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

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