Phonon Calculations Research Articles

Specific heat analyses on optical-phonon-derived uniaxial negative thermal expansion system TrZr2 (tr = fe and Co1− xNix)

A large uniaxial negative thermal expansion (NTE) along the c-axis has recently been observed in the transition metal (Tr) zirconides TrZr2 with a tetragonal CuAl2-type structure. A recent study on FeZr₂ [M. Xu et al., Nat. Commun. 14, 4439 (2023)] suggests that optical phonons play a critical role in inducing the NTE along the c-axis. In this study, we investigate the thermophysical properties of TrZr₂ compounds (Tr = Fe and Co1− xNix(x = 0, 0.2, 0.4, 0.6, 0.8, 1)) using specific heat measurements, sound velocity data, and theoretical phonon calculations to achieve our aim of clarifying the contribution of optical phonons to the uniaxial NTE along the c-axis observed in both FeZr₂ and CoZr₂. We found that FeZr2 shows a lattice-specific heat peak structure at 8.90 meV, which corresponds to optical phonon energy with a high population of negative Grüneisen parameter along the c-axis in the phonon dispersion curves in FeZr2. In an examination of a chemical substitution effect on the Co1− xNixZr2, we found that the lattice-specific heat peak structure disappeared for x ≥ 0.4 and the oscillator intensity decreased. Phonon calculations revealed the existence of low-energy optical phonon branches at the Γ point for CoZr2 and FeZr2 with uniaxial NTE along the c-axis. However, the low-energy phonon branches were not found in NiZr2 with uniaxial positive thermal expansion along the c-axis. The increase in phonon density of states near the above optical phonon energy in CoZr2 and FeZr2 is consistent with the lattice-specific heat analyses, and we propose that low-energy optical phonons are essential for the exhibiting of uniaxial NTE along the c-axis in TrZr2.

Impact of Electric Field and Biaxial Strain on the Structural, Phonon, and Optical Features of Bulk and Monolayer Potassium Nitride

First-principles calculations are used within the framework of density functional theory to investigate the electronic, structural, magnetic and optical properties of Potassium Nitride (KN) in the bulk and monolayer states. This compound is dynamically stable according to phonon calculations. The results show that the energy gap decreases from the bulk to the monolayer. The equilibrium lattice constant increases when changing from bulk to monolayer, and the half-metallic (HM) character remains preserved in that case. According to the Slater–Pauling statute (Zt-4), the total magnetic moment equals 2 µB per unit cell. The electric field and biaxial strain affect the monolayer's electronic and magnetic characteristics were investigated. The magnitude of the spin-up channel concerning the energy gap changes under the biaxial strain. In particular, it decreases under tensile strain and increases under compression strain. Given that the values of magnetic moments remain unchanged, the HM property can be preserved for significant strains. When the electric field reaches -0.6 V/nm, the half-metallic property of this compound will be destroyed. It affects the energy gap and eliminates the HM trait since the magnetic moment of the K grew significantly greater than the moment of the N, and the N played a significant role in the realization of the half-metallic characteristic.

Training machine learning interatomic potentials for accurate phonon properties

Abstract One of the major challenges in the development of universal machine learning interatomic potentials is accurately reproducing phonon properties. This issue appears to arise from the limitations of available datasets rather than the models themselves. To address this, we develop an extensive dataset of phonon calculations using density-functional perturbation theory (DFPT). We then show how this dataset can be used to train neural-network force fields, by implementing the training and the prediction of force constants in periodic crystals. This approach improves the quality of phonon properties prediction while reducing the number of structures needed for neural network training. We demonstrate the efficiency of this method using two examples of ternary phase diagrams: Ti–Nb–Ta and Li–B–C. In both cases, neural network predictions for the energy and forces show a considerable improvement, while phonon properties are predicted with high precision for all structures across the entire phase diagrams.

