Partial Phonon Density Of States Research Articles

Piezo-optical properties and infrared spectra of Rb2SO4 crystals

The paper deals with the dispersion dependences of birefringence Δni(λ) of the mechanically free and uniaxially clamped Rb2SO4 crystal along different crystal-physical axes at room temperature. It was found that the crystal has an insignificant normal dispersion d(Δnx)/dλ χ ≤ 0, and uniaxial compressions σm do not change the character, but only the slope of the dependences Δni(λ). Uniaxial loads shift the position of the optical isotropic point both in the short-wavelength (σz) and in the long-wavelength part of the spectrum (σy). The spectral dependences of the combined piezo-optical coefficients πkmo(λ) were calculated and it was found that they have an insignificant dispersion dependence, and the absolute values of the piezo-optical coefficients π31o and π21o are equal to each other in the vicinity of the optical isotropic point at the light wavelength λ = 490 nm. The reflection spectra of Rb2SO4 crystals were measured in the infrared range at room temperature using FT-IR spectrometer and synchrotron radiation and the corresponding ab initio calculations of the phonon partial density of states and the dielectric function were performed.

Lattice dynamics of 119Sn impurity in a bcc-Cr crystal

The chromium crystal doped with119Sn isotope was studied using the nuclear resonance inelastic x-ray scattering and first principles calculations. The Sn partial phonon density of states (PDOS) was obtained for three temperatures that correspond to different magnetic states of Cr. At all temperatures, the energy spectrum consists of a broad band around 18 meV and a narrow peak at 43 meV. The additional peak around 39 meV is observed only in the magnetically ordered phases, indicating the influence of magnetic order in chromium on lattice dynamics. The partial PDOS calculated with the antiferromagnetic order on Cr atoms show a very good agreement with the experimental data. It is revealed that the high-energy peak is lying above the phonon spectra of the pure bcc-Cr crystal. These are the local modes with the increased energies due to a strongly reduced distance between Sn and the nearest-neighbor Cr atoms.

Generation of thermal neutron scattering data for GH3535 alloy

Recent studies related to the thermal neutron scattering (TNS) data in reactors were generally focused on fuels and moderators, while the TNS data for structural materials has received little attention. The TNS effect of the GH3535 alloy, a nickel-based alloy widely used in molten salt reactor (MSR), is studied in this work. The partial phonon densities of states (PDOS) for the main metals (Ni, Mo, Cr and Fe) in the alloy, which are required for the TNS data generation, are calculated using the density functional perturbation theory (DFPT) based on a special quasi-random structure (SQS) of the alloy. The TNS cross sections for the main metals in the alloy are generated using the modified NJOY code. To preliminarily validate the TNS data, the calculated scattering cross section, combined with the evaluated absorption cross section of the alloy, is compared with the experimental total cross section, which shows a good agreement. Moreover, an integral neutronics experiment of neutron leakage spectrum is designed to further validate the TNS data. The neutron leakage spectrum is simulated by the Monte Carlo method, which can be compared with the future experimental result. Based on the experiment geometry modeling, the TNS effects of the GH3535 alloy are analyzed at multiple incident neutron energies, sample thicknesses and temperatures, respectively. It is found that the TNS effect of the GH3535 alloy on the neutron leakage spectrum cannot be ignored, and the scattering cross section of the alloy with high precision should be included in designs of MSR.

Uncovering the structural and photoelectric properties of perovskite crystals by joint multifaceted characterization and theoretical analysis

The potential of organic-inorganic hybrid perovskites, owing to their distinct photoelectric attributes and boosted photoluminescence stability, has garnered substantial attention. Despite comprehensive examinations of the synthesis and optical characteristics of FAPbBr3 crystals, the understanding of their microstructure's implication on photoelectric applications, their electronic configuration, and electron-phonon interplay remains vague. In this study, we detailed the electronic configuration, phonon dispersion, and vibrational properties using density functional theory. We further calculated the formation enthalpy stability diagram. Characterization and calculation of the crystalline structure were accomplished via Rietveld refinement. Phonon dispersion curves, as well as total and partial phonon densities of states, were also calculated. The theoretical results align well with experimental Fourier infrared and Raman spectra. Beyond elementary characterization, we explored the photoelectric qualities of FAPbBr3 crystals, thereby reaffirming the outstanding photoelectric attributes and extensive application potential. A non-contact fixed-point lighting device was designed based on the principle of electromagnetic induction coils, which proved the uniformity of crystal luminescence. According to the first principle, FAPbBr3 possesses a Poisson ratio of 0.32, indicative of high flexibility, emphasizing their superb photoelectric characteristics and vast application scope.

