Iron silicides are promising materials for optoelectronic and thermoelectric applications; however, their thermal transport properties and the role of anharmonic lattice effects remain incompletely understood. In this work, we present a comprehensive first-principles study of lattice dynamics and phonon-mediated heat transport in the semiconductor beta-FeSi2. The phonon dispersions are obtained using the temperature-dependent effective potential method and the self-consistent phonon theory, while phonon lifetimes and frequency shifts are evaluated through the perturbation theory. Based on the calculated phonon frequencies and anharmonic line profiles, we obtain the Raman spectrum and demonstrate that strong anharmonic renormalization of selected high-frequency optical modes is essential for reproducing experimentally observed linewidths and their temperature evolution. The lattice thermal conductivity is evaluated as a function of temperature and crystallite size within the relaxation-time approximation. We identify weak anisotropy in phonon heat transport arising from the orthorombic crystal structure and show that quartic anharmonic contributions play only a minor role in determining the thermal conductivity. The calculated thermal conductivity shows a good agreement with available experimental data for the finite-grain systems.

In this research, we examine the electronic, magnetic, and lattice properties of the Fe$_4$(P$_2$O$_7$)$_3$ compound using the first principles calculations based on the density functional theory. The crystal lattice has a monoclinic structure, belonging to the P$2_1/n$ space group. The optimized lattice parameters are a=7.406 \AA, b=21.425 \AA, c=9.529 \AA, and agree very well with the experimental data, thanks to the local Coulomb interactions and van der Waals forces included in the calculations. The investigation considers several magnetic orderings. The lowest total energy was found for the antiferromagnetic configuration with the magnetic moment of $\sim4.6~\mu_{\text{B}}$ per Fe atom. The electronic structure calculation shows the Mott insulating state with the energy gap $E_g=2.87$~eV. For the relaxed crystal structure, the elastic properties were obtained and analyzed. The phonon dispersion relations and density of states were calculated within the temperature-dependent effective potential methodusing atomic multidisplacements obtained by high efficiency configuration space sampling.

In this research, we examine the electronic, magnetic, and lattice properties of the Fe_4(P_2O_7)_3 compound using the first principles calculations based on the density functional theory. The crystal lattice has a monoclinic structure, belonging to the P2_1/n space group. The optimized lattice parameters are a=7.406 Å, b=21.425 Å, c=9.529 Å, and agree very well with the experimental data, thanks to the local Coulomb interactions and van der Waals forces included in the calculations. The investigation considers several magnetic orderings. The lowest total energy was found for the antiferromagnetic configuration with the magnetic moment of ∼4.6 μ_B per Fe atom. The electronic structure calculation shows the Mott insulating state with the energy gap E_g=2.87 eV. For the relaxed crystal structure, the elastic properties were obtained and analyzed. The phonon dispersion relations and density of states were calculated within the temperature-dependent effective potential methodusing atomic multidisplacements obtained by high efficiency configuration space sampling.

In this research, we examine the electronic, magnetic, and lattice properties of the Fe4(P2O7)3 compound using the first principles calculations based on the density functional theory. The crystal lattice has a monoclinic structure, belonging to the P21/n space group. The optimized lattice parameters are a = 7.406 Å, b = 21.425 Å, c = 9.529 Å, and agree very well with the experimental data, owing to the local Coulomb interactions and van der Waals forces included in the calculations. The investigation considers several magnetic orderings. The lowest total energy was found for the antiferromagnetic configuration with the magnetic moment of ∼4.6μB per Fe atom. The electronic structure calculation shows the Mott insulating state with the energy gap Eg=2.87eV. For the relaxed crystal structure, the elastic properties were obtained and analyzed. The phonon dispersion relations and density of states were calculated within the temperature-dependent effective potential method using atomic multidisplacements obtained by high efficiency configuration space sampling. We revealed the essential role of anharmonicity in the dynamic stability of the iron pyrophosphate crystal structure.

Cubic silicon carbide phonon thermal conductivity has been calculated using anharmonic phonon analysis. The atomic interaction model was built using displacement-force data obtained with the High Efficiency Configuration Space Sampling (HECSS) technique and density functional theory calculated forces. In the new version of HECSS we replaced the Markov chain scheme of Metropolis-Hastings Monte-Carlo with weighting of the final sampling according to the target distribution. This increased the efficiency of the method and allowed to use – with appropriate weight – all generated and ab-initio evaluated samples. The quality of the proposed method is confirmed by the accuracy with which the experimental results taken from the literature were reproduced.

