8 research outputs found
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 Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub> Revisited: A Neutron Diffraction and Solid-State NMR Study
The
structure of Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub> 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>)Ā(<i>hhc</i>)Ā(<i>chh</i>) sequence with BaO<sub>3āĪ“</sub> 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 <sup>93</sup>Nb
wideline NMR measurements. A two-component line shape was detected:
a narrow featureless resonance with an isotropic chemical shift of
Ī“<sub>iso</sub> ā928 Ā± 5 ppm consistent with regular
Nb octahedra, and a much broader featureless resonance with an approximate
isotropic chemical shift in the range Ī“<sub>iso</sub> ā¼
ā944 to ā937 Ā± 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 Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub> make it a potential candidate
for dielectric and photocatalytic applications
Crystal Chemistry of Vanadium-Bearing Ellestadite Waste Forms
Vanadate
ellestadites Ca<sub>10</sub>(SiO<sub>4</sub>)<sub><i>x</i></sub>(VO<sub>4</sub>)<sub>6ā2<i>x</i></sub>(SO<sub>4</sub>)<sub><i>x</i></sub>Cl<sub>2</sub>, 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>6<sub>3</sub>/<i>m</i> structure, while the vanadate endmember Ca<sub>10</sub>(VO<sub>4</sub>)<sub>6</sub>Cl<sub>2</sub> is triclinic with space group <i>P</i>1Ģ
. 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 <sup>17</sup>O, <sup>23</sup>Na, and <sup>27</sup>Al triple quantum magic angle
spinning solid-state nuclear magnetic resonance spectroscopy to synthetic <sup>17</sup>O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate
gel predominantly comprises Q<sup>4</sup>(4Al), Q<sup>4</sup>(3Al),
Q<sup>4</sup>(2Al), and Q<sup>4</sup>(1Al) Si units charge-balanced
by Na<sup>+</sup> ions that are coordinated by either 3 or 4 framework
oxygen atoms. A significant proportion of Al<sup>3+</sup> 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<sup>IV</sup>āOāAl<sup>IV</sup> bond angles for each type of Al<sup>IV</sup> environments are highly consistent, with compositional changes
dictating the relative proportions of individual Al<sup>IV</sup> species
but not altering the local structure of each individual Al<sup>IV</sup> 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>ā²āDialkyldithiophosphate Ligands
Polycrystalline
bisĀ(dialkyldithiophosphato)ĀPtĀ(II) complexes of
the form [PtĀ{S<sub>2</sub>PĀ(OR)<sub>2</sub>}<sub>2</sub>] (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 <sup>31</sup>P and <sup>195</sup>Pt NMR spectroscopy,
to determine the influence of R to the structure of the central chromophore.
The measured anisotropic chemical shift (CS) parameters for <sup>31</sup>P and <sup>195</sup>Pt 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 spināorbit
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 <sup>195</sup>Pt anisotropic parameters due to the high requirements posed
on the pseudopotential quality in such calculations. Though negligible
differences were found between the <sup>195</sup>Pt CS tensors with
different substituents R, the <sup>31</sup>P 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 BrĆønsted acid sites are presently achieved.
It appears that Lewis acid sites can arise from lower coordinate NbO<sub>5</sub> and in some cases NbO<sub>4</sub> sites, which are due to
the formation of oxygen vacancies in thin and flexible NbO<sub>6</sub> systems. Such structural flexibility of NbāO systems is particularly
pronounced in high surface area nanostructured materials, including
few-layer to monolayer or mesoporous Nb<sub>2</sub>O<sub>5</sub>Ā·<i>n</i>H<sub>2</sub>O 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 BrĆønsted
acid sites on HNb<sub>3</sub>O<sub>8</sub> arise from a protonic structure
due to the water content, whereas no acid sites are detected for anhydrous
crystalline H-Nb<sub>2</sub>O<sub>5</sub>
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 FeSb<sub>2</sub>O<sub>4</sub>
The
structure of the mineral schafarzikite, FeSb<sub>2</sub>O<sub>4</sub>, has one-dimensional channels with walls comprising Sb<sup>3+</sup> cations; the channels are separated by edge-linked FeO<sub>6</sub> 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 ā¼2% by
mass) at low temperatures (ca. 350 Ā°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 Fe<sup>2+</sup> to Fe<sup>3+</sup> within the octahedral
chains but also Sb<sup>3+</sup> to Sb<sup>5+</sup> in the channel
walls. The formation of a defect cluster comprising one 5-coordinate
Sb<sup>5+</sup> ion (which is very rare in an oxide environment),
two interstitial O<sup>2ā</sup> ions, and two 4-coordinate
Sb<sup>3+</sup> 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