Machine learning assisted search for Fe–Co–C ternary compounds with high magnetic anisotropy

We employ a machine learning (ML)-guided framework to explore rare earth free magnetic materials, specifically focusing on Fe–Co–C ternary compounds for potential use in permanent magnets. Utilizing a specifically trained crystal graph convolutional neural network model, we efficiently screen a vast space of nearly a million substitutional structures to select 620 promising structures for further investigation by first-principles calculation. We predict five low-energy metastable Fe–Co–C compounds with formation energy less than 150 meV/atom above the convex hull. These compounds exhibit high magnetization (Js > 1.0 T) and significant magnetic anisotropy (K1 > 1.0 MJ/m3), making them promising candidates for permanent magnet applications. The phonon calculations indicate these compounds are dynamically stable. Our ML-guided framework demonstrates the utility of rapidly identifying novel materials with tailored magnetic properties.

DFT based investigations on inherent CO2 adsorption and direct dissociation capability of Ti2C and Nb2C mxenes

Transition metal carbide MXenes, specifically Ti2C and Nb2C, have been investigated for their ability to efficiently adsorb and dissociate CO2 using First Principles Calculations based on Density Functional Theory (FPS-DFT). Pure Ti2C and Nb2C sheets were studied with the adsorption of 1 to 4 CO2 molecules, and the results were analyzed for each case. For 4 CO2 molecules adsorbed on Ti2C and Nb2C sheets, the weight percentage (wt%) ratio was found to be 21.39% and 14.33%, respectively. The Density of States (DOS) analysis revealed that, upon CO2 adsorption, the CO2 molecule dissociates into CO and O atoms on the MXene surface. The CO molecule showed no significant hybridization with the host sheet atoms, whereas the O atom exhibited strong hybridization with the MXene surface. Transition State (TS) profiles illustrated the dissociation steps on both Ti2C and Nb2C surfaces. Phonon calculations confirmed the dynamic stability of both pure and CO2 adsorbed MXenes. Molecular Dynamics (MD) simulations conducted at 900 K indicated uniform temperature and Mean Square displacement (MSD) profiles, suggesting the stability of both pure and CO2 adsorbed MXenes at elevated temperatures, as well as uniform CO2 adsorption behavior. The adsorption energies for Ti2C and Nb2C sheets were calculated to be in the range of -1.89 to -2.77eV, suggesting that the adsorption process is favorable, spontaneous, and predominantly involves chemisorption. These findings indicate that Ti2C and Nb2C MXenes, in their intrinsic forms, are promising candidates for CO2 adsorption and dissociation technologies.

A new activity model for biotite and its application

A new activity model for biotite is formulated in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (KFMASHTO), which extends that for the KFMASH system by introducing a titanium-biotite and a ferric-biotite end-member (tbio: K(TiMg2)[(O)2(AlSi3)O10] and fbio: K(Fe3+Mg2)[(OH)2(Al2Si2)O10]), as well as a pyrophyllite end-member (pyp: Al2[(OH)2Si4O10]) that accounts for the presence of octahedral excess-Al in natural biotites. Phonon calculations applying density functional theory (DFT) using the software Castep yielded the standard entropies of tbio and fbio as Sotbio = 328.06 J/(mol·K) and Sofbio = 301.69 J/(mol·K), and their heat capacity functions. From experimental phase-equilibrium data, the enthalpy of formation value of tbio was constrained as ΔHf,tbioo\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {H}_{f,tbio}^{o}$$\\end{document} = −6124.68 ± 3.33 kJ/mol. Natural data were used to derive ΔHf,fbioo\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {H}_{f,fbio}^{o}$$\\end{document}= −5935.3 ± 6.6 kJ/mol. The single-defect DFT method was applied to parameterize important macroscopic mixing properties (macro-W’s) involving tbio and pyp end-members in the model (fbio was treated ideal). Castep-derived microscopic interaction energies (micro-w’s) are presented herein for KFMASH-biotite. The octahedral same-site (M1) Mg–Al mixing micro-w (wMgAl(M1)), the same-site tetrahedral Si-Al mixing parameter (wSiAl(T1)) and the related cross-site term are: wMgAl(M1) = 82.5 kJ/mol, wSiAl(T1) = 95.6 kJ/mol (two T1-sites) and wMgAlAlSi(M1T1)=\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${w}_{MgAlAlSi(M1T1)}=$$\\end{document} 175.1 kJ/mol. The linear combination of these micro-w’s gives a macroscopic Wphleas = 18.8 kJ/mol, that is not transferable to other mineral groups. Micro w’s for Mg-Fe mixing in biotite (wMgFe(M1), wMgFe(M2),wMgMgFeFe(M1M2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${w}_{MgMgFeFe(M1M2)}$$\\end{document}), are all close to ideality. The biotite activity model of this study is thus a first example of next-generation activity models that use DFT- and thus physically based micro-w’s and reassembled macro-W’s for petrological calculations. Test calculations on 5 samples from low- to high-grade metamorphic environments covering metapelite to greywacke bulk-compositions using Perple_X suite of programs illustrate the performance of the new biotite activity model. Computed mineral-chemistries are in all cases in better agreement with measured compositions than resulting from published activity models of biotite.

