The assessment of local geological factors for the construction of a Geogenic Radon Potential map using regression kriging. A case study from the Euganean Hills volcanic district (Italy)

The assessment of potential radon-hazardous environments is nowadays a critical issue in planning, monitoring, and developing appropriate mitigation strategies. Although some geological structures (e.g., fault systems) and other geological factors (e.g., radionuclide content, soil organic or rock weathering) can locally affect the radon occurrence, at the basis of a good implementation of radon-safe systems, optimized modelling at territorial scale is required. The use of spatial regression models, adequately combining different types of predictors, represents an invaluable tool to identify the relationships between radon and its controlling factors as well as to construct Geogenic Radon Potential (GRP) maps of an area. In this work, two GRP maps were developed based on field measurements of soil gas radon and thoron concentrations and gamma spectrometry of soil and rock samples of the Euganean Hills (northern Italy) district. A predictive model of radon concentration in soil gas was reconstructed taking into account the relationships among the soil gas radon and seven predictors: terrestrial gamma dose radiation (TGDR), thoron (²²⁰Rn), fault density (FD), soil permeability (PERM), digital terrain model (SLOPE), moisture index (TMI), heat load index (HLI). These predictors allowed to elaborate local spatial models by using the Empirical Bayesian Regression Kriging (EBRK) in order to find the best combination and define the GRP of the Euganean Hills area. A second GRP map based on the Neznal approach (GRP_NEZ) has been modelled using the TGDR and ²²⁰Rn, as predictors of radon concentration, and FD as predictor of soil permeability. Then, the two GRP maps have been compared. Results highlight that the radon potential is mainly driven by the bedrock type but the presence of fault systems and topographic features play a key role in radon migration in the subsoil and its exhalation at the soil/atmosphere boundary.

Negative linear compressibility in nanoporous metal-organic frameworks rationalized by the empty channel structural mechanism

Zinc squarate tetrahydrate (ZnC₄O₄·4H₂O) and titanium oxalate trioxide dihydrate (Ti₂(C₂O₄)O₃·2H₂O) are nanoporous metal-organic frameworks possessing empty channels in their crystal structures. The crystal structures and mechanical properties of these materials are studied using first principles solid-state methods based on Density Functional Theory. The results show that they exhibit the negative linear compressibility (NLC) and negative Poisson's ratio (NPR) phenomena. The absolute value of the negative compressibilities are significant and the range of pressure for which NLC effects are shown is very wide. The detailed study of the deformation of the crystal structures under pressure reveals that the NLC effect in these compounds can be rationalized using the empty channel structural mechanism. Under isotropic compression, the channels are elongated along the direction of minimum compressibiity, leading to NLC. Furthermore, under compression along the direction of minimum compressibity, the unit-cell volume increases leading to negative volumetric compressibilty. The effect of hydration on the NLC effect in titanium oxalate trioxide dihydrate is investigated by studying the parent compound titanium oxalate trioxide trihydrate (Ti₂(C₂O₄)O₃·3H₂O). The NLC effect in this material is reduced due to the reinforcement of the walls of the structural channels.

The magnesium uranyl tricarbonate octadecahydrate mineral, bayleyite: Periodic DFT study of its crystal structure, hydrogen bonding, mechanical properties and infrared spectrum

