theory, Judd-Ofelt intensity parameters (omega2, omega4 and omega6) are obtained. Radiative and non-radiative transition rates (A(T) and W(MPR)), radiative lifetimes (tauR), branching ratios (beta) and integrated absorption cross-sections (sigma) have been computed for certain excited states of Er3+ in these mixed alkali borate glasses. Emission spectra have been studied for all the three Er3+ doped mixed alkali borate glasses. The present paper throws light on the trends observed in the intensity parameters, radiative lifetimes, branching ratios and emission cross-sections as a function of x in these borate glasses, keeping in view the effect of mixed alkalies in borate glasses.     

Oxidation state and coordination environment of uranium in sodium iron aluminophosphate glasses

An analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of uranium determined the oxidation state and coordination environment of uranium atoms in glasses containing 40 mol % Na₂O, 10 mol % Al₂O₃, 10 mol % Fe₂O₃, and 40 mol % P₂O₅ to which uranium oxides were added to a concentration of 50 wt % (above 100%). If the added amount of UO₂ was small, uranium occurred as U(IV) in a near-octahedral oxygen environment with an average U–O distance in the first coordination sphere of 2.25 Å. At higher concentrations of uranium oxides introduced both as UO₂ and as UO₃, uranium occurred as U(V) and U(VI); the first coordination sphere is split; shorter (~1.7–1.8 Å) and longer (2.2–2.3 Å) distances were observed, which corresponded to the axial and equatorial U–O bonds in uranyl ions, respectively; and the redox equilibrium shifted toward U(VI). The glass with the maximal (~33 wt %) UO₃ concentration contained mainly U(VI). The existence of low-valence uranium species can be related to the presence of Fe(II) in glasses. The second coordination sphere of uranium manifests itself only at high concentrations of uranium oxides.     

The mixed network former effect in glasses: solid state NMR and XPS structural studies of the glass system (Na2O)(x)(BPO4)(1-x)

The structural organization of sodium borophosphate glasses with composition (Na(2)O)(x)(BPO(4))(1-x) (0.25 ≤x≤ 0.55) has been investigated by differential scanning calorimetry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), as well as single- and double resonance (11)B and (31)P magic-angle spinning (MAS) NMR. (11)B MAS-NMR data indicate the dominance of anionic four-coordinated boron units, and (31)P MAS NMR reveals the successive transformation of neutral P(3) into singly charged P(2) units and their further transformation into doubly charged P(1) units at high Na(2)O contents. The quantification of these units provides detailed insight into the competition of the network formers borate and phosphate for the network modifier oxide. At low modifier content (x < 0.35), the anionic species are almost exclusively borate (B(4)) units, whereas at higher sodium concentrations, large numbers of anionic phosphate (P(2) and P(1)) species are formed. O-1s XPS data provide a quantitative distinction between B-O-B, B-O-P, and P-O-P linkages as well as non-bridging oxygen atoms, and comparable numbers can be extracted from (11)B and (31)P MAS-NMR experiments. Both XPS as well as (31)P{(11)B} and (11)B{(31)P} rotational echo double resonance (REDOR) NMR results reveal strong interactions between the two network formers boron oxide and phosphorus oxide, resulting in a preferred formation of B-O-P linkages. For higher Na(2)O contents, however, the successive network modification diminishes this preference, resulting in close-to-statistical network connectivities. Compositional trends of T(g) in the Na(2)O-B(2)O(3)-P(2)O(5) glass forming system can be correlated with the overall network connectedness, expressed by the total number of bridging oxygen atoms per network former species. However, separate linear correlations are observed for different compositional lines, indicating also the relevance of the type of network former linkages present.     