Ultralow Lattice Thermal Conductivity of Zintl‐Phase CaAgSb Induced by Interface and Superlattice Scattering

Zintl phases attract extensive attention due to the characteristic of “electron‐crystal, phonon glass”. In this work, an ultralow lattice thermal conductivity ≈0.59 W m−1 K−1 at 300 K and ≈0.3 W m−1 K−1 at 623 K is obtained in CaAgSb Zintl phase, which is much lower than that of other well‐known Zintl compounds. The origin of this ultralow lattice thermal conductivity is explored through first‐principles calculations and Cs‐corrected scanning transmission electron microscopy. Theoretical phonon calculations provide evidence for complex phonon characteristics such as avoided‐crossing effect and low‐frequency flat band that favor the low lattice thermal conductivity. Moreover, subsequent microstructure results reveal abundant structural defects created in the CaAgSb sample, including superlattice structure and interface structure, which further contribute to the ultralow lattice thermal conductivity.

The Effect of Silicon Substitution by Boron for the α-Nb5Si3: INSIGHTS into the Constitutive Properties of Nb5Si2B Through Theory and Experimental Approach

We investigated the structural, elastic, vibrational, and electronic properties of the Nb5Si2B compound combining density functional theory (DFT) calculations and experimental methods. We compared our results with the parent compound Nb5Si3 with two non-equivalent Si sites namely Si(4a) and Si(8h). The analysis of elastic constants and phonon spectra indicate that Nb5Si2B is respectively mechanically and dynamically stable. Based on the phonon calculation we evaluate the theoretical constant volume lattice specific heat (CV) for different site occupancies and compare it with experimental specific heat (Cp) measurements. We found an excellent agreement between theoretical and experimental results for Nb5Si2B with the B at the Si(8h) site, which agrees with the calculated formation energy. In addition, we also performed DFT calculations aimed at showing and comparing the total DOS near the Fermi level (EF) for Nb5Si3 and Nb5Si2B. The XPS valence band (VB) of the Nb5Si2B is largely dominated by two characteristic peaks at − 8.7 and − 1.7 eV, respectively. Based on DFT calculations, it follows that the main sharp peak at − 1.7 eV comes as a contribution from Nb 4d states, while the smaller and broader one located at − 8.7 eV results mainly from Si 3s states weakly hybridized with Nb 4d states. In this connection, the majority contribution in the binding energy range from − 12 eV to the EF comes from Nb 4d states, while the contribution from Si and B atoms is very small in this region. The core levels of Nb 3d, Si 2s, 2p, and B 1s were also identified using the XPS technique.

Extensive screening of novel BaXH3 (X = V, Cr, Co, Ni, Cu, and Zn) perovskites for physical properties and hydrogen storage application: A DFT study

A successful transition to a sustainable economy depends on effective hydrogen storage. To achieve this, we investigate novel inorganic transition metal-based BaXH3 (X = V, Cr, Co, Ni, Cu, Zn) perovskites using Density Functional Theory (DFT). We compute key properties such as structural, electrical, magnetic, optical, mechanical, and hydrogen storage using the GGA-PBE functional. We calculate lattice constants and unit cell volume, indicate thermochemical stability with the negative formation energy of all compounds, and confirm thermodynamic stability with phonon calculations. Investigations into the band structure and density of states (DOS) show that all compounds are metallic, with BaVH3 and BaCrH3 exhibiting magnetic characteristics. We calculate the optical properties, including refractive index, dielectric function, reflectivity, loss function, conductivity, and absorption. We verified mechanical stability using the Born stability criteria. We calculated the hydrogen storage capacities and desorption temperatures. These results indicate that the studied compounds are potential candidates for hydrogen storage in a green economy.

Machine Learning for Predicting Ultralow Thermal Conductivity and High ZT in Complex Thermoelectric Materials.

Efficient and precise calculations of thermal transport properties and figures of merit, alongside a deep comprehension of thermal transport mechanisms, are essential for the practical utilization of advanced thermoelectric materials. In this study, we explore the microscopic processes governing thermal transport in the distinguished crystalline material Tl9SbTe6 by integrating a unified thermal transport theory with machine learning-assisted self-consistent phonon calculations. Leveraging machine learning potentials, we expedite the analysis of phonon energy shifts, higher-order scattering mechanisms, and thermal conductivity arising from various contributing factors, such as population and coherence channels. Our finding unveils an exceptionally low thermal conductivity of 0.31 W m-1 K-1 at room temperature, a result that closely correlates with experimental observations. Notably, we observe that the off-diagonal terms of heat flux operators play a significant role in shaping the overall lattice thermal conductivity of Tl9SbTe6, where the ultralow thermal conductivity resembles that of glass due to limited group velocities. Furthermore, we achieve a maximum ZT value of 3.17 in the c-axis orientation for p-type Tl9SbTe6 at 600 K and an optimal ZT value of 2.26 in the a-axis and b-axis direction for n-type Tl9SbTe6 at 500 K. The crystalline Tl9SbTe6 not only showcases remarkable thermal insulation but also demonstrates impressive electrical properties owing to the dual-degeneracy phenomenon within its valence band. These results not only elucidate the underlying reasons for the exceptional thermoelectric performance of Tl9SbTe6 but also suggest potential avenues for further experimental exploration.

Ab Initio Study of the Crystalline Structure of HgS under Low and High Pressure

This study analyzes the lattice dynamics of HgS under various pressures using ab initio self-consistent calculations based on the plane-wave method (PW) and generalized gradient approximation (GGA). The static study, performed by enthalpy calculations, predicts that the transition from the cinnabar phase (α-HgS) to the zinc-blende B3 (β-HgS) or wurtzite (2H) structures occurs at very low pressures, at 0.65 or 0.70 GPa, respectively. Furthermore, the transition from β-HgS to the rocksalt (B1) phase occurs at 7 GPa, and at high pressure, specifically at 110 GPa, HgS can adopt the CsCl (B2) phase. The mechanical study confirms the stability of the β and 2H phases at 0 GPa. Phonon calculations corroborate the results of the static and mechanical studies regarding stability (α→0.7GPa2H→0.9GPaβ), and the results indicate that the instabilities of the transverse acoustic (TA) modes, induced by the application of pressures of 10.5 GPa, 21 GPa, and 190 GPa, are responsible for the observed phase transitions in part of the Brillouin.

Computational investigation on physical properties of lead based perovskite RPbBr3 (R = Cs, Hg, and Ga) materials for photovoltaic applications

In the modern era, the major problem is solving energy production and consumption. For this purpose, perovskite materials meet these issues and fulfill energy production at a low cost. Density functional theory and the Cambridge Serial Total Energy Package (CASTEP) are used to examine the characteristics of the cubic inorganic perovskites RPbBr3 (R = Cs, Hg, and Ga). In the context of the generalized gradient approximation (GGA), the ultrasoft pseudo-potential plane wave technique and the Perdew–Burke–Ernzerhof exchange–correlation functional are used for investigations. Structural, mechanical, electronics, and optical properties are investigated using CASTEP code. According to structural properties, compounds have a cubic nature with space 221 (Pm3m). Compounds formation energy (− 3.46, − 2.21, and − 3.14 eV)of (CsPbBr3, HgPbBr3, and GaPbBr3) and phonon calculations are studied and find that compounds are stable. The results of our investigation show that the compounds have narrow bandgaps of direct kind, with 1.85 eV for CsPbBr3, 1.56 eV for HgPbBr3, and 1.71 eV for GaPbBr3, respectively, indicating that they may be used to improve conductivity. Additionally, anisotropy (2.135, 3.651, 10.602), Pugh’s ratio (1.87, 2.25, 2.14), and Poison’s ratio (0.27, 0.31, 0.29) are traits that the compounds (CsPbBr3, HgPbBr3, GaPbBr3) display a ductile nature. The CsPbBr3 compound showed significant optical conductivity and absorption in terms of their optical properties, especially in the visible region, which makes them suitable for use in solar cell applications as well as for LED applications.

