RAPIDLY RUNNING TRANSITIONS AT HIGH PRESSURE

Environmental Remediation with Functional Aerogels and Xerogels

Several different types of aerogels and xerogels are demonstrated as effective sorbents for the capture and/or immobilization of radionuclides and other contaminants in gaseous form [e.g., Hg(g), I2(g), Xe, Kr] as well as ionic form (e.g., Cd2+, Ce4+, Cs+, Cu2+, Fe2+, Hg2+, I-, IO3 -, Kr, Pb2+, Rb+, Sr2+, 99Tc7+, U6+, Zn2+). These sorbents have unique properties, which include high specific surface areas, high pore volumes, a range of pore sizes, and functionalities that provide methods for binding radionuclides and other contaminants, generally through physisorption, chemisorption, or a combination thereof. This combination of properties and functionalities makes these types of materials ideal for use as sorbents for capturing radionuclides. The primary base materials that will be discussed in this paper include Ag0-functionalized silica aerogels, Ag+-impregnated aluminosilicate aerogels, Ag0-functionalized aluminosilicate aerogels, metal-impregnated (non-Ag) aluminosilicate aerogels and xerogels, sulfide-based aerogels, and carbon-based aerogel composites. For the capture of I2(g), the materials reported herein show some of the highest iodine loadings ever reported for inorganic sorbents. For the capture of ionic species, these materials also show promise as next-generation materials for active radionuclide remediation. This progress report describes materials fabrication, general properties, and environmental remediation applications.

Silver-Loaded Aluminosilicate Aerogels As Iodine Sorbents

In this paper, aluminosilicate aerogels were used as scaffolds for silver nanoparticles to capture I₂(g). The starting materials for these scaffolds included Na-Al-Si-O and Al-Si-O aerogels, both synthesized from metal alkoxides. The Ag⁰ particles were added by soaking the aerogels in aqueous AgNO₃ solutions followed by drying and Ag⁺ reduction under H₂/Ar to form Ag⁰ crystallites within the aerogel matrix. In some cases, aerogels were thiolated with 3-(mercaptopropyl)trimethoxysilane as an alternative method for binding Ag⁺. During the Ag⁺-impregnation steps, for the Na-Al-Si-O aerogels, Na was replaced with Ag, and for the Al-Si-O aerogels, Si was replaced with Ag. The Ag-loading of thiolated versus nonthiolated Na-Al-Si-O aerogels was comparable at ∼35 atomic %, whereas the Ag-loading in unthiolated Al-Si-O aerogels was significantly lower at ∼7 atomic % after identical treatment. Iodine loadings in both thiolated and unthiolated Ag⁰-functionalized Na-Al-Si-O aerogels were >0.5 m_I m_s^-1 (denoting the mass of iodine captured per starting mass of the sorbent) showing almost complete utilization of the Ag through chemisorption to form AgI. Iodine loading in the thiolated and Ag⁰-functionalized Al-Si-O aerogel was 0.31 m_I m_s^-1. The control of Ag uptake over solution residence time and [Ag] demonstrates the ability to customize the Ag-loading in the base sorbent to regulate the loading capacity of iodine.

Crystal Structure of AgBi2I7 Thin Films

Synthesis of cubic-phase AgBi₂I₇ iodobismuthate thin films and fabrication of air-stable Pb-free solar cells using the AgBi₂I₇ absorber have recently been reported. On the basis of X-ray diffraction (XRD) analysis and nominal composition, it was suggested that the synthesized films have a cubic ThZr₂H₇ crystal structure with AgBi₂I₇ stoichiometry. Through careful examination of the proposed structure and computational evaluation of the phase stability and bandgap, we find that the reported "AgBi₂I₇" films cannot be forming with the ThZr₂H₇-type structure, but rather more likely adopt an Ag-deficient AgBiI₄ type. Both the experimental X-ray diffraction pattern and bandgap can be better explained by the AgBiI₄ structure. Additionally, the proposed AgBiI₄ structure, with octahedral bismuth coordination, removes unphysically short Bi-I bonding within the [BiI₈] hexahedra of the ThZr₂I₇ model. Our results provide critical insights for assessing the photovoltaic properties of AgBi₂I₇ iodobismuthate materials.

On the effective ionic radii for ammonium

A set of effective ionic radii corresponding to different coordination numbers (CNs) and compatible with the radii system by Shannon [Acta Cryst. (1976), A32, 751-767] has been derived for ammonium: 1.40 Å (CN = IV), 1.48 Å (CN = VI), 1.54 Å (CN = VIII) and 1.67 Å (CN = XII). The bond-valence parameters r0 = 2.3433 Å and B = 0.262 Å have been determined for ammonium-fluorine bonds.

Crystalline forms of silver iodide II: Determination of phase transformations

In order to obtain appropriate forms of AgI (?- and ?-), a procedure was developed to synthesize AgI at room temperature (23?C), whereby samples of varying crystallographic purity and of varying crystallographic contents of the different forms were obtained. This paper presents the results of investigations of the influence of the manner of preparation of ?-AgI and ?-AgI samples and the sample heating rate on the phase transformations and their temperatures. During the heating of non-ground, ground and pressed synthesized AgI samples, the phase transformations and the corresponding temperatures for one ?-AgI and four ?-AgI samples with different ?-AgI contents (representing a crystallographic impurity) were identified. The following phase transformations were observed for the non-ground AgI samples: ?-AgI ? ?-AgI at 149.6?C (for the ?-AgI sample) and ?-AgI ? ?-AgI at 148.7?C or 148.2?C for the ?-AgI samples with a minimum content of ?-AgI (less than 7 %), as a crystallographic impurity. The phase transition ?-AgI ? ?-AgI was irreversible because ?-AgI was obtained whenever the samples were heated up to 260?C. Manual sample grinding, as well as pressing at p1 = 650 MPa and p2 = 900 MPa resulted in the ?-AgI ? ?-AgI phase transition in all the investigated cases. .

Lattice Parameters and Lattice Energies of High-Pressure Polymorphs of Some Alkali Halides

Lattice parameters of the high-pressure forms of the alkali halides were determined. The lattice parameters were used to compare the lattice energies of the NaCl and CsCl type structures at the transition pressure. An analysis of the effect of experimental uncertainties on the calculated lattice energies showed that in almost every instance the BornMayer theory adequately accounts for the lattice energy of the high-pressure structure.