3 research outputs found
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>6–2<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>1Ì…. 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>–O–Al<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
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 ∼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 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>2–</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