Bioactive sol-gel glasses at the atomic scale: the complementary use of advanced probe and computer modelling methods

Sol-gel synthesised bioactive glasses may be formed via a hydrolysis condensation reaction, silica being introduced in the form of tetraethyl orthosilicate (TEOS) and calcium is typically added in the form of calcium nitrate. The synthesis reaction proceeds in an aqueous environment; the resultant gel is dried, before stabilisation by heat treatment. These materials, being amorphous, are complex at the level of their atomic-scale structure, but their bulk properties may only be properly understood on the basis of that structural insight. Thus, a full understanding of their structure : property relationship may only be achieved through the application of a coherent suite of leading-edge experimental probes, coupled with the cogent use of advanced computer simulation methods. Using as an exemplar a calcia-silica sol-gel glass of the kind developed by Larry Hench, to whose memory this paper is dedicated, we illustrate the successful use of high-energy x-ray and neutron scattering (diffraction) methods, magic-angle spinning solid state NMR, and molecular dynamics simulation as components to a powerful methodology for the study of amorphous materials

The Crystal Structure of Ba₃Nb₂O₈ Revisited: A Neutron Diffraction and Solid-State NMR Study

The structure of Ba₃Nb₂O₈ has been investigated using high resolution neutron powder diffraction. Our results show that, while the structure has some features in common with the 9R perovskite and palmierite structures, it is a new and distinct structure. It is shown to follow a (<i>chh</i>)­(<i>hhc</i>)­(<i>chh</i>) sequence with BaO_3−δ packing layers and is a cation- and anion-deficient 9H perovskite polytype. Nb atoms occupy octahedral sites with vacancies between hexagonal close-packed layers. Isolated, corner-sharing and face-sharing Nb-O octahedra all occur within the unit cell. The identification of purely octahedral Nb is supported by solid-state ⁹³Nb wideline NMR measurements. A two-component line shape was detected: a narrow featureless resonance with an isotropic chemical shift of δ_iso −928 ± 5 ppm consistent with regular Nb octahedra, and a much broader featureless resonance with an approximate isotropic chemical shift in the range δ_iso ∼ −944 to −937 ± 10 ppm consistent with Nb octahedra influenced by O vacancies. These are both characteristic of 6-fold oxo-coordinated Nb environments. The highly distorted octahedral environments in Ba₃Nb₂O₈ make it a potential candidate for dielectric and photocatalytic applications

Crystal Chemistry of Vanadium-Bearing Ellestadite Waste Forms

Vanadate ellestadites Ca₁₀(SiO₄)_<i>x</i>(VO₄)_{6–2<i>x</i>}(SO₄)_<i>x</i>Cl₂, serving as prototype crystalline matrices for the fixation of pentavalent toxic metals (V, Cr, As), were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance (SS-NMR). The ellestadites 0.19 < <i>x</i> < 3 adopt the <i>P</i>6₃/<i>m</i> structure, while the vanadate endmember Ca₁₀(VO₄)₆Cl₂ is triclinic with space group <i>P</i>1̅. A miscibility gap exists for 0.77 < <i>x</i> < 2.44. The deficiency of Cl in the structure leads to short-range disorder in the tunnel. Toxicity characteristic leaching testing (TCLP) showed the incorporation of vanadium increases ellestadite solubility, and defined a waste loading limit that should not exceed 25 atom % V to ensure small release levels

New Structural Model of Hydrous Sodium Aluminosilicate Gels and the Role of Charge-Balancing Extra-Framework Al

A new structural model of hydrous alkali aluminosilicate gel (N-A-S-H) frameworks is proposed, in which charge-balancing extra-framework Al species are observed in N-A-S-H gels for the first time. This model describes the key nanostructural features of these gels, which are identified through the application of ¹⁷O, ²³Na, and ²⁷Al triple quantum magic angle spinning solid-state nuclear magnetic resonance spectroscopy to synthetic ¹⁷O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate gel predominantly comprises Q⁴(4Al), Q⁴(3Al), Q⁴(2Al), and Q⁴(1Al) Si units charge-balanced by Na⁺ ions that are coordinated by either 3 or 4 framework oxygen atoms. A significant proportion of Al³⁺ in tetrahedral coordination exist in sites of lower symmetry, where some of the charge-balancing capacity is provided by extra-framework Al species which have not previously been observed in these materials. The mean Si^IV–O–Al^IV bond angles for each type of Al^IV environments are highly consistent, with compositional changes dictating the relative proportions of individual Al^IV species but not altering the local structure of each individual Al^IV site. This model provides a more advanced description of the chemistry and structure of alkali aluminosilicate gels and is crucial in understanding and controlling the molecular interactions governing gel formation, mechanical properties, and durability

