[en] Using powerful computational resources and state-of-the-art methods of computational chemistry we contribute to the research on novel nuclear waste forms by providing atomic scale description of processes that govern the structural incorporation and the interactions of radionuclides in host materials. Here we present various results of combined computational and experimental studies on La_1_−_xEu_xPO_4 monazite-type solid solution. We discuss the performance of DFT + U method with the Hubbard U parameter value derived ab initio, and the derivation of various structural, thermodynamic and radiation-damage related properties. We show a correlation between the cation displacement probabilities and the solubility data, indicating that the binding of cations is the driving factor behind both processes. The combined atomistic modeling and experimental studies result in a superior characterization of the investigated material.

[en] When considering f elements, solvent extraction is primarily used for the removal of lanthanides from ore and their recycling, as well as for the separation of actinides from used nuclear fuel. Understanding the complexation mechanism of metal ions with organic extractants, particularly the influence of their molecular structure on complex formation is of fundamental importance. Herein, we report an extraordinary (up to two orders of magnitude) change in the extraction efficiency of f elements with two diastereomers of dimethyl tetraoctyl diglycolamide (Me${}_2$‐TODGA), which only differ in the orientation of a single methyl group. Solvent extraction techniques, extended X‐ray absorption fine structure (EXAFS) measurements, and density functional theory (DFT) based ab initio calculations were used to understand their complex structures and to explain their complexation mechanism. We show that the huge differences observed in extraction selectivity results from a small change in the complexation of nitrate counter‐ions caused by the different orientation of one methyl group in the backbone of the extractant. The obtained results give a significant new insight into metal–ligand complexation mechanisms, which will promote the development of more efficient separation techniques. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

[en] We present near-infrared photometric observations of 15 and spectroscopic observations of 38 cool white dwarfs (WDs). This is the largest near-infrared spectroscopic survey of cool WDs to date. Combining the Sloan Digital Sky Survey photometry and our near-infrared data, we perform a detailed model atmosphere analysis. The spectral energy distributions of our objects are explained fairly well by model atmospheres with temperatures ranging from 6300 K down to 4200 K. Two WDs show significant absorption in the infrared, and are best explained with mixed H/He atmosphere models. Based on the up-to-date model atmosphere calculations by Kowalski and Saumon, we find that the majority of the stars in our sample have hydrogen-rich atmospheres. We do not find any pure helium atmosphere WDs below 5000 K, and we find a trend of increasing hydrogen to helium ratio with decreasing temperature. These findings present an important challenge to understanding the spectral evolution of WDs.

[en] The first aim of this study was to determine the role of the structurally different kink sites at these steps for the crystal growth kinetics by computing the activation energies of the Ba²⁺, Ra²⁺ and SO₄^2- ion attachment processes. Here, we have combined several advanced methods of computational quantum mechanics to optimize accuracy and efficiency of the simulations: a hybrid Density Functional Theory (DFT) and Soft-sphere Continuum Solvation (SSCS) approach for computation of barite-aqueous phase interface, and a Nudged Elastic Band (NEB) approach for calculation of activation energies. A barite (001) surface structure was constructed, including steps and all relevant attachment positions for kink-site nucleation, and verified by comparison with experimental data. Our simulations show that Ba²⁺ attachment to the [120] steps of the (001) surface is a complicated multistep process, involving several water detachment-steps and the formation of outer, inner sphere and bidentate complexes with the barite surface. Two mechanisms mainly determine the shape of the energy path, the creation of bonds and the dehydration of the attaching ion. Energy differences between ion attachment processes at different sites are predominantly caused by the influence of the different [120] steps. The rate-limiting steps for Ba²⁺ attachment were the formation of the first bond to the barite surface and complete uptake. The Ba²⁺ ion completely attached to the barite surface is the configuration with the lowest energy, and the Ba²⁺ ion completely dissolved is the minimum energy configuration with the highest energy. The energy pathway derived from this study can explain the barite growth exclusively by the driving forces of surface processes, which are also postulated to be responsible for the anisotropic barite (001) surface growth. This is not the case with the classical force-field-based approach of Stack et al. 2012, which shows the same basic minimum energy structures but with different relative energies and the inner sphere complex as the configuration with the lowest energy. The DFT-NEB-SSCS approach also provides more detailed energy paths that allow for an in-depth comparison of the different ion attachment processes at a certain site. On-going simulations for the uptake of ²²⁶Ra into the (001) barite surface indicate distinct differences between Ra²⁺ and Ba²⁺, related to the different water coordination numbers, and slight variations within the attachment path. Ra²⁺ could therefore be kinetically favored during recrystallization due to an easier dehydration compared to Ba²⁺ at the barite (001) surface

[en] We present Spitzer 5-15 μm spectroscopy of one cool white dwarf and 3.6-8 μm photometry of 51 cool white dwarfs with T _eff < 6000 K. The majority of our targets have accurate BVRIJHK photometry and trigonometric parallax measurements available, which enables us to perform a detailed model atmosphere analysis using their optical, near- and mid-infrared photometry with state-of-the-art model atmospheres. We demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our best-fit models are consistent with the observations within 5% in all filters except the IRAC 8 μm band, which has the lowest signal-to-noise ratio photometry. Excluding the ultracool white dwarfs, none of the stars in our sample show significant mid-infrared flux deficits or excesses. The nondetection of mid-infrared excess flux around our ∼2-9 Gyr old targets constrain the fraction of cool white dwarfs with warm debris disks to 0.8^+1.5_-0.8 %.