Charge density wave transition and unusual resistance hysteresis in vanadium disulfide (1T-VS2) microflakes

We report existence of charge density wave (CDW) transition with unusual resistivity hysteresis in stable micro-flakes of 2D transition metal dichalcogenide (TMDC) vanadium disulfide (1T-VS2) as ascertained by structural studies, Raman spectroscopy, heat capacity, resistivity measurements and supported by phonon calculations based on density functional theory. The CDW transition occurs at around 296 K on cooling and manifests itself by the onset of resistivity with a negative temperature coefficient, which we identify as the transition temperature (TCDW). The transition is identified by a distinct peak in the heat capacity at T ≈ T CDW along with anomalies in temperature variation of the lattice parameters and complimentary signatures in electron diffraction and temperature-dependent Raman spectroscopy. The temperature-dependent resistivity measurements done with different ramp rates of cooling and heating show strong hysteresis and a low temperature relaxation pointing to the existence of metastable states below the transition. The Raman spectroscopy data show a hysteresis loop below the onset of the CDW transition. The phonon band structure calculations carried out on the 1T-VS2 system show the existence of the phonon mode softening along the diagonal of the Brillouin zone, which is pronounced near the transition temperature implying the prominent role of the mode softening as a driver of the transition. The results imply that the formation of CDW is sensitive to phonon mode softening rather than energy gap opening at the Fermi level.

X[formula omitted]CoH[formula omitted] (X = Ca, Sr) for hydrogen storage: First-principles computations

The structural, electronic, elastic, optical, phonon, and thermo-physical properties of the tetragonal X2CoH5 (X = Ca, Sr) hydrogen storage compounds were thoroughly examined using first-principles calculations. The negative formation enthalpies (−76.03 kJ/mol.H2 for Ca2CoH5 and −72.55 kJ/mol.H2 for Sr2CoH5) highlight the structural stability of these materials. Phonon calculations revealed no imaginary frequency modes, confirming the dynamic stability of X2CoH5. The mechanical stability of Ca2CoH5 and Sr2CoH5 was evidenced by the compliance of the elastic constants with Born stability criteria. The electronic band structure indicates that Ca2CoH5 and Sr2CoH5 are direct bandgap semiconductors with narrow bandgaps of ∼0.27 and 0.34 eV, respectively. The appraisal of Poisson’s ratio (ν = 0.22 for X = Ca and 0.23 for Sr) and Pugh’s ratio (B/G = 1.47 for X = Ca and 1.56 for Sr) suggests that both metal hydrides exhibit brittle mechanical behavior. Moreover, 3D plots of Young’s modulus (E), shear modulus (G), linear compressibility (β), and Poisson’s ratio (ν) uncover anisotropy within X2CoH5. The hydrogen storage capabilities were scrutinized, and the present materials feature excellent volumetric hydrogen density (95.65 gH2l−1 for Ca2CoH5 and 80.24 gH2l−1 for Sr2CoH5), moderate gravimetric hydrogen density (3.38 wt% for Ca2CoH5 and 2.06 wt% for Sr2CoH5) and high hydrogen desorption temperature (584 K for Ca2CoH5 and 558 K for Sr2CoH5). Notably, the obtained volumetric hydrogen densities are in line with the volumetric capacity criteria set by the U.S. Department of Energy (DOE) for 2025. Finally, the optical and thermo-physical characteristics of X2CoH5 compounds were also investigated in this work.