Experimental and First-Principles NMR Analysis of Pt(II) Complexes With <i>O</i>,<i>O</i>′‑Dialkyldithiophosphate Ligands

Polycrystalline bis­(dialkyldithiophosphato)­Pt­(II) complexes of the form [Pt­{S₂P­(OR)₂}₂] (R = ethyl, <i>iso</i>-propyl, <i>iso</i>-butyl, <i>sec</i>-butyl or <i>cyclo</i>-hexyl group) were studied using solid-state ³¹P and ¹⁹⁵Pt NMR spectroscopy, to determine the influence of R to the structure of the central chromophore. The measured anisotropic chemical shift (CS) parameters for ³¹P and ¹⁹⁵Pt afford more detailed chemical and structural information, as compared to isotropic CS and <i>J</i> couplings alone. Advanced theoretical modeling at the hybrid DFT level, including both crystal lattice and the important relativistic spin–orbit effects qualitatively reproduced the measured CS tensors, supported the experimental analysis, and provided extensive orientational information. A particular correction model for the non-negligible lattice effects was adopted, allowing one to avoid a severe deterioration of the ¹⁹⁵Pt anisotropic parameters due to the high requirements posed on the pseudopotential quality in such calculations. Though negligible differences were found between the ¹⁹⁵Pt CS tensors with different substituents R, the ³¹P CS parameters differed significantly between the complexes, implying the potential to distinguish between them. The presented approach enables good resolution and a detailed analysis of heavy-element compounds by solid-state NMR, thus widening the understanding of such systems

Structural Studies of Bulk to Nanosize Niobium Oxides with Correlation to Their Acidity

Hydrated niobium oxides are used as strong solid acids with a wide variety of catalytic applications, yet the correlations between structure and acidity remain unclear. New insights into the structural features giving rise to Lewis and Brønsted acid sites are presently achieved. It appears that Lewis acid sites can arise from lower coordinate NbO₅ and in some cases NbO₄ sites, which are due to the formation of oxygen vacancies in thin and flexible NbO₆ systems. Such structural flexibility of Nb–O systems is particularly pronounced in high surface area nanostructured materials, including few-layer to monolayer or mesoporous Nb₂O₅·<i>n</i>H₂O synthesized in the presence of stabilizers. Bulk materials on the other hand only possess a few acid sites due to lower surface areas and structural rigidity: small numbers of Brønsted acid sites on HNb₃O₈ arise from a protonic structure due to the water content, whereas no acid sites are detected for anhydrous crystalline H-Nb₂O₅

Deoxygenation of Graphene Oxide: Reduction or Cleaning?

We show that the two-component model of graphene oxide (GO), that is, composed of highly oxidized carbonaceous debris complexed to oxygen functionalized graphene sheets, is a generic feature of the synthesis of GO, independent of oxidant or protocol used. The debris present, roughly one-third by mass, can be removed by a base wash. A number of techniques, including solid state NMR, demonstrate that the properties of the base-washed material are independent of the base used and that it contains similar functional groups to those present in the debris but at a lower concentration. Removal of the oxidation debris cleans the GO, revealing its true monolayer nature and in the process increases the C/O ratio (i.e., a deoxygenation). By contrast, treating GO with hydrazine both removes the debris and reduces (both deoxygenations) the graphene sheets

Oxygen Insertion Reactions within the One-Dimensional Channels of Phases Related to FeSb₂O₄

The structure of the mineral schafarzikite, FeSb₂O₄, has one-dimensional channels with walls comprising Sb³⁺ cations; the channels are separated by edge-linked FeO₆ octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe²⁺ to Fe³⁺ within the octahedral chains but also Sb³⁺ to Sb⁵⁺ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb⁵⁺ ion (which is very rare in an oxide environment), two interstitial O^2– ions, and two 4-coordinate Sb³⁺ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications