Nonlinear optical properties of tellurium oxide glasses: ab initio calculations

International audienc

Identification of anhydrous CaCl2 and KCaCl3 in natural inclusions by Raman spectroscopy

Anhydrous chlorides - CaCl2, and KCaCl3 (chlorocalcite) were identified as mineral inclusions in halite from the Siberian Large Igneous Province at the contact of magmatic intrusions and evaporates. Chlorocalcite was also found as daughter mineral in polyphase hypersaline inclusions. While Raman spectra of KCaCl3 (chlorocalcite) in natural inclusions are similar to spectra of synthetic analogue, the Raman spectra of natural CaCl2 do not correspond to the published Raman spectra of synthetic CaCl2. Simulations of Raman spectra using ab initio density-functional theory (DFT) allowed us to calculate the spectra of individual polymorphs of CaCl2 and to discriminate anhydrous CaCl2 phases in natural inclusions and synthetic CaCl2. In the spectrum of the Pbcn polymorph of CaCl2 twelve different peaks could be identified at 74, 95, 99, 107, 124, 158, 164, 179, 212, 236, 244, 256 cm−1 in contrast to five peaks in the spectrum of the Pnnm polymorph of CaCl2 at 115, 157, 160, 211 and 252 cm−1. Naturally occurring CaCl2 in inclusions in halite consist of Pbcn polymorph only, which probably results from a mechanical stress on cooling from magmatic to ambient temperatures. However, the Raman spectra of the synthetic CaCl2 corresponds to the Pnnm phase with small contributions of the Pbcn phase.Raman spectra of synthetic KCaCl3 with main peaks at 58, 67, 90, 97, 133, 139, 147, 193 cm−1 agrees well with the spectra of chlorocalcite in the natural inclusions. Positions of each atom in the KCaCl3 structure were refined using the density-functional theory. There are no imaginary phonon modes for the optimized structure, indicating that the structure of KCaCl3 is stable. Calculated Raman spectrum is in a good agreement with the Raman spectrum of synthetic and natural KCaCl3 samples

Identification of anhydrous CaCl2 and KCaCl3 in natural inclusions by Raman spectroscopy

Anhydrous chlorides - CaCl2, and KCaCl3 (chlorocalcite) were identified as mineral inclusions in halite from the Siberian Large Igneous Province at the contact of magmatic intrusions and evaporates. Chlorocalcite was also found as daughter mineral in polyphase hypersaline inclusions. While Raman spectra of KCaCl3 (chlorocalcite) in natural inclusions are similar to spectra of synthetic analogue, the Raman spectra of natural CaCl2 do not correspond to the published Raman spectra of synthetic CaCl2. Simulations of Raman spectra using ab initio densityfunctional theory (DFT) allowed us to calculate the spectra of individual polymorphs of CaCl2 and to discriminate anhydrous CaCl2 phases in natural inclusions and synthetic CaCl2. In the spectrum of the Pbcn polymorph of CaCl2 twelve different peaks could be identified at 74, 95, 99, 107, 124, 158, 164, 179, 212, 236, 244, 256 cm−1 in contrast to five peaks in the spectrum of the Pnnm polymorph of CaCl2 at 115, 157, 160, 211 and 252 cm−1. Naturally occurring CaCl2 in inclusions in halite consist of Pbcn polymorph only, which probably results from a mechanical stress on cooling from magmatic to ambient temperatures. However, the Raman spectra of the synthetic CaCl2 corresponds to the Pnnm phase with small contributions of the Pbcn phase. Raman spectra of synthetic KCaCl3 with main peaks at 58, 67, 90, 97, 133, 139, 147, 193 cm−1 agrees well with the spectra of chlorocalcite in the natural inclusions. Positions of each atom in the KCaCl3 structure were refined using the density-functional theory. There are no imaginary phonon modes for the optimized structure, indicating that the structure of KCaCl3 is stable. Calculated Raman spectrum is in a good agreement with the Raman spectrum of synthetic and natural KCaCl3 samples

Unraveling the Structure–Raman Spectra Relationships in V₂O₅ Polymorphs via a Comprehensive Experimental and DFT Study

Vanadium pentoxide polymorphs (α-, β-, γ′-, and ε′-V₂O₅) have been studied using the Raman spectroscopy and quantum-chemical calculations based on density functional theory. All crystal structures have been optimized by minimizing the total energy with respect to the lattice parameters and the positions of atoms in the unit cell. The structural optimization has been followed by the analysis of the phonon states in the Γ-point of the Brillouin zone, and the analysis has been completed by the computation of the Raman scattering intensities of the vibrational modes of the structures. The optimized structural characteristics compare well with the experimental data, and the calculated Raman spectra match the experimental ones remarkably well. With the good agreement between the spectra, a reliable assignment of the observed Raman peaks to the vibrations of specific structurals units in the V₂O₅ lattices is proposed. The obtained results support the viewpoint on the layered structure of vanadium pentoxide polymorphs as an ensemble of V₂O₅ chains held together by weaker interchain and interlayer interactions. Similarities and distinctions in the Raman spectra of the polymorphs have been highlighted, and the analysis of the experimental and computational data allows us, for the first time, to put forward spectrum–structure correlations for the four V₂O₅ structures. These findings are of the utmost importance for an efficient use of Raman spectroscopy to probe the changes at the atomic scale in the V₂O₅-based materials under electrochemical operation

The Effect of Interface Diffusion on Raman Spectra of Wurtzite Short-Period GaN/AlN Superlattices

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