Effects of Si substitution on ferromagnetic order and off-center rattling of Eu ions in the type-I clathrate Eu8Ga16Ge30

In the ferromagnetic type-I clathrate Eu8Ga16Ge30, the divalent Eu ions are known to rattle among off-center positions in the tetrakaidecahedral cage. We have studied effects of Si substitution on the ferromagnetic state and the rattling motion of the Eu ions in Eu8Ga16Ge30 by measuring the magnetization M, electrical resistivity ρ, thermopower, specific heat C, and Eu partial phonon density of states (Eu pDOS) of Eu8Ga16−xGe30+x−y−zSiy□z (□ = vacancy, 0 ≤y≤ 7.3). Upon increasing y from 0 to 7.3, the lattice parameter decreases by 0.7%, which has no effect on the divalent state of Eu. The Curie temperature TC decreases from 36.2 K to 32.6 K, whereas the specific heat jump at TC, ΔC, increases twofold. For y= 0, both ρ (T) and dM(T)/dT exhibit anomalies below TC at T∗∼ 24 K, which are suppressed by applying magnetic field of 1 T. The anomalies at T∗ disappear by Si substitution at a low level, y = 3.7. The measured Eu pDOS indicates that the low-lying phonon energy of the Eu ions decreases with Si substitution. This softening means that the off-center rattling is changed to on-center one with large amplitude in an anharmonic potential. Based on these findings, we discuss relationship between the off-center Eu rattling and their collective magnetic behavior.

Grain-boundary thermodynamics with artificial-neural-network potential: Its ability to predict the atomic structures, energetics, and lattice vibrational properties for Al

An artificial neural network (ANN) potential for Al, trained with density-functional-theory (DFT) data, is constructed to accurately predict lattice vibrational properties and thermodynamics of grain boundaries (GBs) in Al. The ANN potential is demonstrated to accurately predict not only atomic structures and energetics of the GBs at 0 K but also partial phonon densities of states and vibrational entropies, even for GBs absent in the training data sets. In addition, their total potential energies and atomic forces by DFT at elevated temperatures up to 800 K can also be well reproduced by molecular dynamics with the ANN potential. In contrast, a modified embedded atom method (MEAM) potential shows larger errors in phonon frequencies and atomic forces for atoms at GBs, as well as in the bulk, than the ANN potential. The MEAM potential is thus likely to be inadequate to quantitatively predict thermodynamic properties of GBs, particularly at high temperature. The present ANN potential is also applied to systematically examine thermodynamic stability of asymmetric tilt GBs. It is predicted that for the \ensuremath{\Sigma}9 system, the GB free-energy profile as a function of inclination angle exhibits a cusp at elevated temperatures, due to its larger vibrational entropies of asymmetric tilt GBs than those of \ensuremath{\Sigma}9 symmetric tilt GBs.

Effects of fission product doping on the structure, electronic structure, mechanical and thermodynamic properties of uranium monocarbide: A first-principles study

A first-principle approach within the framework of density functional theory was employed to study the effect of vacancy defects and fission products (FPs) doping on the mechanical, electronic, and thermodynamic properties of uranium monocarbide (UC). Firstly, the calculated vacancy formation energies confirm that the C vacancy is more stable than the U vacancy. The solution energies indicate that FPs prefer to occupying in U site rather than in C site. Zr, Mo, Th, and Pu atoms tend to directly replace U atom and dissolve into the UC lattice. Besides, the results of the mechanical properties show that U vacancy reduces the compressive and deformation resistance of UC while C vacancy has little effect. The doping of all FPs except He has a repairing effect on the mechanical properties of U1-xC. In addition, significant modifications are observed in the phonon dispersion curves and partial phonon density of states (PhDOS) of UC1-x, ZrxU1-xC, MoxU1-xC, and RhxU1-xC, including narrow frequency gaps and overlapping phonon modes, which increase the phonon scattering and lead to deterioration of thermal expansion coefficient (αV) and heat capacity (Cp) of UC predicted by the quasi harmonic approximation (QHA) method.

