Solving the problem of the arrangement of pyridinium cations in one-dimensional PyPbBr3 perovskite-related compound, the possible configurations with different orientation of the pyridinium cations relative to the anionic framework were theoretically proposed. Their structural characteristics and formation energy values were estimated using density functional theory (DFT) calculations. The calculated and experimental Raman spectra were compared in the wide spectral range, and the correspondence between the observed spectral lines and the characteristic modes in the calculated vibrational spectra of the PyPbBr3 models was established. The presence of several orientations of Py cations in the PyPbBr3 sample at room temperature was suggested.

This paper reports on a systematic first-principles molecular dynamics investigation of binary (TlO0.5)(y) - (TeO2)(1-y) and ternary (TiO2)(x) - (TlO0.5)(y) - (TeO2)(1-x-y) tellurite glasses. The obtained structural models are validated against available measured X-ray pair distribution functions. In the binary system, increasing TlO0.5 content induces network depolymerization through the reduction of Te coordination number, the substitution of Te-O-Te bridges with Te=O-center dot center dot center dot Tl+ units, and the proliferation of non-bridging oxygens. In addition, rings analysis demonstrates a loss of the network connectivity via the opening of small n-membered rings. In contrast, TiO2 acts as a network former in ternary glasses, preserving Te coordination number, and promoting a high fraction of bridging oxygens. Ti atoms induces a network repolymerization that manifests through the formation of smaller Ti-containing n-membered rings thereby balancing the strong effect of Tl2O modifier. Beside the structural analysis, we also computed Raman spectra and non-linear optical properties on the obtained large periodic models. Our results reproduce experimental trends in Raman band shifts with composition, while nonlinear optical calculations show that remains stable with TlO0.5 addition in binary glasses, consistent with experiment. In the case of ternary systems, we find that the inclusion of a small fraction of TiO2 preserves the high optical nonlinearity of the TeO2 network while maintaining the overall network connectivity. These results establish a predictive framework for tailoring the atomic structure and nonlinear optical response of tellurite glasses through the controlled interplay of the nature and concentration of modifiers.

Cosalite (), a layered sulfosalt mineral with an orthorhombic structure, is investigated through a combined experimental and first-principles theoretical approach to elucidate its vibrational, nonlinear optical, and thermal transport properties. The unambiguous determination of the non-centrosymmetric space group for the most stable phase of cosalite is performed using DFT calculations, supported by experimental EBSD data. The experimental Raman spectra recorded at liquid nitrogen temperature reveal eight intense modes in the range 50-300 cm, assigned to Bi-S and Pb-S stretching vibrations. The direct optical transition is established by calculation of the band structure. The optical absorption spectrum calculated by TDDFT including SOC effects yields a bandgap value of = 1.1 eV, which is substantially similar to the fundamental Shockley-Queisser limit value, thus opening the possibility of using cosalite as a photovoltaic element. The strong third-order nonlinear optical response is obtained in cosalite ( m/V) and is attributed to the dynamics of the electronic lone pair localized on Pb and Bi atoms. The lattice thermal conductivity is established by analysis of the heat flux current using classical molecular dynamics, employing a machine-learning technique for constructing the interatomic potential. The estimated ultralow value gives rise to potential applications of the crystal as a thermoelectric generator.

The nonlinear optical properties of amorphous tellurium oxide is studied in a framework of computational technique based on the ab initio quantum-chemical calculations. The calculated values of refractive index and third-order nonlinear susceptibility are in a very good agreement with experimental data. The origin of the high nonlinear properties of TeO2 is attributed, in general, to the hyperpolarizability of Te atoms electron lone pairs and to the hyperpolarizability of electronic density localized on bonds of the asymmetric Te-O-Te bridges in part. The computational technique is extended on Hyper Raman spectroscopy including scattering on longitudinal modes. The spectra were calculated for both crystalline and glassy TeO2 with a very good reproduction of experimental data. An extra bands inactive in classical Raman spectroscopy are studied using this approach. The most intense high-frequency band in the spectra of paratellurite is attributed to anti-symmetric stretching of long-short bonds in TeO4 disphenoid with A2 symmetry. The influence of the structural unit configuration on the band position in Hyper Raman spectra is established. The best agreement between calculated phonon spectrum and experimental data is obtained using hybrid functional approach, which explains the strong correlation on valence electron states of Te atoms. The strong dependence of LO modes intensity in Hyper Raman spectra on third order nonlinear susceptibility is obtained and explained. The reported technique can be easily extended on a big family of binary and ternary compounds.