Bayleyite is a highly hydrated uranyl tricarbonate mineral containing eighteen water molecules per formula unit. Due to this large water content, the correct description of its crystal structure is a great challenge for the first principles solid state methodology. In this work, the crystal structure, hydrogen bonding, mechanical properties and infrared spectrum of bayleyite, Mg₂[UO₂(CO₃)₃] · 18 H₂O, have been investigated by means of Periodic Density Functional Theory methods using plane wave basis sets and pseudopotentials. The computed unit-cell parameters, interatomic distances, hydrogen bonding network geometry and the X-ray powder diffraction pattern of bayleyite reproduce successfully the experimental data, thus confirming the crystal structure determined from X-ray diffraction data. From the energy-optimized structure, the elastic properties and infrared spectrum have been determined using theoretical methods. The calculated elastic properties include the bulk modulus and its pressure derivatives, the Young and shear moduli, the Poisson ratio and the ductility, hardness and anisotropy indices. Bayleyite is shown to be a very isotropic ductile mineral possessing a bulk modulus of B ~28 GPa. The infrared spectrum of bayleyite is obtained experimentally from a natural mineral sample from the Jáchymov ore district, Czech Republic, and determined employing density functional perturbation theory. Since both spectra show a high degree of consistence, the bands in the observed spectrum are assigned using the theoretical methodology. The atomic vibrational motions localized in the uranyl tricarbonate units are described in detail, using appropriate normal coordinate analyses based on accurate vibrational computations, since the vibrational normal modes have not been hitherto studied for any uranyl tricarbonate mineral.

Hydrogen bonding in the crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O: single-crystal X-ray study and TORQUE calculations

The crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) Å3, space group Pbca, Z = 8 has been refined from single-crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen-bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca2+ cations and H2O groups. In contrast to previous studies the approach here reveals five transformer H2O groups (compared to three expected by a previous study) and two non-transformer H2O groups. One of the transformer H2O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H2O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca2(H2[3]O)5(H2[4]O)2}[(UO2)3O2(PO4)2], Z = 8.

Full crystal structure, hydrogen bonding and spectroscopic, mechanical and thermodynamic properties of mineral uranopilite

The determination of the full crystal structure of the uranyl sulfate mineral uranopilite, including the positions of the H atoms in the corresponding unit cell, has not been feasible to date due to the poor quality of its X-ray diffraction pattern.

Thermodynamic properties of the uranyl carbonate minerals roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite

The thermodynamic properties of six important uranyl carbonate minerals, roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite, are determined as a function of temperature using first principles methods.

The crystal structures and mechanical properties of the uranyl carbonate minerals roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite

The structure, hydrogen bonding, X-ray diffraction pattern and mechanical properties of six important uranyl carbonate minerals, roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite, are determined using first principles methods.

Structural, mechanical, spectroscopic and thermodynamic characterization of the copper-uranyl tetrahydroxide mineral vandenbrandeite

The full crystal structure of the copper-uranyl tetrahydroxide mineral (vandenbrandeite), including the positions of the hydrogen atoms, is established by the first time from X-ray diffraction data taken from a natural crystal sample from the Musonoi Mine, Katanga Province, Democratic Republic of Congo. The structure is verified using first-principles solid-state methods. From the optimized structure, the mechanical and dynamical stability of vandenbrandeite is studied and a rich set of mechanical properties are determined. The Raman spectrum is recorded from the natural sample and determined theoretically. Since both spectra have a high-degree of consistence, all spectral bands are rigorously assigned using a theoretical normal-coordinate analysis. Two bands in the Raman spectra, located at 2327 and 1604 cm-1, are recognized as overtones and a band at 1554 cm-1 is identified as a combination band. The fundamental thermodynamic functions of vandenbrandeite are computed as a function of temperature using phonon calculations. These properties, unknown so far, are key-parameters for the performance-assessment of geological repositories for storage of radioactive nuclear waste and for understanding the paragenetic sequence of minerals arising from the corrosion of uranium deposits. The thermodynamic functions are used here to determine the thermodynamic properties of formation of vandenbrandeite in terms of the elements and the Gibbs free-energies and reaction constants for a series of reactions involving vandenbrandeite and a representative subset of the most important secondary phases of spent nuclear fuel. Finally, from the thermodynamic data of these reactions, the relative stability of vandenbrandeite with respect to these phases as a function of temperature and in the presence of hydrogen peroxide is evaluated. Vandenbrandeite is shown to be highly stable under the simultaneous presence of water and hydrogen peroxide.