[en] We present extensive spectral and photometric observations of the recycled pulsar/white dwarf binary containing PSR J0437–4715, which we analyzed together with archival X-ray and gamma-ray data, to obtain the complete mid-infrared to gamma-ray spectrum. We first fit each part of the spectrum separately, and then the whole multi-wavelength spectrum. We find that the optical-infrared part of the spectrum is well fit by a cool white dwarf atmosphere model with pure hydrogen composition. The model atmosphere (T_eff = 3950 ± 150 K, log g = 6.98 ± 0.15, R_WD = (1.9 ± 0.2) × 10⁹ cm) fits our spectral data remarkably well for the known mass and distance (M = 0.25 ± 0.02 M_☉, d = 156.3 ± 1.3 pc), yielding the white dwarf age (τ_WD = 6.0 ± 0.5 Gyr). In the UV, we find a spectral shape consistent with thermal emission from the bulk of the neutron star surface, with surface temperature between 1.25 × 10⁵ and 3.5 × 10⁵ K. The temperature of the thermal spectrum suggests that some heating mechanism operates throughout the life of the neutron star. The temperature distribution on the neutron star surface is non-uniform. In the X-rays, we confirm the presence of a high-energy tail which is consistent with a continuation of the cutoff power-law component (Γ = 1.56 ± 0.01, E_cut = 1.1 ± 0.2 GeV) that is seen in gamma rays and perhaps even extends to the near-UV.

[en] Highlights: • Simulated Ed values for LaPO₄ monazite-type ceramics. • Found correlation between Ed value and the energy barrier. • First experimental results on ion beam irradiation of La_0.2Gd_0.8PO₄ material. We simulated the threshold displacement energies ($E_d$), the related displacement and defect formation probabilities, and the energy barriers in LaPO₄ monazite-type ceramics. The obtained $E_d$ values for La, P, O primary knock-on atoms (PKA) are 56 eV, 75 eV and 8 eV, respectively. We found that these energies can be correlated with the energy barriers that separate the defect from the initial states. The $E_d$ values are about twice the values of energy barriers, which is explained through an efficient dissipation of the PKA kinetic energy in the considered system. The computed $E_d$ were used in simulations of the extent of radiation damage in La_0.2Gd_0.8PO₄ solid solution, investigated experimentally. We found that this lanthanide phosphate fully amorphises in the ion beam experiments for fluences higher than ∼10¹³ ions/cm².

[en] The composition dependence of the lattice constant, a, in A_xB_1-xO_2-0.5xV_0.5x, fluorite-type solid solutions (B = Zr, A = {Nd-Yb, Y}, V = oxygen vacancy) is characterized by changes in slope at x ~ 1/3, which cannot be described by existing models. Moreover, over the range of 0.15 ≤ x ≤ 1/3 the a vs. x data on all systems can be fitted with a linear equation, a = q + rx, with the same intercept, q, however, over the range of 1/3 ≤ x ≤ 1/2 the data bifurcate requiring different intercepts for A={Nd-Gd] and for A={Dy-Yb,Y} systems, implying steeper slopes for the first group. An adequate description is achieved via close-packing models involving two anion species, O^2- and V, and three combinations of cation species, namely ⁸A, ⁷B, ⁸B (0 ≤ x ≤ 1/3), ⁷A, ⁸A, ⁷B (1/3 ≤ x ≤ 1/2) and ⁸A, ⁶B, ⁷B (1/3 ≤ x ≤ 1/2), that take into an account the constraint on the average coordination number, K(x), of K(x) = 8 – 2x and two additional short-range order (SRO) constrains referring to vacancy-vacancy avoidance and vacancy-to-Zr association. The consistent fit to a vs. x data with these models requires the effective size of the oxygen vacancy to be larger than the radius of the oxygen anion by ~11%. The latter observation seems to play an important role in the understanding of structure-property relationships in A_xB_1-xO_2-0.5xV_0.5x systems.