Ab Initio Study of the Electronic Structure and Lattice Dynamics of Scheelite Type AgTcO4

To build a safe, sustainable, and reliable disposal system for radioactive waste from the nuclear industry, researchers need to understand the physical and chemical aspects of technetium (Tc)‐based oxides. Here, structure–property correlation studies of scheelite‐type are performed using ab initio calculations. The full phonon dispersion studies reveal the dynamical stability of this compound. The partial phonon density of states, including the complete mode assignment of the vibrational frequencies, shows the maximum involvement of silver (Ag) atoms in the low‐frequency range, while high contribution of oxygen atoms and average contribution from Tc atoms are found throughout the frequency region. The electronic band structure shows that the main contribution to the bottom of the conduction band is Tc d states with a minor contribution from oxygen p states, while the top of the valence band is from Ag d states, which are responsible for the electronic transitions. The frequency‐dependent dielectric functions and computed optical properties show near‐isotropy behavior and, together with absorption peak in the low‐energy range, indicate that may be a good phosphor when doped with a suitable activator. The estimations are grounded by the available experimental data and also deliver direction for distinct and futuristic applications of .

Computational Analysis of Vibrational Spectra of Hydrogen Bonds in sII and sH Gas Hydrates.

The amount of energy in natural gas hydrates is thought to be equivalent to twice that of all other fossil fuels combined. However, economic and safe energy recovery has remained a challenge till now. To develop a novel method of breaking the hydrogen bonds (HBs) surrounding the trapped gas molecules, we investigated the vibrational spectra of the HBs of gas hydrates with structure types II and H. Two models of 576-atom propane-methane sII hydrate and 294-atom neohexane-methane sH hydrate were built. A first-principles density functional theory (DFT) method was employed using the CASTEP package. The simulated spectra were in good agreement with the experimental data. Compared with the partial phonon density of states of guest molecules, we confirmed that the experimental infrared absorption peak in the terahertz region mainly arose from HB vibrations. By removing the components of guest molecules, we found that the theory of two kinds of hydrogen bond vibrational modes applies. The use of a terahertz laser to enable resonance absorption of HBs (at about 6 THz, to be tested) may therefore lead to the rapid melting of clathrate ice and release of guest molecules.

Scanning the latent phases and superconductivity in the Nb-Pb system at high pressure.

So far, few literature studies have been reported on niobium-lead binary intermetallic compounds, which are expected to have very different properties compared to existing niobium-carbon binary compounds, due to the distinct electronic properties of lead when compared to other carbon-group elements. Herein, we carry out a global structure search for the Nb-Pb system based on the evolutionary algorithm and density functional theory. Based on the dynamical and mechanical stability analyses, we unveiled five new phases, P4/m-Nb9Pb, Cmcm-Nb3Pb, I4/mmm-Nb2Pb, Pmm2-Nb5Pb3, and I4/mmm-NbPb2, that are promising candidates for experimental synthesis. Moreover, the superconducting transitions of all Nb-Pb binary intermetallic compounds are performed with electron-phonon calculations. As Nb9Pb exhibited the maximum Tc in the Nb-Pb intermetallics, greater than 3.0 K at 20 GPa, the phonon band structures, partial phonon density of states (PHDOS), the corresponding Eliashberg spectral functions α2F(ω), and integral electron-phonon coupling (EPC) parameters λ as a function of frequency of Nb9Pb were also studied. This work filled the gap in the pressure-tuned Nb-Pb phase transitions from a systematic first principles study for the first time.