Triaxial strain enhanced thermoelectric performance and conversion efficiency in Tl3TaSe4

Thermoelectric (TE) materials have garnered significant attention due to their potential in converting waste heat into useful electrical energy. Strain engineering has emerged as a promising strategy for tailoring the properties of materials, offering opportunities to optimize TE performance. In this study, we employ first-principles calculations to investigate the electronic and thermal transport properties of cubic Tl3TaSe4 under triaxial strain. Our results demonstrate that tensile strain can reduce the effective mass of carriers and contribute to high carrier mobility. The phonon calculation and AIMD simulation reveal excellent dynamic stability and thermal stability of Tl3TaSe4 both with and without strain. The red shift phenomenon of phonon vibration modes and the higher portion of phonon DOS in the low frequency region indicate that tensile strain enhances the anharmonicity of Tl3TaSe4. In addition, triaxial tensile strain can effectively reduce the heat capacity, group velocity, and phonon lifetime, while enhancing the anharmonicity of Tl3TaSe4, which contributes to the lower κl. Compared to the condition without strain, the room temperature κl of Tl3TaSe4 under 1 % and 2 % tensile strain is reduced by about 43 % and 79 %, respectively. Furthermore, the tensile strain exhibits a positive enhancement effect on the ZT value of Tl3TaSe4. At 700 K, the optimal ZT values of p-type and n-type Tl3TaSe4 under 2 % tensile strain are 1.31 and 0.94, respectively, which are three times higher than that of 0.4 and 0.33 without strain. Finally, the TE conversion efficiency of Tl3TaSe4 under 2 % tensile strain is 6.8 %, which is higher than BiCuSeO and α-Cu2+xSe, and comparable to SnSe. Our results demonstrate the effectiveness of triaxial tensile strain in enhancing the phonon scattering, reducing the κl, and improving the TE properties of Tl3TaSe4.

Is the Presence of Sn2+ a Crucial Factor for the Generation of Low Thermal Conductivity in Tin-Based Sulfides?

Comprehending the relationship between crystal structures and transport properties is crucial to develop materials with improved electrical and thermal properties for thermoelectric applications. In this article, we report on the complex crystal structure and physical properties of Cr2Sn3S7, a n-type magnetic semiconductor with a low energy band gap and low thermal conductivity. Importantly, we demonstrate that the high level of structural complexity is related to the coexistence of two sublattices: a host lattice, [Cr4Sn2S11]2-, characterized by a mixed Sn4+/Cr3+ occupancy of its cationic sites, and a guest lattice characterized by [Sn4S3]2+ chains, containing Sn2+ cations only, closely related to the ones encountered in the orthorhombic SnS compound. By combining experiments including X-ray diffraction with ab initio calculations of electrons and phonons, we succeeded to elucidate the origin of the low thermal conductivity in Cr2Sn3S7. We demonstrate that the low dimensionality of the [Sn4S3]2+ chains, which generates weak Sn···S interactions with the 3D host lattice, is induced by the lone pair stereochemical activity of Sn2+. This lattice softening favors strong anisotropic vibrations at low frequencies, highlighting the primordial role played by Sn2+ cations in both crystal structure dimensionality and low thermal conductivity in tin-based sulfides.

Substitution of europium (III) in hydroxyapatite lattice for luminescence applications: Integrating experimental and theoretical insights

Europium(III) (Eu3+/Eu(III)) substitution in the hydroxyapatite (HAp) lattice imparts luminescence properties, enabling its application in diagnostics and bioimaging. Despite numerous experimental studies, there has been a lack of comprehensive discussion on the substitution behavior and local structural stability of Eu3+ within HAp. In this study, experimental and first-principles investigations were integrated to explore and understand the substitution of Eu3+ in HAp. Optimization of various HAp models revealed the dependency of Eu3+ substitution site on the charge compensation mechanism. Eu3+ prefers the Ca(2) site when OH is converted to O and the Ca(1) site in the presence of a Ca vacancy. Phonon calculations showed that temperature influences charge compensation and site preference, favoring OH to O compensation and Eu3+ in Ca(2) at higher temperatures and Ca vacancy and Eu3+ in Ca(1) at lower temperatures. Electronic structure simulations explained that the luminescence in Eu(III)-substituted HAp primarily arises from charge transfer between Eu3+ and O atoms, supported by experimental photoluminescence measurements. Correspondence between simulations and experimental photoluminescence data emphasizes the reliability of the computational approach in this study. The integration of insights from experimental and simulation results enhances the understanding of substitution of Eu3+ in the HAp lattice, providing valuable guidance for designing and optimizing HAp-based luminescent and optical materials.