We present the pressure dependence of the electronic and dynamical properties of six different CoGe phases with orthorhombic Cmmm, hexagonal P6/mmm and P (6) over bar 2m, monoclinic C2/m, cubic P2(1) 3, and orthorhombic Pnma symmetries. Using first-principles DFT calculations and the direct force-constants method, we study the dynamical stability of individual phases under external pressure. We show that the orthorhombic (Cmmm) and hexagonal (P6/mmm) structures are unstable over a broad pressure range and most pronounced imaginary phonon soft mode in both cases leads to a stable hexagonal (P (6) over bar 2m) structure of the lowest ground-state energy of all studied phases at ambient and low (below similar to 3 GPa) external pressure. Under these conditions, the cubic structure has the highest energy, however, together with monoclinic and orthorhombic phases, it is dynamically stable and all these three structures can potentially coexist as meta-stable phases. Above similar to 3 GPa, the cubic phase becomes the most energetically favorable. Fitting the Birch-Murnaghan equation of state, we derive bulk modulus for all mentioned phases. The results indicate relatively high resistance of CoGe to compression. Such conclusions are confirmed by band structure calculations. Additionally, we show that electronic bands of the hexagonal (P (6) over bar 2m) phase reveal characteristic features of the kagome-like structure, while in the cubic phase the electronic bands contain spin-1 and double Weyl fermions. In both cases, the external pressure induces the Lifshitz transition, related to the modification of the Fermi surface topology.

Electron-correlation-driven phonon soft modes have been recently reported in the antiferromagnetic kagome FeGe compound and associated with the observed charge density wave (CDW). In this paper, we present a systematic investigation of the CDW origin in the context of the ab initio lattice dynamics study. Performing the group theory analysis of the aforementioned soft modes, we found that the stable structure has the Immm symmetry and can be achieved by shifts of Ge atoms. The occurrence of two soft modes induces the first-order phase transition, which leads to the CDW order. Additionally, we show that the final structure realizes a distorted honeycomb Ge lattice, as well as a nonflat kagome-like Fe net. For completeness, we present the electronic properties calculations. From the theoretical STM topography simulation, we indicate that the observed CDW occurs in the deformed honeycomb Ge sublattice.

RhPb was initially recognized as one of CoSn-like compounds with P6/mmm symmetry, containing an ideal kagome lattice of d-block atoms. However, theoretical calculations predict the realization of the phonon soft mode, which leads to the kagome lattice distortion and stabilization of the structure with P6 over bar 2m symmetry [A. Ptok et al., Phys. Rev. B 104, 054305 (2021)]. Here, we present the single crystal x-ray diffraction results supporting this prediction. Furthermore, we discuss the main dynamical properties of RhPb with P6 over bar 2m symmetry, i.e. phonon dispersions and surface Green's functions using the modern theoretical methods based on density functional theory. The bulk phononic dispersion curves contain several flattened bands, Dirac nodal lines, and triple degenerate Dirac points. As a consequence, the phononic drumhead surface state is realized for the (100) surface, terminated by the zigzaglike edge of Pb honeycomb sublattice.

Thermodynamical stability of different variants of FePt nanoparticles has been studied using DFT molecular dynamics. The melting temperature and general stability at elevated temperatures have been estimated from both energy difference and atomic root-mean-square displacement functions. The investigated systems include multi-shell nanoparticles of iron and platinum with icosahedral symmetry and a magic number of atoms (55): iron-terminated Fe43Pt12 and platinum-terminated Fe12Pt43. Additionally, the cuboctahedral Fe24Pt31 particle, cleaved-out of the bulk structure, has been studied using the same procedure. Molecular dynamics simulations have been performed for a range of temperatures reaching above the melting points. The calculations confirmed high structural instability of the Fe-terminated nanoparticles and a strong stabilizing effect of the Pt-termination in the shell-type icosahedral particles. The advantage of the presented study is the self-consistency of the nanoparticle band structure in each time step, including magnetic interactions among local magnetic moments.

Ab initio investigations of the structural, electronic, and dynamical properties of the high-temperature 0 phase of copper pyrophosphate were performed using density functional theory. The electronic band structure shows the Mott insulating state due to electron correlations in the copper ions. By calculating phonon dispersion relations, the soft mode at the A point of the Brillouin zone was revealed, showing the dynamical instability of the 0 phase at low temperatures. The double-well potential connected with the soft mode is derived and the mechanism of the structural phase transition to the & alpha; phase is discussed. The self-consistent phonon calculations based on the temperature-dependent effective potential show the stabilization of the 0 phase at high temperatures, due to the anharmonic effects. The pronounced temperature dependence and the large linewidth of the soft mode indicate an essential role of anharmonicity in the structural phase transition.