Single crystals of the layered EuRECuTe3 series with RE = Nd, Sm, Tb and Dy are obtained for the first time, completing the series of studies on quaternary tellurides synthesized using the halide flux method. These compounds crystallize in the orthorhombic space group Pnma (no. 62) with unit cell parameters ranging from a = 11.5634(7) Å, b = 4.3792(3) Å and c = 14.3781(9) Å for EuNdCuTe3 to a = 11.2695(7) Å, b = 4.3178(3) Å and c = 14.3304(9) Å for EuDyCuTe3. The influence of prismatic polyhedra [EuTe6+1]7− structural units on the stabilization of 3d framework composed by 2d layered fragments [RECuTe3]2−, which have a key role in the interlayer interaction, is established. A comparative analysis of structural and magnetic properties dependence on the rare-earth element radius ri(RE3+) in the EuRECuTe3 series (RE = Sc, Y, Nd–Lu) is carried out. The structural contraction, including decrease in degree of tetrahedral polyhedra distortion, bond lengths shortening and unit cell volume shrinking with increasing ri(RE3+), is established. It is shown that the structural alternation leads to transition from ferromagnetic to ferrimagnetic ordering. It was established that changes in the cationic sublattice have a more significant impact on structural transitions in the series of quaternary tellurides than changes in the anionic sublattice. The electronic structure and elastic and dynamic properties were estimated using ab initio calculations. The exfoliation energy for each compound is obtained by estimation of monolayer ground state energy as a result of structure relaxation. The symmetry and structural properties of monolayer EuRECuTe3 (RE = Nd, Sm, Tb, Dy) compound are established and the orthorhombic symmetry is obtained with layer group pm2_1b.

A study was carried out of the dielectric properties of planar Si/SiO2 heterostructures, which play an important role in modern electronics. Using the model of dielectric continuum, the spectra of polar phonons in Si/SiO2 binary superlattices have been studied. Quartz and cristobalite lattices are considered as a structural model of the oxide layer. The dependences of polar optical phonons frequencies and the high-frequency dielectric constant tensor elements on the ratio of layer thicknesses were obtained. The results obtained open up the possibility of using spectroscopic data to characterize the structure of superlattices.

The electronic structure, optical and dynamical properties of the natural phenakite Be2SiO4, are studied using ab initio calculations completed by Raman spectroscopy experiments. The calculated structural properties are in excellent agreement with experimental data and are accompanied by chemical bonds analysis using Maximally Localized Wannier Functions method. The theoretical analysis of projected density of states reveals the nature of the valence band as composed by electronic states of oxygen atoms, while conducting band is composed by electronic states of Si-atoms. Using linear response method the optical properties are calculated and compared with optical properties of quartz. It was shown the similarity of rare phenakite crystal and quartz. The calculated phonon dispersion structure reveals mostly delocalized nature of phonon states with quasilocalized phonons in high-frequency range, where the most intense band in Raman spectrum is observed. The band is ambiguously assigned to the breathing mode in SiO4 tetrahedra. Basing on such peculiarity in the spectra the rapid nondestructive diagnostic method of the sample is purposed.

