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 Eg = 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 Γ-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.
•Raman spectra of hygroscopic Li7MnN4 (LMN) in 293 K and 130 K are reported for the first time.•DFT calculations well reproduce LMN experimental structure and Raman spectra.•Calculated electronic structure characterizes LMN as indirect band gap semiconductor with a narrow gap.•The calculations provide a complete explanation of the vibrational dynamics of the compound.•The oxidation state of Mn in LMN and the reactivity of LMN in air can be monitored by Raman spectroscopy.
Nanocomposite materials which are silicate nanoporous matrices filled with KNO
3
are studied by the calorimetric method. The anomalies observed in the specific heat related to the ferroelectric phase ...transitions are studied in the framework of the smeared phase transition theory. The extrapolation of results confirms the assumption made before that there is the minimum pore size in the nanocomposite at which KNO
3
at room temperature will be completely in the ferroelectric state.
In the present study, the unit cell parameters and atomic coordinates are predicted for the Pbcm orthorhombic structure of Si(oP32) modification. This new allotrope of silicon is mechanically stable ...and stable with respect to the phonon states. The electronic structure of Si(oP32) is calculated for LDA and HSE06 optimized structures. The band gap value Eg = 1.361 eV predicted for Si(oP32) is extremely close to the Shockley–Queisser limit and it indicates that the Si(oP32) modification is a promising material for efficient solar cells. The frequencies of Raman and Infrared active vibrations is calculated for allotrope Si(oP32).
•The atomic coordinates are predicted for the orthorhombic structure of Si(oP32) modification.•The electronic structure of Si(oP32) is calculated for LDA and HSE06 optimized structures.•The frequencies of Raman and Infrared vibrations are calculated for allotrope Si(oP32).
Raman spectra of (NH4)3TiF7 within spectral range 10–3,400 cm−1 at temperatures 8–370 K are recorded, which revealed 2 phase transitions at temperatures T1 = 355 K and T2 = 283 K. First principles ...calculations of lattice dynamics and assignment of the observed lines to specific vibrational types are performed. The first phase transition is classified as an “order–disorder” one. Manifestation of 97 cm−1 vibration (mixed translational NH4 + F7) evidences the creation of the hydrogen bond between ammonium group and free fluorine atom, indicating to reconstructive type of phase transition an T2 = 283 K. The emergence of a large number of vibrational lines in Raman spectra at low temperatures proves symmetry lowering of NH4 tetrahedra with the simultaneous ordering of TiF6 octahedra.
Raman spectroscopy and lattice dynamics simulation has been used to study the phase transitions in ammonium heptafluorotitanate. The first phase transition is accompanied by disordering of TiF6 octahedra and ordering of NH4. The second phase transition determined earlier as a reconstructive one is accompanied by ordering of TiF6 octahedra and a symmetry lowering of NH4 tetrahedra.
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.
•Calculations on H-capped clusters reproduce well the main characteristics of the structure of glassy tellurium oxide.•Atypical TeO5 unit formation involves the appearance of 3-fold coordinated O atoms.•Vibrational properties of H-capped clusters are in line with experimental data.•Average polarizability of the clusters agree well with the linear dielectric susceptibility of the tellurium oxide.
The structural, electronic, and nonlinear optical properties of a γ-TeO
2
crystal have been studied using nonempiric quantum-mechanical calculations. The electron localization on the 5
d
orbital is ...taken into account using the Hubbard corrections to the density functional (the LDA +
U
approximation). The use of this approach enables a fairly correct reproducibility of the experimental structural parameters. The electronic structure is studied using the
G
0
W
0
quasi-particle approximation that recommended itself as one of most exact methods of calculating the band structure. The γ-TeO
2
crystal is found to be a wide bandgap semiconductor with indirect optical transition. Using the maximally localized Wannier functions, the chemical binding in this oxide is analyzed and it is shown that valent electrons of oxygen atoms are in sp3 hybridization, and the tellurium atom valence is four.
Crystal structure of LaF3 single crystal is refined in tysonite-type trigonal unit cell Pc1 using density functional theory calculations and Raman spectroscopy. It is shown that trigonal structure ...with Pc1 space group is more energy-efficient than hexagonal structure with space group P63cm. Simulated Raman spectra obtained using LDA approximation is in much better agreement with experimental data than that obtained with PBE and PBEsol functionals of GGA. The calculated frequency value of silent mode B2 in case of hexagonal structure P63cm was found to be imaginary (unstable mode), thus the energy surface obtains negative curvature with respect to the corresponding normal coordinates of the mode which leads to instability of the hexagonal structure in harmonic approximation. The A1g line at 214 cm−1 in Raman spectra of LaF3 related to the translation of F2 ions along c axis can be connected with F2 ionic conductivity.
The hypothesis of the huge optical nonlinearity of the crystalline TeO3, recently advanced on the basis of the quantum-mechanical simulations, is tested. Electronic band gaps of α-TeO2 and β-TeO3 ...crystals are determined by diffuse reflectance measurements. The DFT+U method is applied to calculate electronic band gap E g and the third-order nonlinear dielectric susceptibility χ(3). The χ(3)(TeO3) is about two times lower than the χ(3)(TeO2) in spite of the fact that the E g(TeO3) is narrower than E g(TeO2). It is shown that this peculiarity is related to the 5s(Te) electronic states which are occupied in TeO2 and are vacant in TeO3. This distinction is due to the specific electronic state related to the electron lone pairs localized on the Te(IV) atoms.
The finite field method using ab initio hybrid functional approach was applied to estimate nonlinear properties of the cluster based on paratellurite crystal structure. The good agreement was found ...with previously reported data.The method demonstrate high efficiency in prediction of nonlinear properties for a wide ensemble of sufficiently large polyatomic clusters.
Abstract
The results of joint theoretical and experimental studies aimed at revealing features in the Raman spectra, which can be used for evaluation of the interface quality between GaN and AlN ...layers in short-period GaN/AlN superlattices (SLs) are presented. The Raman spectra for SLs with sharp interface and with different degree of interface diffusion are simulated by
ab initio
calculations and within the framework of random-element isodisplacement model, respectively. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, leads to conclusion that the spectral region of the A
1
(LO) confined phonons is very sensitive to the degree of interface sharpness. As a result of comprehensive studies, the correlations between the parameters of the A
1
(LO) confined phonons and the structure of SLs are obtained. The results of the complex studies can be used to optimize the parameters of the growth process in order to form structurally perfect short-period GaN/AlN SLs.
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