The synthesis of stable yttrium oxyhydride-type compounds raised a principal question regarding the key factors that may be responsible for formation routes and structural features of these attractive materials. For solving this challenge, the interplay between chemical composition and crystalline architectures has been theoretically explored via modeling structural transformations caused by the gradual oxidation of the host metal hydride system. The close combination of grouptheory methods, mixed-anion chemistry arguments, and relevant density functional theory calculations provided us with the opportunity to suggest and characterize the candidate models for most probable stoichiometric versions of yttrium oxyhydrides. The predicted chemical compositions along with the crystallization results have been summarized in the phase diagram. It is shown that the cation−ligand interaction governs the structural stability which is achieved by matching favorable crystallographic positions of the oxygen and hydrogen atoms at the metal center.

When exposed to air, metallic yttrium dihydride YH 2 films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of YH 2 and the emergence of photochromic properties, i.e., YHO darkens reversibly when illuminated with light of adequate energy and intensity. However, the adequate bleaching of the photodarkened samples once the illumination has stopped is much faster in air than in inert atmosphere. According to this experimental evidence, the photochromic mechanism has to be related to an oxygen diffusion and exchange process. Since this process is accompanied by a lattice expansion/contraction, it can be said that YHO "breathes" when subjected to illumination/darkness cycling. Another interesting side effect of the breathing is the unexpected enhancement of the hydrophobicity of the YHO samples under illumination. A theoretical model able to explain the breathing in YHO is presented, together with the discussion of other alternative explanations.

We report the results of experimental studies related to implantation of thorium ions into thin silicon dioxide by pulsed plasma fluxes expansion. Thorium ions were generated by laser ablation from a metal target, and the ionic component of the laser plasma was accelerated in an electric field created by the potential difference (5, 10 and 15 kV) between the ablated target and SiO 2 /Si (001) sample. Laser ablation system installed inside the vacuum chamber of the electron spectrometer was equipped with YAG:Nd3+ laser having the pulse energy of 100 mJ and time duration of 15 ns in the Q-switched regime. Depth profile of thorium atoms implanted into the 10 nm thick subsurface areas together with their chemical state as well as the band gap of the modified silicon oxide at different conditions of implantation processes were studied by means of X-ray photoelectron spectroscopy (XPS) and Reflected Electron Energy Loss Spectroscopy (REELS) methods. Analysis of chemical composition showed that the modified silicon oxide film contains complex thorium silicates. Depending on local concentration of thorium atoms, the experimentally established band gaps were located in the range of 6.0 -9.0 eV. Theoretical studies of optical properties of the SiO 2 and ThO 2 crystalline systems have been performed by ab initio calculations within hybrid functional. Optical properties of the SiO 2 /ThO 2 composite were interpreted on the basis of Bruggeman effective medium approximation. A quantitative assessment of the yield of isomeric nuclei in "hot" laser plasma at the early stages of expansion has been performed. The estimates made with experimental results demonstrated that the laser implantation of thorium ions into the SiO 2 matrix can be useful for further research of low-lying isomeric transitions in 229 Th isotope with energy of 7.8 ± 0.5 eV.

One of the most significant aspects of crystal chemistry of multianionic oxyhydrides is the possibility of flexible regulation of the composition-structure-function relationships. In the context of competitive coordinations of different anions in the crystal lattice, this may afford formation of a number of stable stoichiometric phases without inversion symmetry. In the present work, we demonstrated that semiconducting yttrium and lantanium oxyhydrides with the composition Ln₂H₄O (Ln=Y, La) have an attractive potential for the design of novel lead-free ferro- and piezoelectric systems. By means of advanced DFT-based computational simulations we predicted that several polar monoclinic and orthorhombic phases of Ln₂H₄O may exhibit exceptional ferro- and piezoelectric properties as well as electromechanical coupling characteristics that are especially suitable for the piezoelectric devices working in a shear mode. Structure-dependent theoretical evaluations of the relevant physical responses demonstrated estimates of ferro- and piezoelectric characteristics that are comparable with the specifications of advanced ferroelectric solid solutions. Thus, our prediction of lead-free piezoelectric systems forms a solid and technologically reliable basis for the future development of effective and non-hazardous materials.

The synthesis of stable yttrium oxyhydride-type compounds raised a question regarding the key factors that may be responsible for formation routes and structural features of these attractive materials. For solving this problem the interplay of chemical composition and crystalline architectures has been theoretically explored in terms of possible structural transformations caused by the gradual oxidation of the host metal-hydride system. The combination of group-theory methods, mixed-anion chemistry arguments, and relevant DFT calculations provided us with the opportunity to predict and characterize the candidate models for most probable stoichiometric versions of yttrium oxyhydrides. The predicted chemical compositions along with the crystallization results have been summarized in the phase diagram. It is shown that structural stability is achieved by matching favorable crystallographic positions of the nearest oxygen and hydrogen atoms at the metal center.

Rare‐earth metal oxyhydride compositions are currently attracting increasing attention to develop materials with unusual optical responses. Herein, using computer simulations of the electronic and optical properties, the optical responses of two stable yttrium oxyhydride compounds, normalY4normalH10O and YHO, are studied for the visible light range. The emphasis is on modeling macroscopic optical characteristics, which are numerically derived within a conventional scheme using refractive indices, and absorption, transmittance, and reflection spectra. The main goal is twofold: first, to simulate spectral behavior of different single‐phase and two‐phase oxyhydride compositions and second, to conduct a comparative analysis that could explain the features of the transmission spectra measured for different samples. Based on the obtained results, models of new optical coatings are proposed in which yttrium oxyhydrides play the key role. In the context of nonlinear optics, the frequency profile of the second‐order susceptibility χ(2)(2ω) for the noncentrosymmetric cubic structure of normalY4normalH10O is evaluated and it is shown that this system could exhibit large optical nonlinearity.

Design of inorganic compounds containing different anions attracts a lot of attention because it affords a great opportunity to develop new functionality across the whole range of material properties. Based on the results of structure-modeling studies of a mixed-anion system, we predicted novel derivatives of oxyhydrides – chiral hydroxyhydrides M₂H₃O(OH) (M = Y, Sc, La, and Gd) that are characterized by the coexistence of three anionic species, H^–, O2^–, and OH^– inside the crystal lattice. The materials demonstrate a specific charge ordering, which is connected with the chiral organization of atoms where both the metal cations and the anions are standing in positions that form helical curves spreading along the tetragonal axis. Moreover, the twisting of the H^– and H⁺ sites gives rise to their linking via strong dihydrogen bonds. Unusual structural, electron and optical features caused by the P4₁ crystal structure have been investigated in the Y₂H₃O(OH) comprehensive case study.

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.