Rare-earth orthoferrites are a promising platform for antiferromagnetic spintronics with a rich variety of terahertz spin and lattice dynamics phenomena. For instance, it has been experimentally demonstrated that the light-driven optical phonons can coherently manipulate macroscopic magnetic states via nonlinear magnetophononic effects. Here using TbFeO3 as an example, we reveal the origin of the mode mixing between the LO and TO phonons, which is important for understanding of nonlinear phononics. We performed a comprehensive study of the lattice dynamics of the TbFeO3 single crystal by polarized infrared and Raman scattering spectroscopic techniques, and experimentally obtained and carefully analyzed the spectra of anisotropic complex dielectric functions in the far-infrared spectral range. This allowed us to reliably identify the symmetries and parameters of most infrared- and Raman-active phonons. Next, the experimental studies were supplemented by the lattice dynamics calculations which allowed us to propose the normal mode assignments. We reveal that the relation between LO and TO polar phonons is complex and does not strictly follow the "LO-TO rule" due to the strong mode mixing. We further analyze how displacements of different ions contribute to phonon modes and reveal that magnetic Fe ions are not involved in Raman-active phonons, thus shedding light on a lack of spin phonon coupling for such phonons. The obtained results establish a solid basis for further in-depth experimental research in the field of nonlinear phononics and magnetophononics in rare-earth orthoferrites.

The paper reports on theoretical analysis of structural and non-linear optical properties of different size and shape TeO2 clusters using ab initio hybrid functional approximation to density functional theory. The obtained structural properties reveal uncommon structural units for crystal, which are common for amorphous tellurium oxide glass. The pair distribution function reproduces the experimental data in very good agreement, as well as the calculated phonon density and the Raman spectra. The finite field method was applied to evaluate the values of third order nonlinear susceptibility. The obtained values are in line with experimental data. This opens possibility to use such clusters to analyze the structural organization, vibrational and dielectric properties of the tellurium oxide glass.

Herein, a computational technique that combines density functional theory and the finite difference method is presented to enable the calculation of the Raman spectra of large models of oxide glasses. The calculated Raman spectra of amorphous are found to be in excellent agreement with the experimental data. A strong peak in the low‐frequency range of the Raman spectra is observed and attributed to the Boson peak. According to atomic‐scale analysis, this peak is assigned to collective vibrations of nanoclusters that are formed by the structural units of the glass. Two general factors that influence the Boson peak intensity are established. The first factor concerns the intensity of the low‐frequency peak in vibrational density of states. The second factor is related to the low‐frequency vibrational state occupancy at fixed temperature, which obeys the Bose–Einstein statistic. It is found that even a small shift toward high frequencies leads to a significant decay of the vibrational state occupancy. This correlates quite well when the Raman spectra of glass are compared to the spectra of fused silica. The technique can be readily applied to the large set of amorphous systems.

Raman spectroscopy has proven its effectiveness as a highly informative and sensitive method for the nondestructive analysis of layered nanostructures and their interfaces. However, there is a lack of information concerning the characteristic phonon modes and their activity in Si/SiO2 nanostructures. In order to overcome this problem, the phonon states and Raman spectra of several Si/SiO2 superlattices (SL) with layer thicknesses varied within 0.5–2 nm are studied using DFT-based computer modeling. Two types of structures with different interfaces between crystalline silicon and SiO2 cristobalite were studied. A relationship between the phonon states of heterosystems and the phonon modes of the initial crystals was established. Estimates of the parameters of deformation potentials are obtained, with the help of which the shifts of phonon frequencies caused by elastic strains in the materials of the SL layers are interpreted. The dependence of intense Raman lines on the SL structure has been studied. Several ways have been proposed to use this information, both for identifying the type of interface and for estimating the structural parameters. The obtained information will be useful for the spectroscopic characterization of the silicon/oxide interfaces.

α-moganite is a recently discovered polymorph of silica, commonly intergrown with quartz in natural microcrystalline silica samples. An important challenge is finding an effective method for estimating its amount in a sample under study, which is important for its applications, related to the technology of growth of dielectric layers, as well as for fundamental problems, related to the formation of both terrestrial and lunar mineral deposits and biogenic formation. One of these methods is vibrational spectroscopy, with the help of which the presence of a particular compound is determined by the presence of characteristic spectral lines. In this work, the search for such lines is carried out using density functional theory calculations and comparisons of the IR and Raman spectra of α-quartz and α-moganite. With the help of such calculations, the stability of the moganite structure has been proven for the first time, and its spectral characteristics have been determined over the entire range of vibrational frequencies. Several new spectral lines characteristic of α-moganite were discovered in the 65–85 cm−1 region. Moreover, the evolution of spectral peculiarities under hydrostatic pressure was studied.

New polymorphic modifications of double sulfates β-AEu(SO4)2 (A-Rb+, Cs+) were obtained by the hydrothermal method, the structure of which differs significantly from the monoclinic modifications obtained earlier by solid-state methods. According to single-crystal diffraction data, it was found that the compounds crystallize in the orthorhombic system, space group Pnna, with parameters β-RbEu(SO4)2: a = 9.4667(4) Å, b = 13.0786(5) Å, c = 5.3760(2) Å, V = 665.61(5) Å3; β-CsEu(SO4)2: a = 9.5278(5) Å, b = 13.8385(7) Å, c = 5.3783(3) Å, V = 709.13(7) Å3. The asymmetric part of the unit cell contains one-half Rb+/Cs+ ion, one-half Eu3+ ion, both in special sites, and one SO42- ion. Both compounds exhibit nonlinear negative thermal expansion. According to the X-ray structural analysis and theoretical calculations, the polarizing effect of the alkali metal ion has a decisive influence on the demonstration of this phenomenon. Experimental indirect band gaps of β-Rb and β-Cs are 4.05 and 4.11 eV, respectively, while the direct band gaps are 4.48 and 4.54 eV, respectively. The best agreement with theoretical calculations is obtained using the ABINIT package employing PAW pseudopotentials with hybrid PBE0 functional, while norm-conserving pseudopotentials used in the frame of CASTEP code and LCAO approach in the Crystal package gave worse agreement. The properties of alkali ions also significantly affect the luminescent properties of the compounds, which leads to a strong temperature dependence of the intensity of the 5D0 → 7F4 transition in β-CsEu(SO4)2 in contrast to much weaker dependence of this kind in β-RbEu(SO4)2.

The oxide-semiconductor interface is a key element of MOS transistors, which are widely used in modern electronics. In silicon electronics, SiO2 is predominantly used. The miniaturization requirement raises a problem regarding the growing of heterostructures with ultrathin oxide layers. Two structural models of interface between crystalline Si and cristobalite SiO2 are studied by using DFT-based computer modelling. The structures of several Si/SiO2 superlattices (SL), with layer thicknesses varied within 0.5–2 nm, were optimized and tested for stability. It was found that in both models the silicon lattice conserves its quasi-cubic structure, whereas the oxide lattice is markedly deformed by rotations of the SiO4 tetrahedra around axes perpendicular to the interface plane. Based on the analysis of the calculated total energy of SLs with different thicknesses of the layers, an assessment of the interface formation energy was obtained. The formation energy is estimated to be approximately 3–5 eV per surface Si atom, which is close to the energies of various defects in silicon. Elastic strains in silicon layers are estimated at 5–10%, and their value rapidly decreases as the layer thickens. The elastic strains in the oxide layer vary widely, in a range of 1–15%, depending on the interface structure.

Structure, phonon states and vibrational spectra of binary Si/SiO2 superlattices (SL) formed by junction of crystalline silicon and β-cristobalite are investigated with the use of ab-initio quantum-mechanical computational methods. Several stable SL structures with ultra-narrow interfaces consisted of only one monolayer of Si2+ atoms are found. For these SLs, we have simulated the infrared and Raman spectra in which some characteristic spectral features are detected

The electronic structure of the promising Li-ion battery anode material Li7MnN4 synthesized by a solid-state reaction is studied using ab initio calculations completed by Raman spectroscopy experiments. The structural optimization reliably reproduces the experimental one, hence validating the accuracy of the chosen Density Functional Theory method. The theoretical analysis of the electronic structure reveals the nature of the valence band as composed from band filled by electrons with spin-up states only, which allows refuting literature data about the claimed electronic character of Li7MnN4 . Actually, the calculated electronic band gap E-g = 0.95 eV is found to be in good agreement with available experimental data. A careful experimental approach provides the first experimental Raman spectra of hygroscopic Li7MnN4 at 293 K and 130 K. The analysis of the phonon states in the gamma-point of the Brillouin zone, completed by the computation of the Raman scattering intensities of the vibrational modes of the Li7MnN4 structure give a remarkable agreement between simulated and experimental Raman spectra. With such a good matching, a reliable assignment of all the observed Raman peaks to the vibrations of specific structural units in the Li7MnN4 lattices is proposed. In particular, the most intense band in the Raman spectrum is ascribed to a totally symmetric MnN4 breathing mode. We also show that, using different wavelengths of exciting radiation, the transition from off-resonance to resonance Raman scattering process can be observed. Furthermore, Raman spectroscopy is revealed as an efficient in situ diagnostic tool to control the degradation of the Li7MnN4 powder in open air through the observation of extra bands in the Raman spectra. Results of this study shed a light on the understanding of the fundamental properties of Li7MnN4 and pave a way for the upcoming operando Raman spectroscopy investigation of the atomic-scale induced structural changes of this negative electrode material for Li-ion battery. (C) 2022 Elsevier B.V. All rights reserved.

Structure, phonon states and vibrational spectra of binary Si/SiO 2 superlattices (SL) formed by junction of crystalline silicon and β-cristobalite are investigated with the use of ab-initio quantum-mechanical computational methods. Several stable SL structures with ultra-narrow interfaces consisted of only one monolayer of Si 2+ atoms are found. For these SLs, we have simulated the infrared and Raman spectra in which some characteristic spectral features are detected. Keywords: oxide-semiconductor heterostructures, superlattices, computer simulation, density functional method, vibrational spectra.

The electronic structure of alpha-V2O5, gamma'-V2O5, and gamma-MeV2O5 (Me = Li, Na) bronzes is studied by quantum-chemical calculations completed by spectroscopic experiments. The calculations are performed using the G(0)W(0) method with the DFT+U self-consistent wave function as an initial approximation. The electronic band gap E-g = 2.89 eV calculated for alpha-V2O5 is found to be in fair agreement with available experimental data. The strategy was then applied to studying the electronic structure of the. gamma'-V2O5 phase and gamma-MeV2O5 bronzes for which no experimental band gap data exist in the literature. Computed E-g values are equal to 3.17, 1.21 and 1.18 eV for gamma'-V2O5, gamma-LiV2O5, and gamma-NaV2O5, respectively. The nature of the alkali metal atom is determined to have little influence on the structure and electronic states of the bronzes. Raman spectra recorded with different wavelengths of exciting radiation have allowed the determination of the energy threshold corresponding to the transition from off-resonance to resonance Raman scattering process. In this way, a band gap value in the range 2.54-2.71 eV for alpha-V2O5 and gamma'-V2O5 is obtained in good agreement with the experimental values for the alpha-phase. Raman spectra of gamma-MeV2O5 suggest the band gap smaller than 1.58 eV in these materials, whereas the photoluminescence measurements yield E-g approximate to 0.95 eV for the gamma-LiV2O5 bronze. Remarkably, the result of the G(0)W(0) calculations lies in between the experimental estimates. The strong similarity of structures and electronic states of gamma-LiV2O5 and gamma-NaV2O5 accounts for their the same operating voltage when used as cathodes in Li(Na)-ion batteries.

We present an extensive theoretical and experimental study to identify the effect on the Raman spectrum due to interface interdiffusion between GaN and AlN layers in short-period GaN/AlN superlattices (SLs). The Raman spectra for SLs with sharp interfaces and with different degree of interface diffusion are simulated by ab initio calculations and within the framework of the random-element isodisplacement model. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, showed that the bands related to A1(LO) confined phonons are very sensitive to the degree of interface diffusion. As a result, a correlation between the Raman spectra in the range of A1(LO) confined phonons and the interface quality in SLs is obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy.

Калориметрическим методом изучены нанокомпозиционные материалы, представляющие собой силикатные нанопористые матрицы, заполненные KNO3. Обнаруженные аномалии теплоемкости, связанные с сегнетоэлектрическими фазовыми переходами, изучены в рамках теории размытых фазовых переходов. Экстраполяция полученных результатов подтверждает высказанное ранее предположение о том, что существует минимальный размер пор в нанокомпозите, при котором KNO3 при комнатной температуре будет находиться полностью в сегнетоэлектрическом состоянии. Ключевые слова: фазовые переходы, сегнетоэлектрик, нитрат калия, теплоемкость.