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Spinel ferrites demonstrate extensive applications in different areas, like electrodes for electrochemical devices, gas sensors, catalysts, and magnetic adsorbents for environmentally important processes. However, defects in the real spinel structure can change the many physical and chemical properties of spinel ferrites. Although the number of defects in a crystal spinel lattice is small, their influence on the vast majority of physical properties could be really decisive. This review provides an overview of the structural characteristics of spinel compounds (e.g., CoFe2O4, NiFe2O4, ZnFe2O4, Fe3O4, γ–Fe2O3, Co3O4, Mn3O4, NiCo2O4, ZnCo2O4, Co2MnO4, etc.) and examines the influence of defects on their properties. Attention was paid to the classification (0D, 1D, 2D, and 3D defects), nomenclature, and the formation of point and surface defects in ferrites. An in-depth description of the defects responsible for the physicochemical properties and the methodologies employed for their determination are presented. DFT as the most common simulation approach is described in relation to modeling the point defects in spinel compounds. The significant influence of defect distribution on the magnetic interactions between cations, enhancing magnetic properties, is highlighted. The main defect-engineering strategies (direct synthesis and post-treatment) are described. An antistructural notation of active centers in spinel cobalt ferrite is presented. It is shown that the introduction of cations with different charges (e.g., Cu(I), Mn(II), Ce(III), or Ce(IV)) into the cobalt ferrite spinel matrix results in the formation of various point defects. The ability to predict the type of defects and their impact on material properties is the basis of defect engineering, which is currently an extremely promising direction in modern materials science. [ABSTRACT FROM AUTHOR]

Isomers of stilbenes containing phenalenyl substituents in the para- and meta-positions of phenyl rings have been studied by means of the density functional theory method. Exchange coupling in the structures with the trans-form of stilbenes has been absent. In the cis-isomer of a meta-substituted compound, the state with a closed electron shell has been stabilized due to two-electron multicenter interactions, as confirmed by the CASSCF calculations. The predicted transition between the triplet and singlet states due to the trans-cis isomerization opens the prospects for using the discovered effect in the development of organic spin switches. [ABSTRACT FROM AUTHOR]

Cycloarenes such as kekulenes, septulenes and extended kekulenes are of considerable interest as they are precursors to holey nanographene with pores of variable sizes with interesting electronic and magnetic properties. These systems have also been of interest due to aromaticity, ring currents arising from π -electrons and intriguing topological properties. This study's primary objective is to determine the topological properties including entropies through analytical expressions of degree-based entropy metrics for two well-known classes of tessellations of kekulenes. The various topological indices and entropies obtained here to shed light on the underlying topological connectivities and molecular structural features. It is shown that the rhomboidal tessellations exhibit greater entropies compared to triangular tessellations. Applications of the developed techniques to machine learning of NMR and ESR spectra of these tessellations are pointed out. [ABSTRACT FROM AUTHOR]

By establishing Ising model, we investigated the thermodynamic properties and magnetocaloric effect of a diluted Kekulene-like structure with defects. Under the influence of different exchange coupling and the concentration of magnetic atoms, the changes in some magnetic and thermodynamic quantities including the magnetocaloric effect were discussed. For the change of concentration and exchange coupling, which parameter is more crucial to the magnetic properties of the structure was also given. The results indicate that when the exchange coupling and the concentration of magnetic atoms increase, the temperature range corresponding to the entropy change becomes wider. Besides, the relative cooling power (RCP) increases as the exchange coupling decreases or the concentration of magnetic atoms increases. [ABSTRACT FROM AUTHOR]

We investigate the magnetic, electronic, and transport properties of chemically modified Clar's goblets and laterally connected graphene quantum dots (GQDs) using density functional theory. The chemical modification includes full or partial edge passivation with 2H, O, and F atoms. We report transformation of antiferromagnetic spin ordering of Clar's goblet to ferromagnetic ordering with a nonzero net spin such as S = 1 by full passivation with 2H or S = 2.5 by partial passivation with 2H. The full and partial passivation of Clar's goblet with 2H isolates correspondingly 2 and 5 electrons with the same spin alignment which boosts the ferromagnetism. We also report the spin density localization on one side of the goblet by doping the other side with Li-atom. For laterally connected triangular GQDs, we reveal unconventional net spin. Contrary to Lieb's rule, the laterally connected triangular GQDs host not S = 0 but S = 4 net spin. This anomalous ferromagnetic state occurs due to the twisting of GQDs with respect to one another. Due to the symmetry breaking between spin up and spin down energy states, the corresponding spin up and down energy gaps are strongly affected by the modification. For instance, the spin down energy gap increased/decreased by partial passivation with F/O atoms leading to tunable transport properties. I-V characteristics indicate that the edge modification increases conduction due to the enhanced dipole moment and the number of conducting charges. Transport through latterly connected GQDs show quantum tunneling effect due to the overlap between spin up and spin down molecular orbitals which makes these systems promising as ferromagnetic tunneling diodes. [ABSTRACT FROM AUTHOR]