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.     

Local structure of magnesium in silicate glasses: a 25Mg 3QMAS NMR study

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.     

MAS-NMR studies of lithium aluminum silicate (LAS) glasses and glass-ceramics having different Li2O/Al2O3 ratio

Emergence of phases in lithium aluminum silicate (LAS) glasses of composition (wt%) xLi₂O-71.7SiO₂-(17.7−x)Al₂O₃-4.9K₂O-3.2B₂O₃-2.5P₂O₅ (5.1≤x≤12.6) upon heat treatment were studied. ²⁹Si, ²⁷Al, ³¹P and ¹¹B MAS-NMR were employed for structural characterization of both LAS glasses and glass-ceramics. In glass samples, Al is found in tetrahedral coordination, while P exists mainly in the form of orthophosphate units. B exists as BO₃ and BO₄ units. ²⁷Al NMR spectra show no change with crystallization, ruling out the presence of any Al containing phase. Contrary to X-ray diffraction studies carried out, ¹¹B (high field 18.8 T) and ²⁹Si NMR spectra clearly indicate the unexpected crystallization of a borosilicate phase (Li,K)BSi₂O₆, whose structure is similar to the aluminosilicate virgilite. Also, lithium disilicate (Li₂Si₂O₅), lithium metasilicate (Li₂SiO₃) and quartz (SiO₂) were identified in the ²⁹Si NMR spectra of the glass-ceramics. ³¹P NMR spectra of the glass-ceramics revealed the presence of Li₃PO₄ and a mixed phase (Li,K)₃PO₄ at low alkali concentrations.     

Structure-directing roles and interactions of fluoride and organocations with siliceous zeolite frameworks

Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) (1)H, (19)F, and (29)Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F(-) anions, imidazolium structure-directing agents (SDA(+)), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA(+) species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F(-) and SDA(+) ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F(-) anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong (19)F-(29)Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F(-) anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA(+) ions. Such differences manifest the importance of interactions between the imidazolium and F(-) ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D (29)Si{(29)Si} double-quantum NMR measurements establish (29)Si-O-(29)Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures.     

Combined solid state NMR and X-ray diffraction investigation of the local structure of the five-coordinate silicon in fluoride-containing as-synthesized STF zeolite

The local structure of the [SiO(4/2)F]- unit in fluoride-containing as-synthesized STF zeolite has been experimentally determined by a combination of solid-state NMR and microcrystal X-ray diffraction to be very close to trigonal bipyramidal. Because the fluoride ions are disordered over two sites, the resulting local structure of the [SiO(4/2)F]- unit from a conventional XRD refinement is an average between tetrahedral SiO(4/2) and five-coordinate [[SiO(4/2)F]-, giving an apparent F-Si distance longer than expected. The correct F-Si distance was determined by slow spinning MAS and fast spinning (19)F/(29)Si CP and REDOR solid-state NMR experiments and found to be between 1.72 and 1.79 A. In light of this, the X-ray structure was re-refined, including the disorder at Si3. The resulting local structure of the [SiO(4/2)F]- unit was very close to trigonal bipyramidal with a F-Si distance of 1.744 (6) A, in agreement with the NMR results and the prediction of Density Functional Theory calculations. In addition, further evidence for the existence of a covalent F-Si bond is provided by a (19)F-->(29)Si refocused INEPT experiment. The resonance for the five-coordinate species at -147.5 ppm in the (29)Si spectrum is a doublet due to the (19)F/(29)Si J-coupling of 165 Hz. The peaks in this doublet have remarkably different effective chemical shift anisotropies due to the interplay of the CSA, dipolar coupling, and J-coupling tensors. The distortions from tetrahedral geometry of the neighboring silicon atoms to the five-coordinate Si3 atom are manifested in increased delta(aniso) values. This information, along with F-Si distances measured by (19)F-->(29)Si CP experiments, makes it possible to assign half of the (29)Si resonances to unique tetrahedral sites. As well as determining the local geometry of the [SiO(4/2)F]- unit, the work presented here demonstrates the complementarity of the solid-state NMR and X-ray diffraction techniques and the advantages of using them together.     