Electronic effect on protonated hydrogen-bonded imidazole trimer and corresponding derivatives cationized by alkali metals (Li(+), Na(+), and K(+))

The electronic effects on the protonated hydrogen-bonded imidazole trimer (Im)(3)H(+) and the derivatives cationized by alkali metals (Li(+), Na(+), and K(+)) are investigated using B3LYP method in conjunction with the 6-311+G( *) basis set. The prominent characteristics of (Im)(3)H(+) on reduction are the backflow of the transferred proton to its original fragment and the remoteness of the H atom from the attached side bare N atom. The proton transfer occurs on both reduction and oxidation for the corresponding hydrogen-bonded imidazole trimer. For the derivatives cationized by Li(+), (Im)(3)Li(+), the backflow of the transferred proton occurs on reduction. The electron detachment from respective highest occupied molecular orbital of (Im)(3)Na(+) and (Im)(3)K(+) causes the proton transferring from the fragment attached by the alkali metal cation to the middle one. The order of the adiabatic ionization potentials of (Im)(3)M(+) is (Im)(3)H(+)>(Im)(3)Li(+)>(Im)(3)Na(+)>(Im)(3)K(+); the order of (Im)(3)M indicates that (Im)(3)H is the easicst complex to be ionized. The polarity of (Im)(3)M(+) (M denotes H, Li, Na, and K) increases on both oxidation and reduction. The (Im)(3)M(+) complexes dissociate into (Im)(3) and M(+) except (Im)(3)H(+), which dissociates preferably into (Im)(3) (+) and H atom, while the neutral complexes [(Im)(3)M] dissociate into (Im)(3) and M. The stabilization energy of (Im)(3)Li(2+), (Im)(3)Na(2+), and (Im)(3)K(2+) indicate that their energies are higher as compared to those of the monomers.     

Quadrupole moments of the alkali dimers,Li2, Na2, and K2

The quadrupole moments (Θ) of Li₂, Na₂, and K₂ have been determined using SCF, B3LYP, and CCSD(T). Included in this study is the effect of valence and core–valence correlation. The variation of Θ as a function of bond length has also been evaluated from which vibrational contributions have been determined. Additionally, the shape and relative magnitude of Θ vs. bond length are attributed to the s–s nature of the bonding. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Synthesis and characterisation of new bimetallic alkali metal-magnesium mixed diisopropylamide-acetylides: structural variations in bimetallic lithium- and sodium-heteroleptic magnesiates

The reactivity of the Br?nsted basic mixed-metal tris-amide compounds of empirical formula [MMg(N(i)Pr2)3] [where M = Li (1), Na (2)] towards phenylacetylene (HC[triple bond, length as m-dash]CPh) has been investigated and has led to the synthesis of a series of mixed-metal acetylido-amido-magnesiates. Thus, 1 and 2 molar equivalents of the alkyne with [MMg(N(i)Pr2)3] produce heteroanionic bis(amido)-mono(acetylido) [LiMg(N(i)Pr2)2(C[triple bond, length as m-dash]CPh)]2 (3) and mono(amido)-bis(acetylido) [(TMEDA) x Na(C[triple bond, length as m-dash]CPh)2Mg(N(i)Pr2)](2) (4) (TMEDA = N,N,N',N'-tetramethylethylenediamine) respectively. X-Ray crystallographic studies reveal that the new compounds adopt a different structural motif. Complex can be defined as an inverse crown structure, having a cationic eight-atom [(NaNMgN)2]2+ ring which hosts in its core two acetylido ligands. On the other hand, adopts a tetranuclear NaMgMgNa near-linear chain arrangement, held together by acetylido and amido bridges. The metal coordination geometries in both structures are distorted tetrahedral, and the sodium cations at the end of the mixed-metal chain carry terminal chelating TMEDA ligands. 1H and 13C NMR spectral data recorded in C6D6 solutions are also reported for and , and are consistent with the solid-state structures being retained in solution.     

