RESUMEN
The main result of our investigation is the prediction of a new family of multianion compounds - Ln2OF2-xClxH2 (Ln = Y, La, Gd) which due to anomalous elastic behaviour could present interest for the design and development of electromechanical devices. The composition Ln2OF2-xClxH2 utilizes a complex heteroatomic anion [OF2-xClxH2]6-; in a solid state, as it follows from the DFT calculations, the system crystallizes into a columnar-type layered structure of P3m1 or R3m trigonal symmetries in which the LnO(F,Cl)H and Ln(F,Cl)FH layers are uniformly stacked in an alternating order along the high-symmetry c axis. In the trigonal lattice without an inversion center, the resulting two-layer geometry puts groups of the anionic species together in a way that gives rise to a strong localization of valence charge density. We showed that being globally stable, such specific crystal architecture may lead to a high asymmetry of mechanical and electrical responses with respect to imposed loads. Moreover, small dynamic changes of the equilibrium charge and bonding configurations may cause rather enhanced structural sensitivity of the elastic responses at low pressures. Comparison of electromechanical characteristics showed that the predicted materials can serve as direct successors of the line of polyvinylidene fluoride (PVDF) piezopolymers.
RESUMEN
Examination of possible pathways of how oxygen atoms can be added to a yttrium oxyhydride system allowed us to predict new derivatives such as hydroxyhydrides possessing the composition M2H3O(OH) (M = Y, Sc, La, and Gd) in which three different anions (H-, O2-, and OH-) share the common chemical space. The crystal data of the solid hydroxyhydrides obtained on the base of DFT modeling correspond to the tetragonal structure that is characterized by the chiral space group P 4 1 . The analysis of bonding situation in M2H3O(OH) showed that the microscopic mechanism governing chemical transformations is caused by the displacements of protons which are induced by interaction with oxygen atoms incorporated into the crystal lattice of the bulk oxyhydride. The oxygen-mediated transformation causes a change in the charge state of some adjacent hydridic sites, thus forming protonic sites associated with hydroxyl groups. The predicted materials demonstrate a specific charge ordering that is associated with the chiral structural organization of the metal cations and the anions because their lattice positions form helical curves spreading along the tetragonal axis. Moreover, the effect of spatial twisting of the H- and H+ sites provides additional linking via strong dihydrogen bonds. The structure-property relationships have been investigated in terms of structural, mechanical, electron, and optical features. It was shown that good polar properties of the materials make them possible prototypes for the design of nonlinear optical systems.
RESUMEN
Results of periodic DFT simulations have been used to gain a new view of the crystal chemistry of LiBSi2 in terms of computational modeling of the LiB + 2Si â LiBSi2 synthesis reaction. It was shown that both the strong alkali-metal-[BSi2]- interaction and the rich behavior of B-Si couplings are the main distinctive features of the chemical bonding picture in LiBSi2. In particular, an interplay between charge transfer from easily ionizable lithium linear chains to the boron atoms and strong covalent connectivities in the tetrahedral B-Si framework is of great importance for the bonding architecture in LiBSi2. The activation of positively ionized silicon species Si+ and the formation of electron-rich B3- anions in the [BSi2]- group were found to play a key role in providing the stability of boron-silicon polar covalent bonds. It was suggested that the Si+-B3--Si+ bonding pattern featuring the high anionic state of the boron atom can be identified as a boryl anion.