MAS-NMR investigations of the crystallization behaviour of lithium aluminum silicate (LAS) glasses containing P2O5 and TiO2 nucleants

Lithium aluminum silicate (LAS) glass of composition (mol%) 20.4Li₂O-4.0Al₂O₃-68.6SiO₂-3.0K₂O-2.6B₂O₃-0.5P₂O₅-0.9TiO₂ was prepared by melt quenching. The glass was then nucleated and crystallized based on differential thermal analysis (DTA) data and was characterized by ²⁹Si, ³¹P, ¹¹B and ²⁷Al MAS-NMR. XRD and ²⁹Si NMR showed that lithium metasilicate (Li₂SiO₃) is the first phase to c form followed by cristobalite (SiO₂) and lithium disilicate (Li₂Si₂O₅). ²⁹Si MAS-NMR revealed a change in the network structure already for the glasses nucleated at 550 °C. Since crystalline Li₃PO₄, as observed by ³¹P MAS-NMR, forms concurrently with the silicate phases, we conclude that crystalline Li₃PO₄ does not act as a nucleating agent for lithium silicate phases. Moreover, ³¹P NMR indicates the formation of M-PO₄ (M=B, Al or Ti) complexes. The presence of BO₃ and BO₄ structural units in all the glass/glass-ceramic samples is revealed through ¹¹B MAS-NMR. B remains in the residual glass and the crystallization of silicate phases causes a reduction in the number of alkali ions available for charge compensation. As a result, the number of trigonally coordinated B (BO₃) increases at the expense of tetrahedrally coordinated B (BO₄). The ²⁷Al MAS-NMR spectra indicate the presence of tetrahedrally coordinated Al species, which are only slightly perturbed by the crystallization.     

Fluorinated hydrotalcites: the addition of highly electronegative species in layered double hydroxides to tune basicity

Hydrotalcite-like compounds were synthesized by coprecipitation using a constant-pH method. Aluminum was introduced in the form of an octahedral aluminum fluorine complex during the synthesis. Controlling the pH during the synthesis ensured that aluminum fluorine did not decompose to tetrahedral species but remained in the octahedral (AlF(6))(3-) form to be incorporated into brucite-like sheets. The physicochemical, thermal, and spectroscopic characterizations showed significant modifications by fluorine introduction regarding structural, textural, and adsorption properties. The memory effect of fluorinated hydrotalcites differed from the memory effect commonly observed in fluorine-free hydrotalcites. Nitromethane adsorption showed that the basicity of hydrotalcite was greatly modified by the fluorine loading.     

Reversible photoswitching of the coordination numbers of silicon in organosilicon compounds bearing a 2-(phenylazo)phenyl group

Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen-lithium transmetalation of 2-iodoazobenzene. Their structures were determined by (1)H, (13)C, (19)F, and (29)Si NMR spectra, UV-vis spectra, and X-ray crystallographic analyses. In the UV-vis spectra, silyl groups caused red shifts of both the n-pi and pi-pi transitions of the azo group compared with the transitions of the unsubstituted azobenzene. The E-isomers of the fluorosilanes showed an intramolecular interaction between a nitrogen atom of the azo group and the silicon atom, leading their intermediate structures between a distorted trigonal bipyramidal structure and a tetrahedral structure around the silicon atoms, which were revealed by the X-ray crystallographic analyses and the NMR spectra. On the other hand, silanes without fluorine atoms showed tetrahedral structures in the absence of such an interaction. The photoirradiation of the E-isomers of the fluorosilanes afforded reversibly the corresponding Z-isomers in good yields. The silicon atoms of the Z-isomers were found to be tetracoordinate in the absence of Si-N interactions by the (29)Si NMR spectra. The coordination numbers of the silicon atom of the fluorosilanes were reversibly switched between four and five by photoirradiation. These properties were compared to those of a tetrafluoro[2-(phenylazo)phenyl]silicate.     

Distribution of isomorphous substitutions in silicates by NMR spectroscopy

The use of spectroscopic methods permits in certain cases to investigate the relative distribution of cations and anions in silicate structures. In particular, in micas (phyllosilicates 2:1) with very heterogeneous composition, the use of multinuclear (²⁹Si, ²⁷Al, ¹H and ¹⁹F) NMR spectroscopy has demonstrated the existence of local ordering in both the tetrahedral and octahedral sheets.From the study of NMR spectra associated to OH⁻ and F⁻ ions we have concluded that: a) the Fe⁺² ions have not any preference to occupy M1 or M2 sites in the octahedral sheet of micas, b) the F⁻ ions are located in homogeneous domains composed only by F⁻ and Mg⁺² ions, and c) the Fe⁺² are in close association with OH⁻ ions. As the content in Fe⁺² and F⁻ increases Fe⁺² ions have a net tendency to cluster around the OH groups.The analysis of ²⁹Si and ²⁷Al NMR spectra showed that in the tetrahedral sheet of micas: a) each Al is surrounded by 3Si, and b) Si, Al distribution is more dispersed that imposed by Loewenstein's rule.     

NMR spectroscopy of bismuth fluoride glasses with alkali cations

Results of the ⁷Li, ¹⁹F, and ²³Na NMR studies of ionic mobility in bismuth fluoride glasses in the systems BiF₃-LiF and BiF₃-MF-ZrF₄ (M = Li, Na, K, Cs) are summarized. Analysis of the ⁷Li, ¹⁹F, and ²³Na NMR spectra made it possible to reveal changes in the nature of ion motions in the fluoride, lithium and sodium sublattices of glasses upon temperature variation and to determine their types. The temperature ranges were found where main types of ion motions in the tested glasses are represented by diffusion of lithium ions, reorientations of fluorine-containing groups constituting the glass network, and diffusion of fluorine ions. The role of alkali cations in the formation of ionic mobility in bismuth fluorozirconate glasses is considered.     

Fluorine sites in glasses and transparent glass-ceramics of the system Na2O/K2O/Al2O3/SiO2/BaF2

The transparent glass-ceramics obtained in the silicate system Na₂O/K₂O/SiO₂/BaF₂ show homogeneously dispersed BaF₂ nano crystals with a narrow size distribution. The X-ray diffraction and the nuclear magnetic resonance spectroscopy were applied to glasses and the respective glass-ceramics in order to clarify the crystallization mechanism and the role of fluorine during crystallization. With an increasing annealing time, the concentration and also the number of crystals remain approximately constant. With an increasing annealing temperature, the crystalline fraction increases until a saturation limit is reached, while the number of crystals decreases and the size of the crystals increases. Fluoride in the glassy network occurs as Al-F-Ba, Al-F-Na and also as Ba-F structures. The latter are transformed into crystalline BaF₂ and fluoride is removed from the Al-F-Ba/Na bonds. However, some fluorine is still present in the glassy phase after the crystallization.     

Formation of functional phosphosilicate gels from phytic acid and tetraethyl orthosilicate

Phosphosilicate gels with high phosphorus content (P mol% > Si mol%) have been prepared using phytic acid as the phosphorus precursor, with tetraethyl orthosilicate (TEOS). It is shown that the structure of phytic acid is maintained in both the sols and those gels dried at a low temperature (i.e. ≤120 °C). Solid state ²⁹Si and ³¹P NMR suggest that the gel network is primarily based on tetrahedral silicon and that phosphorus is not chemically incorporated into the silicate network at this point. X-ray diffraction shows the gel to be amorphous at low temperatures. After heat treatment at higher temperatures (i.e. up to 450 °C), P–O–Si linkages are formed and the silicon coordination changes from tetrahedral to octahedral. At the same time, the gel crystallizes. Even after this partial calcination, ³¹P NMR shows that a large fraction of phytic acid remains in the network. The function of phytic acid as chelating agent is also maintained in the gels dried at 120 °C such that its ability to absorb Ca²⁺ from aqueous solution is preserved.     

Studies on the role of fluoride ion vs reaction concentration in zeolite synthesis

This work is an experimental response to an intriguing paper recently published by Catlow and co-workers, which looked at the computational feasibility of fluorine location in three different all-silica zeotypes (Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708). The materials were chosen as representative of three unique host locations. Our present work examined the synthesis of zeotypes AST, IFR, and MTT using organo-cations with a strong preference for crystallizing these structures. We studied the effect of reaction time and the H(2)O/SiO(2) reactant ratio. The latter is probably the most important function in these zeolite crystallizations that use HF. As reaction conditions became more dilute, AST gave way to SGT and IFR to MTW as host structures, while the MTT synthesis was invariant. Our reactions were studied in terms of product yield vs time, product organo-cation content, fluorine content, and the representative (29)Si and (19)F NMR spectra for certain samples. A single crystal study was carried out for a sample of MTT. Our results showed that, consistent with other recent studies, low H(2)O/SiO(2) reactant ratios lead to more open framework host structures (i.e., IFR vs MTW), and there is typically a higher uptake of organo-cation and fluorine. The structure may well contain a higher population of 4-rings within the silicate substructure. While MTT that contains no 4-rings was chosen as the best possible candidate to achieve an ion-pair for the organo-cation and fluoride anion within the silicate host, both NMR and single crystal work confirm that fluoride is bonded to a 5-coordinate silica center within the lattice.     

Glass structure and ion dynamics of lead-cadmium fluorgermanate glasses

Glass structure and fluorine motion dynamics are investigated in lead-cadmium fluorgermanate glasses by means of differential scanning calorimetry, Raman scattering, x-ray absorption (EXAFS), electrical conductivity (EC), and (19)F nuclear magnetic resonance (NMR) techniques. Glasses with composition 60PbGeO(3)-xPbF(2)-yCdF(2) (in mol %), with x+y=40 and x=10, 20, 30, 40, are studied. Addition of metal fluorides to the base PbGeO(3) glass leads to a decrease of the glass transition temperature (T(g)) and to an enhancement of the ionic conductivity properties. Raman and EXAFS data analysis suggest that metagermanate chains form the basic structural feature of these glasses. The NMR study leads to the conclusion that the F-F distances are similar to those found in pure crystalline phases. Experimental results suggest the existence of a heterogeneous glass structure at the molecular scale, which can be described by fluorine rich regions permeating the metagermanate chains. The temperature dependence of the NMR line shapes and relaxation times exhibits the qualitative and quantitative features associated with the high fluorine mobility in these systems.     

Variable temperature 1H, 23Na,and 29Si MAS NMR studies on sodium silicate hydrates of composition Na2O · SiO2 · nH2O (n = 9, 6, 5): Local structure in crystals,melts, supercooled melts,and glasses

The local structure in crystals, melts, supercooled melts, and glasses of sodium silicate hydrates of composition Na₂O · SiO₂ · nH₂O (n = 9, 6, 5) is studied by variable temperature ¹H, ²³Na, and ²⁹Si MAS NMR spectroscopy. Detailed in situ investigations on the melting process of the crystalline materials reveal the importance of H₂O motion in the melting mechanism. Depending on the local coordination, crystallographically distinct Na sites show different behaviour during the melting process. Upon melting, the monomer silicate anions present in the crystalline hydrates undergo condensation reactions to oligomeric silicate anions. No recrystallization but glass formation occurs at low temperature if the melts were heated initially about 10 K above the melting point. In the glasses also oligomeric silicate anions are present with a preference for cyclotrimer species. In situ MAS NMR investigations and electric conductivity measurements of the melts, supercooled melts, and glasses suggest the distinction of three temperature ranges characterized by different local structure and dynamics of the sodium cations, water and silicate anions. These ranges comprise a glass and glass transition range A at low temperatures, an aggregation region B at intermediate temperatures, and a solution or electrolyte region C at high temperatures. In region B aggregation of sodium water complexes to hydrated polycation clusters is suggested, the dynamic behaviour of which is clearly different to that of the silicate anions, indicating that no long-lived contact ion pairs between sodium cations and silicate anions are formed.     

The structure of phosphate and borosilicate glasses and their structural evolution at high temperatures as studied with solid state NMR spectroscopy: Phase separation,crystallisation and dynamic species exchange

In this contribution we present an in-depth study of the network structure of different phosphate based and borosilicate glasses and its evolution at high temperatures. Employing a range of advanced solid state NMR methodologies, complemented by the results of XPS, the structural motifs on short and intermediate length scales are identified. For the phosphate based glasses, at temperatures above the glass transition temperature T_g, structural relaxation processes and the devitrification of the glasses were monitored in situ employing MAS NMR spectroscopy and X-ray diffraction. Dynamic species exchange involving rapid P–O–P and P–O–Al bond breaking and reforming was observed employing in situ ²⁷Al and ³¹P MAS NMR spectroscopy and could be linked to viscous flow. For the borosilicate glasses, an atomic scale investigation of the phase separation processes was possible in a combined effort of ex situ NMR studies on glass samples with different thermal histories and in situ NMR studies using high temperature MAS NMR spectroscopy including ¹¹B MAS, ²⁹Si MAS and in situ ²⁹Si{¹¹B} REAPDOR NMR spectroscopy.     