Raman Spectroscopy of Janus MoSSe Monolayer Polymorph Modifications Using Density Functional Theory.

Two-dimensional transition metal dichalcogenides (TMDs) with Janus structures are attracting increasing attention due to their emerging superior properties in breaking vertical mirror symmetry when compared to conventional TMDs, which can be beneficial in fields such as piezoelectricity and photocatalysis. The structural investigations of such materials, along with other 2D materials, can be successfully carried out using the Raman spectroscopy method. One of the key elements in such research is the theoretical spectrum, which may assist in the interpretation of experimental data. In this work, the simulated Raman spectrum of 1H-MoSSe and the predicted Raman spectra for 1T, 1T’, and 1H’ polymorph modifications of MoSSe monolayers were characterized in detail with DFT calculations. The interpretation of spectral profiles was made based on the analysis of the lattice dynamics and partial phonon density of states. The presented theoretical data open the possibility of an accurate study of MoSSe polymorphs, including the control of the synthesized material quality and the characterization of samples containing a mixture of polymorphs.

Localized vibrational modes of GeV-centers in diamond: Photoluminescence and first-principles phonon study

The vibrational behaviour of the germanium-vacancy (GeV) in diamond is studied through its photoluminescence spectrum and first-principles modelled partial phonon density of states. The former is measured in a region below 600 cm−1. The latter is calculated for the GeV center in its neutral, charged, and excited state. The photoluminescence spectrum presents a previously unobserved feature at 248 cm−1 in addition to the well-known peak at 365 cm−1. In our calculations, two localized modes, associated with the GeV center and six nearest carbon atoms (GeC6 cluster) are identified. These correspond to one vibration of the Ge ion along with the [111] orientation of the crystal and one perpendicular to this direction. We propose these modes to be assigned to the two features observed in the photoluminescence spectrum. The dependence of the energies of the localized modes on the GeV-center and their manifestation in experimental optical spectra is discussed.

Optical Properties and Lattice Dynamics of Pure and S‐Alloyed Cu–Zn–Sn–Te Semiconductors: First‐Principles Calculations and Raman Scattering

The electronic band structure, the optical properties, and the lattice dynamics of the quaternary Cu2ZnSnTe4 (CZTTe) semiconductor with kesterite crystallographic structure are calculated within first principles. The analysis of the phonon partial density of states shows that the highest frequency phonon mode involves mostly heavy Sn and Te atoms due to their exceptionally strong bonding. Experimental Raman spectra of Cu–Zn–Sn–Te nanocrystals, prepared by an aqueous colloidal synthesis, reveal a single characteristic line and its higher order overtones due to specific resonant conditions. In the case of Cu–Zn–Sn–(Te–S) alloy nanoparticles, other Raman features, related to Sn–S vibrations, appear in the spectra indicating a two‐mode type of transformation of their vibrational spectra. The effect of sulfur incorporation on the Te‐related frequency is opposite to the effect observed for the Se‐related frequency in CZTSSe alloys.

First-principles DFT investigations of the vibrational spectra of chloro-quine and hydroxychloroquine

Since the COVID-19 pandemic began, two drugs, chloroquine (CQ) and hydroxychloroquine (HCQ), have received renewed attention. Using the density functional theory method in the CASTEP and DMol3 packages, we calculated both molecules’ infrared spectra and the partial phonon density of states of the hydroxyl group to identify the origin of the differences between the two spectra. Some characteristic vibrational modes of the hydroxyl group in HCQ were analysed individually. We also compared their Fukui functions and found that the oxygen atom in HCQ possesses electrophilic properties. This finding may be related to the large difference in toxicity between these two drugs. The method herein presents a new pathway to investigate organic molecules from the view of physics.

