Layla Martin-Samos

Although fluorine is one of the most important dopants of silica, for several decades fluorine‐related paramagnetic defects have been very elusive. Yet, a recent experimental investigation [L. Skuja et al. Phys. Rev. Lett. 131 , 256 903 (2023)] has claimed the identification of a new paramagnetic center, that is, the E′(F) center, responsible for a 10.5 mT hyperfine doublet in irradiated F‐doped silica glass. Due to the lack of theoretical investigations, the reliability of the assignment proposed for the E′(F) has been under debate so far. In this work two alternative structural models, here labeled as the 2fSiF and models, are investigated and discussed by means of first‐principle calculations of electron paramagnetic resonance parameters, and further evidence in favor of the 2fSi‐F model is provided. The latter consists of a threefold‐coordinated Si atom, which is bonded to two bridging oxygen atoms and to one fluorine atom and that features an unpaired electron in an sp ³ orbital. Furthermore, through first‐principles nudged elastic band calculations it is shown that, under irradiation, generation mechanisms involving an interconversion between and E′(F) centers can occur, with no need for precursors like the tetrahedral unit SiO ₂ F ₂ or twofold Si centers.

Stable ferrofluids of permanently magnetic nanoplatelets of barium hexaferrite (BHF NPLs) in 1-hexanol were obtained using phosphonic acid- and phosphonate ester-based polar ligands. These ligands with the different electron-withdrawing groups and alkyl chain lengths of the terminal chain and linker were synthesized. Their attachment to the surface of the BHF NPLs was studied for various conditions and followed by a combination of spectroscopic techniques, thermogravimetry, and electrokinetic measurements. The results confirmed the theoretically predicted surface condensation of the ligands onto the BHF NPLs surfaces at 120 °C in 1-hexanol, whereas at lower temperatures or in more polar solvents the ligands were mostly physisorbed. The NPL hybrids with chemisorbed ligands having surface densities of at least 0.4 molecules/nm2 formed stable ferrofluids in 1-hexanol. The hybridization of the BHF NPLs via the condensation reaction in 1-hexanol creates sufficient steric-solvation repulsion to overcome the magnetic dipolar attraction between the NPLs and stabilizes the NPLs in 1-hexanol. Due to the relatively low polarity of 1-hexanol, the ligands remain protonated and the ferrofluids have negligible electric conductivity. In addition, an increase in the saturation magnetization of such hybrid BHF NPLs was correlated with their superior sensitivity to a magnetic field compared to the core NPLs. The ferrofluids can be exploited for the development of novel magneto-optic sensors that can operate under an electric field.

Point Group (PG) symmetries play a fundamental role in many aspects of theoretical chemistry and computational materials science. With the objective to automatize the search of PG symmetry operations of generic atomic clusters, we present a new algorithm called Symmetry Operation FInder (SOFI). SOFI addresses the problem of identifying PG symmetry by framing it as a degenerate shape-matching problem, where the multiple solutions correspond to distinct symmetry operations. The developed algorithm is compared against three other algorithms dedicated to PG identification on a large set of atomic clusters. The results, along with some illustrative use cases, showcase the effectiveness of SOFI. The SOFI algorithm is released as part of the iterative rotations and assignments library, accessible at https://github.com/mammasmias/IterativeRotationsAssignments.

Saddle points on high-dimensional potential energy surfaces (PES) play a determining role in the activated dynamics of molecules and materials. Building on approaches dating back more than 50 years, many open-ended transition-state search methods have been developed to follow the direction of negative curvature from a local minimum to an adjacent first-order saddle point. Despite the mathematical justification, these methods can display a high failure rate: using small deformation steps, up to 80% of the explorations can end up in a convex region of the PES, where all directions of negative curvature vanish, while if the deformation is aggressive, a similar fraction of attempts lead to saddle points that are not directly connected to the initial minimum. In high-dimension PES, these reproducible failures were thought to only increase the overall computational cost, without having any effect on the methods’ capacity to find all saddle points surrounding a minimum. Using activation-relaxation technique nouveau (ARTn), we characterize the nature of the PES around minima, considerably expanding on previous knowledge. We show that convex regions can lie on activation pathways and that not exploring beyond them can introduce significant bias in the saddle-point search. We introduce an efficient approach for traversing the convex regions, almost eliminating exploration failures, while multiplying by almost 10 the number of identified unique and connected saddle points as compared to the standard ARTn, thus underlining the importance of correctly handling convex regions for completeness of saddle point explorations.

In this study, MNPs were functionalized with pyrocatechol (CAT), pyrogallol (GAL), caffeic acid (CAF), and nitrodopamine (NDA) at pH 8 and pH 11. The functionalization of the MNPs was successful, except in the case of NDA at pH 11. The thermogravimetric analyses indicated that the surface concentration of the catechols was between 1.5 and 3.6 molecules/nm². The saturation magnetizations (M_s) of the functionalized MNPs were higher than the starting material. XPS analyses showed only the presence of Fe(III) ions on the surface, thus refuting the idea of the Fe being reduced and magnetite being formed on the surfaces of the MNPs. Density functional theory (DFT) calculations were performed for two modes of adsorption of CAT onto two model surfaces: plain and adsorption via condensation. The total magnetization of both adsorption modes remained the same, indicating that the adsorption of the catechols does not affect the M_s. The analyses of the size and the size distribution showed an increase in the average size of the MNPs during the functionalization process. This increase in the average size of the MNPs and the reduction in the fraction of the smallest (i.e., s values.

In this work we present a combined experimental and ab initio simulation investigation concerning the Germanium Lone Pair Center (GLPC), its interaction with molecular oxygen (O₂), and evolution under irradiation. First, O₂ loading has been applied here to Ge-doped optical fibers to reduce the concentration of GLPC point defects. Next, by means of cathodoluminescence in situ experiments, we found evidence that the 10 keV electron irradiation of the treated optical fibers induces the generation of GLPC centers, while in nonloaded optical fibers, the irradiation causes the bleaching of the pre-existing GLPC. Ab initio calculations were performed to investigate the reaction of the GLPC with molecular oxygen. Such investigations suggested the stability of the dioxagermirane (DIOG) bulk defect, and its back conversion into GLPC with a local release of O₂ under irradiation. Furthermore, it is also inferred that a remarkable portion of the O₂ passivated GLPC may form Ge tetrahedra connected to peroxy bridges. Such structures may have a larger resistance to the irradiation and not be back converted into GLPC.