Raman spectrum and phonon thermal transport in van der Waals semiconductor GaPS4

The emergent van der Waals semiconductor GaPS4 is heralding frontiers for gallium-based semiconductors. Despite its potential, the intricacies of its Raman spectrum and phonon heat transport remain elusive. In this research, experimental and theoretical methods are employed to give a comprehensive portrayal. The Raman spectra and phonon calculations obtained were cross-validated, affirming the study's credibility. A total of 28 Raman peaks were identified, with all phonon irreducible representations delineated. Advanced calculations unveiled notable shifts in the transition of GaPS4 from bulk to monolayer. During this process, phonons undergo a red shift, and the vibration contributions of different atoms change. The lifetime and group velocity of low wavenumber phonons are markedly reduced, suppressing the thermal conductivity in the monolayer. The thermal conductivity of GaPS4 bulk at 300 K is 0.5 W/m K, and 0.13 W/m K for monolayer, while the thermal conductivity in the cleavage direction is lower. These findings offer a detailed account of the complex Raman spectra and phonon thermal transport properties of GaPS4, setting the stage for its subsequent exploration and prospective applications in electronic and thermal devices, and contributing to enriching condensed matter theory of phonon thermal transport in van der Waals materials.

Two novel stable phases of polonium oxides from first-principles evolutionary algorithm

Oxygen prominently displays a strong propensity to efficiently form stable molecules by establishing ionic/covalent bonds with a broad array of elements within the Mendeleev Table. That gives rise to a diverse spectrum of oxides that hold pivotal significance in contemporary optoelectronics, mineralogy, biological entities, and atmospheric constitution. We explore here the feasibility of polonium-oxygen compound formation, employing a first-principle evolutionary algorithm. The obtained structural predictions yield two distinct phases of PoO2, exhibiting respectively thermodynamic stability and metastability, specifically the cubic (Fm 3‾ m) and orthorhombic (Pmn21) crystal structures. Phonon calculations have unequivocally substantiated the dynamical stability inherent in these PoO2 structures. The Po–O system, characterized by its unusual physical-chemical attributes, presents noteworthy features in electronic behavior, bonding interactions, and dynamical properties. Both the Fm 3‾ m and the Pmn21 phases exhibit semiconductor behavior, each displaying a relatively substantial indirect bandgap of 2.11 eV and 2.50 eV, respectively, within the cubic and orthorhombic crystals. To unravel the bonding nature of PoO2, we employ a suite of analytical tools, including electronic density of states, Bader charge analysis, and electron localization function. These analyses collectively unveil a transfer of electrons from polonium to oxygen, thereby elucidating the coexistence of significant ionic and partial covalent Po–O bonds within the Po–O system.

Transition Metal (Cu, Pd, and Ag)-Modified Nb2S2C Monolayer for Highly Efficient Catechol Sensing: A First-Principles Investigation.

Motivated by recent advancements and the escalating application of two-dimensional (2D) gas or molecule sensors, this study explores the potential of the 2D Nb2S2C monolayer for detecting biomolecule catechol (Cc), whose excess concentration is highly dangerous to living beings. We use first-principles density functional theory (DFT) calculations to assess the Cc sensing performance of pure and transition metal (TM = Cu, Pd, Ag)-modified Nb2S2C monolayers. The Nb2S2C monolayer belonging to the new class of synthesized 2D materials, TM carbo-chalcogenides (TMCC), combines distinctive properties from both TM dichalcogenides and TM carbides and exhibits physisorption (-0.66 eV) toward the Cc molecule. Notably, the surface modifications with these TMs significantly enhanced the adsorption energy of Cc. The chemisorption of the Cc molecule on the Pd to Cu-modified monolayer is demonstrated with adsorption energies ranging from -1.09 to -1.3 eV and is due to the robust charge transfer and orbital interactions between the valence orbitals of TMs and Cc. In addition, the modification of the surface by TM leads to an increased work function sensitivity toward the Cc molecule. The study establishes the thermal stability at 300 K and dynamic stability of TM-Nb2S2C through ab initio molecular dynamics (AIMD) simulations and Phonon calculations, respectively. The theoretical estimation of achievable recovery time at 400 and 450 K for Pd and Ag and at 500 K for the Cu-modified Nb2S2C monolayer, respectively, confirms the potential practical application of the sensor for Cc detection. These compelling characteristics position the Nb2S2C monolayer as a promising nanomaterial for detecting Cc molecules in the environment.