Vibrational anisotropy of <i>δ</i>-(Al,Fe)OOH single crystals as probed by nuclear resonant inelastic X-ray scattering

Abstract. The formation of high-pressure oxyhydroxide phases spanned by the components AlOOH–FeOOH–MgSiO2(OH)2 in experiments suggests their capability to retain hydrogen in Earth's lower mantle. Understanding the vibrational properties of high-pressure phases provides the basis for assessing their thermal properties, which are required to compute phase diagrams and physical properties. Vibrational properties can be highly anisotropic, in particular for materials with crystal structures of low symmetry that contain directed structural groups or components. We used nuclear resonant inelastic X-ray scattering (NRIXS) to probe lattice vibrations that involve motions of 57Fe atoms in δ-(Al0.87Fe0.13)OOH single crystals. From the recorded single-crystal NRIXS spectra, we calculated projections of the partial phonon density of states along different crystallographic directions. To describe the anisotropy of central vibrational properties, we define and derive tensors for the partial phonon density of states, the Lamb–Mössbauer factor, the mean kinetic energy per vibrational mode, and the mean force constant of 57Fe atoms. We further show how the anisotropy of the Lamb–Mössbauer factor can be translated into anisotropic displacement parameters for 57Fe atoms and relate our findings on vibrational anisotropy to the crystal structure of δ-(Al,Fe)OOH. As a potential application of single-crystal NRIXS at high pressures, we discuss the evaluation of anisotropic thermal stresses in the context of elastic geobarometry for mineral inclusions. Our results on single crystals of δ-(Al,Fe)OOH demonstrate the sensitivity of NRIXS to vibrational anisotropy and provide an in-depth description of the vibrational behavior of Fe3+ cations in a crystal structure that may motivate future applications of NRIXS to study anisotropic vibrational properties of minerals.

Ab initio studies for characterization and identification of nanocrystalline copper pyrophosphate confined in mesoporous silica

Here we employ a novel method for preparing the homogeneous copper pyrophosphate nanocrystals inside silica mesopores. In order to characterize and identify synthesized nanocrystals we performed the ab initio studies of the α phase of Cu2P2O7. The electronic and crystal structure were optimized within the density functional theory with the strong electron interactions in the states on copper atoms and van der Waals corrections included in calculations. The relaxed lattice parameters and atomic positions agree very well with the results of the diffraction measurements for nanocrystalline copper pyrophosphates embedded inside SBA-15 silica pores. The obtained Mott insulating state with the energy gap of 3.17 eV exhibits the antiferromagnetic order with magnetic moments on copper atoms (0.8 ) that is compatible with the experimental studies. The phonon dispersion relations were obtained to study the dynamical properties of the Cu2P2O7 lattice and the element-specific atomic vibrations were analyzed using the partial phonon density of states. The calculated Raman spectrum revealed the consistency of typical bands of Cu2P2O7 with the experimental data. The investigation that combines a new synthesis of nanomaterials with the first-principles calculations is important for better characterization and understanding of the physical properties relevant for nanotechnological applications.

High pressure lattice dynamics and stress-strain responses of LaIn3 and LaTl3

A detailed study of the ground state structure, non-zero elastic constants, high pressure lattice dynamics as well as theoretical tensile and shear strengths of the cubic intermetallic compounds LaIn3 and LaTl3 with AuCu3 (L12)-type has been made through first-principles. The optimized equilibrium crystal constants, cell volumes, bulk modulus and its pressure derivative, and single-crystalline elastic constants are in fair agreement with available experimental data and other calculations. More importantly, high pressure phonon spectrum, total and partial phonon densities of states in LaIn3 and LaTl3 up to 10 GPa are reported by employing Density Functional Perturbation Theory (DFPT), and the present results have demonstrated that both systems are dynamically stability up to 10 GPa. The differences between the dynamical properties of LaIn3 and LaTl3 have also been discussed systematically. Additionally, the tensile and shear stress-strain responses and the theoretical strengths of LaIn3 and LaTl3 are further calculated, we have found that LaIn3 exhibits larger tensile and shear strengths than LaTl3. These investigations are expected to provide beneficial references for further exploring the physical properties of rare earth compounds.