[en] Highlights: • We determined the low temperature heat capacity of two uranyl tungstates. • We determined enthalpy of formation of these phases by HF solution calorimetry. • We calculated Δ_fG° (298 K) of Cs₄[(UO₂)₄(WO₅)(W₂O₈)O₂] and Cs₄[(UO₂)₇(WO₅)₃O₃]. • We perform DFT + U modeling of solid state reaction involving both phases. -- Abstract: A calorimetric and thermodynamic investigation of two alkali-metal uranyl tungstates, ${\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_4\left({\text{WO}}_5\right)\left({\text{W}}_2{\text{O}}_8\right){\text{O}}_2\right](\text{cr})$ and ${\text{Cs}}_4[{\left({\text{UO}}_2\right)}_7{\left({\text{WO}}_5\right)}_3{\text{O}}_3](\text{cr})$, was undertaken. Both phases were synthesized by solid-state sintering of a mixture of cesium nitrate, tungsten (VI) oxide and gamma-uranium (VI) oxide at high temperature. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The enthalpies of formation of both phases were determined using HF-solution calorimetry giving ${\mathrm{\Delta }}_{\text{f}}{H}_{\text{m}}^{\text{o}}\left(T=298.15\text{K},{\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_4\left({\text{WO}}_5\right)\left({\text{W}}_2{\text{O}}_8\right){\text{O}}_2\right],\text{cr}\right)=-\left(9239\pm 17\right)\text{kJ}·{\text{mol}}^{-1}$ and${\mathrm{\Delta }}_{\text{f}}{H}_{\text{m}}^{\text{o}}\left(T=298.15\text{K},{\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_7{\left({\text{WO}}_5\right)}_3{\text{O}}_3\right],\text{cr}\right)=-\left(13008\pm 23\right)\text{kJ}·{\text{mol}}^{-1}$. The low-temperature heat capacity, $C_{p,\text{m}}^{\text{o}}$, was measured using adiabatic calorimetry from T = 5 K to 324 K for ${\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_4\left({\text{WO}}_5\right)\left({\text{W}}_2{\text{O}}_8\right){\text{O}}_2\right]\left(\text{cr}\right)$ and from T = 5 K to 325 K for ${\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_7{\left({\text{WO}}_5\right)}_3{\text{O}}_3\right]\left(\text{cr}\right)$. Using these $C_{p,\text{m}}^{\text{o}}\left(T\right)$ data, the third law entropy at T = 298.15 K, $S_{\text{m}}^{\text{o}}$, is calculated as (931 ± 4) J∙K⁻¹∙mol⁻¹ for ${\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_4\left({\text{WO}}_5\right)\left({\text{W}}_2{\text{O}}_8\right){\text{O}}_2\right]\left(\text{cr}\right)$ and (1262 ± 8) J∙K⁻¹∙mol⁻¹ for ${\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_7{\left({\text{WO}}_5\right)}_3{\text{O}}_3\right]\left(\text{cr}\right)$. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, ${\mathrm{\Delta }}_{\text{f}}{\text{G}}_{\text{m}}^{\text{o}}$, for both phases giving: ${\mathrm{\Delta }}_{\text{f}}{G}_{\text{m}}^{\text{o}}\left(T=298.15\text{K},{\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_4\left({\text{WO}}_5\right)\left({\text{W}}_2{\text{O}}_8\right){\text{O}}_2\right],\text{cr}\right)=-\left(8623\pm 18\right)\text{kJ}·{\text{mol}}^{-1}$ and ${\mathrm{\Delta }}_{\text{f}}{G}_{\text{m}}^{\text{o}}\left(T=298.15\text{K},{\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_7{\left({\text{WO}}_5\right)}_3{\text{O}}_3\right],\text{cr}\right)=-\left(12171\pm 24\right)\text{kJ}·{\text{mol}}^{-1}$. Smoothed $C_{p,\text{m}}^{\text{o}}\left(T\right)$ values between 0 K and 320 K are presented, along with values for $S_{\text{m}}^{\text{o}}$ and the functions $\left[H_{\text{m}}^{\text{o}}\left(T\right)-H_{\text{m}}^{\text{o}}\left(0\right)\right]$ and $\left[G_{\text{m}}^{\text{o}}\left(T\right)-H_{\text{m}}^{\text{o}}\left(0\right)\right]$, for both phases. Based on these data, the enthalpy of the intercalation reaction ${\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_4\left({\text{WO}}_5\right)\left({\text{W}}_2{\text{O}}_8\right){\text{O}}_2\right]\left(\text{cr}\right)+3{\text{UO}}_3\left(\text{cr},\gamma \right)\to {\text{Cs}}_4\left[{\left({\text{UO}}_2\right)}_7{\left({\text{WO}}_5\right)}_3{\text{O}}_3\right]\left(\text{cr}\right)$ at 298 K was determined as $-\left(98\pm 29\right)\text{kJ}·{\text{mol}}^{-1}.$ A value of $-90.3\text{kJ}·{\text{mol}}^{-1}$ at 0 K was obtained from DFT + U calculations. The results show the potential of using DFT + U simulations to understand solid-state reactions involving crystalline phases having 5d- and 5f-elements.

[en] This paper focuses on major phosphate-based ceramic materials relevant for the immobilisation of Pu, minor actinides, fission and activation products. Key points addressed include the recent progress regarding synthesis methods, the formation of solid solutions by structural incorporation of actinides or their non-radioactive surrogates and waste form fabrication by advanced sintering techniques. Particular attention is paid to the properties that govern the long-term stability of the waste forms under conditions relevant to geological disposal. The paper highlights the benefits gained from synergies of state-of-the-art experimental approaches and advanced atomistic modeling tools for addressing properties and stability of f-element-bearing phosphate materials. In conclusion, this article provides a perspective on the recent advancements in the understanding of phosphate based ceramics and their properties with respect to their application as nuclear waste forms.