Untreated and alkali treated fibers from Alfa stem: effect of alkali treatment on structural,morphological and thermal features

Alfa stems are rich in cellulose and they are an inexpensive, easily renewable source of natural fibers with the potential for polymer reinforcement. However, large amounts of non-cellulosic materials, surface impurities and low degradation temperature make natural fibers less attractive for reinforcement of polymeric materials, unless they can be modified in a proper way. In this paper, Alfa stems were treated with NaOH solution with two different concentrations (1 and 5 wt%). Raw and treated stems were crushed to obtain fibers. Stems and fibers were characterized by scanning electron microscopy (SEM) and optical microscopy, respectively. Their crystallinity index was determined by X-ray diffraction, thermal stability by thermogravimetry and structural change by FT-IR and ¹³C NMR spectroscopy. Comparison and analysis of results confirmed some thermal, structural and morphological changes of the fibers after treatment due to removal of some non-crystalline constituents from the plant. SEM showed rougher surfaces after alkalization. FT-IR and ¹³C NMR showed a gradual improvement in cellulose level by alkali treatment with increasing NaOH concentration. The crystallinity index and thermal stability of treated Alfa fibers were also found to be improved.     

Short- and medium-range order in sodium aluminophosphate glasses: new insights from high-resolution dipolar solid-state NMR spectroscopy

The structures of sodium aluminophosphate glasses prepared by both sol-gel as well as melt-cooling routes have been extensively characterized by high-resolution solid-state 23Na, 27Al, and 31P single and double-resonance NMR techniques, including quantitative connectivity studies by 27Al <--> 31P and 23Na <--> 31P rotational echo double-resonance (REDOR) methods. Studies along four compositional lines, I: (AlPO4)x -(NaPO3)1-x, II: (Na2O)x -(AlPO4)1-x, III: (NaAlO2)x -(NaPO3)1-x, and IV: (Al2O3)x (NaPO3)1-x, reveal that the network structures of those glasses that are accessible by either preparation method are essentially identical. However, the significantly extended glass-forming ranges available by the sol-gel route facilitate exploration of the structure/composition relationships in more detail, revealing a number of interesting universal features throughout the whole glass system. Both short- and medium-range order appear to be controlled strongly by the O/P ratio of the glasses studied: Up to an O/P ratio of 3.5 (pyrophosphate composition), aluminum is predominantly six-coordinated and fully connected to phosphorus (Al(OP)6 sites). In the region 3.5 < or = O/P < or = 4.0, a dramatic structural transformation takes place, leading to the appearance of additional four- and five-coordinated aluminum species whose second coordination spheres are also entirely dominated by phosphorus. The structure of glasses with an O/P ratio of precisely 4.0 (orthophosphate) is dominated by Al(OP)4 units. As the O/P ratio increases beyond 4.0, the average extent of Al-O-P connectivity is decreased significantly. Here, new types of five- and six-coordinated aluminum units, which are only weakly connected to phosphorus, are formed, while the network modifier is attracted mainly by the phosphate units.     

DFT study of the cryptand and benzocryptand and their complexes with alkali metal cations: Li+, Na+, K+

In the present work, a theoretical study of the cryptand 4, 7, 13, 16, 21, 24-hexaoxa-1, 10- diazabicyclo [8,8,8] hexacosan (the named [222]) and the cryptand 5, 6-benzo-4, 7, 13, 16, 21, 24-hexaoxa-1, 10-diazabicyclo [8, 8, 8] hexacosan (the nemed [222B]) had been done using density functional theory (DFT) with B3LYP/6-31G* method in order to obtain the electronic and geometrical structure of the cryptands and their complexes with alkali metal ions: Li(+), Na(+), and K(+). The nucleophilicity of cryptands had been investigated by the Fukui function. For complexes, the match between cation and cavity size, the status of interaction between alkali metal ions and donor atoms in the cryptands and the rigidity of the cryptands had been analyzed through the other calculated parameters. In addition, the enthalpies of complexation reaction and cation exchange reaction had been studied by the calculated thermodynamic data. The calculated results are in a good agreement with the experimental data for the complexes.