Application of Nuclear Inelastic Scattering Spectroscopy to the Frequency Scale Calibration of Ab Initio Calculated Phonon Density of States of Quasi-One-Dimensional Ternary Iron Chalcogenide RbFeSe2

This study aims to examine the applicability of nuclear inelastic scattering (NIS) and conventional Mössbauer spectroscopy for calibration of the frequency scale of ab initio calculated phonon density of states (PDOS) of iron ternary chalcogenides. NIS measurements are carried out on the quasi-one-dimensional ternary chalcogenide RbFeSe2 to obtain the partial PDOS of the iron atoms in the compound. We compare the experimental PDOS with our previous results on vibrational properties of RbFeSe2 obtained with density functional theory (DFT) ab initio calculations, conventional Mössbauer, and infra-red spectroscopies. The experimental PDOS measured by NIS is collated with the ab initio calculated one. The frequency correction factor for the ab initio results is determined as 1.077, in good agreement with value of 1.08 obtained previously from the temperature dependence of the Lamb–Mössbauer factor of the iron atoms in RbFeSe2. We conclude that nuclear inelastic scattering and temperature dependence of the Lamb–Mössbauer factor in conventional Mössbauer spectroscopy can be equally applied for evaluation of the frequency correction factor for ab initio calculated phonon density of iron of ternary chalcogenides.

Computational study of lattice dynamics and thermodynamic properties of energetic solid cyanuric triazide

The structural, vibrational and thermodynamic properties of the cyanuric triazide crystal is investigated through density functional theory (DFT) simulations within the dispersion corrected generalized gradient approximation (GGA + G06) by considering the norm conserving pseudopotentials. Infrared spectra and Born effective charges are computed through linear response method using the density functional perturbation theory (DFPT). The experimentally observed vibrational modes are well reproduced in our calculations. The high intensity infrared absorption bands in the range of 1200–1500 cm−1 originate from the vibrations of the N3 group. The Born effective charges indicate that the compound to be asymmetric. The calculated phonon dispersion curve clearly show the absence of imaginary frequencies along the high symmetry directions, which confirms the dynamical stability of C3N12, and the interaction between the acoustic and low lying optical modes results in enhanced phonon scattering enabling low thermal conductivity in this compound. The phonon partial density of states reveals that the low-frequency regions (below 500 cm−1) are dominated by N atoms. In addition, the compound shows covalent nature owing to the hybridization between C and N atoms. The phonon spectra and thermodynamic properties of C3N12 were computed for the first time, which still awaits experimental confirmation. The temperature-dependent heat capacity and Debye temperature has been discussed in detail. The heat capacity CV of C3N12 increases with temperature, being 98.13 cal/cell.K for T = 306 K. Finally, the entropy, enthalpy and Helmholtz free energy, are investigated and discussed in the framework of the harmonic approximation. From the above results, it can be inferred that cyanuric triazide is an effective initiating explosive, as it possess low-thermal conductivity.

Electronic properties, anisotropic elasticity, theoretical strengths, and lattice vibrational mode of orthorhombic YSi

We investigated the crystal structure, electronic structure, elastic stiffness constants, responses to tensile and shear deformations, lattice vibrational modes as well as vibration heat capacity and vibration entropy of the orthorhombic YSi compound using ab initio approach. The relaxed ground-state lattice constants a, b and c of YSi exhibit fair agreement with the available experimental and theoretical data. Nine independent elastic stiffness constants have been computed by increasingly varying minute strains based on “energy-strain” approach, which confirm that the orthorhombic YSi is mechanically stable. Some polycrystalline moduli involving bulk modulus, Young's modulus and shear modulus, Poisson's ratio, brittle/ductile characteristic have also been systematically derived, and YSi can be classified as brittle compound. Both three-directional Young's modulus anisotropy and bulk modulus anisotropy are calculated in detail, and the calculations indicate that the orthorhombic YSi takes on significant elastic anisotropy. We further studied the theoretical tensile and shear strengths under different loads. Also, the phonon spectra, total and partial phonon densities of states are researched by employing Density Functional Perturbation Theory (DFPT) and the finite-displacement method, and the infrared-active and Raman active vibration modes at the center of Brillouin zone are classified using the group theory. Moreover, we predicted the constant volume heat capacity of YSi under high temperature relying on the calculated phonon densities of states as well as quasi-harmonic approximation (QHA) and Debye model.