Distribution of aluminum and boron in the periodical building units of boron-containing Beta zeolites

Various boron only ([B]-BEA) as well as aluminum- and boron-containing beta zeolites ([Al,B]-BEA) have been prepared and modified by ion exchange of ammonium, sodium, and nickel ions. The zeolite samples have been characterized by 11B, 27Al, and 29Si MAS as well as three of them by 11B and 27Al 3Q-MAS NMR spectroscopy. The quantitative contributions of defect-free Si(nX) (n = 2, 1, 0; X = Al, B) and Si(OH)x (x = 2, 1) sites to the NMR signal intensities were calculated from the various Si/(Al + B) ratios and relative 11B, 27Al, and 29Si NMR signal intensities using the special distribution of aluminum and boron in different periodical building units of the zeolite framework. The boron atoms are sitting exclusively in diagonal positions in the four-membered rings of [B]-BEA zeolites, while the aluminum atoms are situated both in diagonal and lone positions in the four-membered rings of [Al,B]-BEA zeolites. A higher part of boron atoms are positioned in framework-related deformed tetrahedral boron species than in lattice positions in the [B]-BEA than in the [Al,B]-BEA zeolites. All extraframework octahedral aluminum species are transformed back to lattice positions due to ion exchange from the protonated form to ammonium-, sodium-, or nickel-ions containing zeolites. Oppositely, trigonal boron leaves the zeolite structure completely during ion exchange.     

Thermal and Chemical Properties of a Glass in the SiO2-CaO-F System for Dental Applications

The crystallization behaviour of a glass in the SiO₂-CaO-F system was analyzed using differential scanning calorimetry (DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Three crystalline phases were detected according to ICDD patterns. The first phase formed at 583°C was identified as CaF₂. The morphology was spherulitic with a diameter of approximately 100 nm. The second phase was formed at 664°C. It was identified as calcium fluoride silicate ‘Ca₂SiO₂F₂’ (ICDD 35-0002). SEM investigation showed that the crystals were spherulitic with a diameter smaller than 100 nm. The crystals were precipitated in the volume of the glass and homogeneously distributed. As a third phase, cristobalite crystallized at 895°C. The simultaneous release of calcium and fluorine ions from the vitreous glass in lactate buffer solution at pH 4.0, simulating an acidic oral environment, was investigated using X-ray photoelectron spectroscopy (XPS). The release of calcium and fluorine ions is of special interest for dental applications. The atomic ratios of the components Si, Ca and F at the glass surface after different leaching periods were determined. In order to investigate the leaching process, concentration profiles were measured using ion beam sputtering with Ar⁺ -ions. The dependence of the atomic ratios of Si, Ca and F on the sputter time was determined in order to measure the depth of the leaching layers. Most probably, the release of calcium and fluoride was controlled by a surface layer rich in calcium and flourine ions which dissolved with increasing leaching time. After 2 min leaching, a fluoride-rich surface layer measuring approximately 10 nm was detected. The atomic ratios of Si, Ca and F were different from the bulk composition ratios in a surface reaction layer of 800 nm thickness. After 30 min leaching time, a calcium- and fluoride-rich surface layer approximately 50 nm thick was formed. The bulk composition was reached at a depth of approximately 500 nm. The main component in the surface layer, after 12 days leaching in acidic environment, was silicon. This revised version was published online in August 2006 with corrections to the Cover Date.     

29Si NMR studies of zeolite precursor solutions

Solution 29Si NMR spectroscopy results of zeolite precursor solutions of composition 1 SiO2:4 C2H5OH:0.36/n R+n[OH-]n:20 H2O are reported. This work employs isotopically enriched 29Si materials to aid in spectral interpretation. Using both 1D and 2D methods, spectra of solutions containing tetrapropylammonium hydroxide are wholly consistent with the existing silicate chemistry literature and indicate that the majority of the species are high-symmetry silicate clusters previously observed in aqueous solutions. The results are inconsistent with the nanoblock or nanoslab model proposed by Kirschhock and co-workers. Mixtures containing the 4,4'-trimethylene-bis(1,1'-dimethylpiperidinium) dihydroxide cation were also studied. These mixtures have similar speciation to the TPA solutions, although the relative populations of the species are different. Preliminary variable temperature 29Si NMR of these mixtures shows that the exchange properties of the high-symmetry silicate species, most notably the tetrahedral tetramer, depend on the organocation identity.