Ab initio calculations were used to determine physical properties of AlN(0001) surface under Al coverage. It was shown that Al atoms are adsorbed in T4 sites for very low Al coverage, up to θAl=1/4 monolayers (ML). For higher Al coverage, θAl=1/4ML up to θAl=1ML the adlayer becomes disordered and corrugated vertically. In this coverage range (0<θAl≤1ML) the Al bonding energy is independent on the coverage and equal to EadsDFT≅5.0eV. For higher coverage, 1ML≤θAl≤7/6ML, the Al adlayer becomes ordered and atomically flat. The Al adsorption energy in this range is EadsDFT≈6.0eV, i.e. much higher. For higher coverage, θAl>1.25ML, the Al adatoms are located in the second layer. The adsorption energy is reduced to EadsDFT=3.97eV at θAl=1.25ML and linearly increasing to EadsDFT=4.88eV for the coverage increase up to θAl=2ML. Full thermodynamic analysis identified two regions in equilibrium with Al vapor: first at very low pressures where θAl≤10-3ML and the second for higher pressures which is 1ML≤θAl≤7/6ML. The second region is relevant for physical vapor transport (PVT) AlN growth. The nitrogen adsorption process, important for PVT AlN growth, takes place at single or double Al layer covered AlN(0001) surface.

The lattice vibrational properties of alpha-Sn (gray tin) were investigated experimentally by temperature-dependent x-ray diffraction and theoretically by density functional theory calculations. Similar to the other elements of group IV, alpha-Sn exhibits a lattice anomaly at low temperatures and negative thermal expansion, with a minimum at similar to 27 K and a magnitude three times larger than in Si. The influence of anharmonic effects up to fourth-order potential terms on the phonon dispersion relations, the lattice parameters, and the thermal expansion coefficient have been tested. The performed analysis gives an excellent agreement with experiment when quartic potential terms are included in the theory. We point out that negative thermal expansion in alpha-Sn is not driven by the anharmonicity of the interatomic potential. This resolves the long-standing puzzle in the thermal behavior of alpha-Sn.

In a public space there are several reports of materials with general stoichiometry CaCoSinO2n+2. Pyroxene CaCoSi2O6 is probably the best-known representative for n = 2 but not much is known about materials with n = 3 and n = 4. In this study, attempts were carried out to synthesize those phantom materials and it was found that they do not exist as a single phase. A quantitative XRD analysis revealed that their chemical composition is correct but the formula should be written as CaCoSi2O6 + (n-2)SiO2. Similar qualitative conclusions were drawn from investigation of magnetic (DC magnetometry) and electronic properties using X-ray Photoelectron Spectroscopy (XPS) and Si K edge X-ray Absorption Spectroscopy (XAS). Additionally, the DFT ab initio calculations were carried out to obtain electronic signature from band structure of CaCoSi2O6. The apparent influence of the excess of SiO2 on magnetic properties of this "series" can be understood in terms of presence and suppression of secondary phases like Ca2CoSi2O7, which form when the starting materials are not homogenized properly. Addition of surplus SiO2 suppresses their formation leaving clear signature from CaCoSi2O6, which also can be synthesized from stoichiometric mixture using proper techniques.

The spatial confinement of atoms at surfaces and interfaces significantly alters the lattice dynamics of thin films, heterostructures and multilayers. Ultrathin films with high dielectric constants (high-k) are of paramount interest for applications as gate layers in current and future integrated circuits. Here we report a lattice dynamics study of high-k Eu2O3 films with thicknesses of 21.3, 2.2, 1.3, and 0.8 nm deposited on YSZ(001). The Eu-partial phonon density of states (PDOS), obtained from nuclear inelastic scattering, exhibits broadening of the phonon peaks accompanied by up to a four-fold enhancement of the number of low-energy states compared to the ab initio calculated PDOS of a perfect Eu2O3 crystal. Our analysis demonstrates that while the former effect reflects the reduced phonon lifetimes observed in thin films due to scattering from lattice defects, the latter phenomenon arises from an ultrathin EuO layer formed between the thin Eu2O3 film and the YSZ(001) substrate. Thus, our work uncovers another potential source of vibrational anomalies in thin films and multilayers, which has to be cautiously considered.

Systems containing the ideal kagome lattice can exhibit several distinct and novel exotic states of matter. One example of such systems is a recently discovered AV(3)Sb(5) (A = K, Rb, and Cs) family of compounds. Here the coexistence of the charge density wave (CDW) and superconductivity is observed. In this paper, we study the dynamic properties of the AV(3)Sb(5) systems in context of origin of the CDW phase. We show and discuss the structural phase transition from P6/mmm to C2/m symmetry that is induced by the presence of phonon soft modes. We conclude that the CDW observed in this family of compounds is a consequence of the atom displacement, from the high-symmetry position of the kagome net, in the low-temperature phase. Additionally, using the numerical ab initio methods, we discuss the charge distribution on the AV(3)Sb(5) surface. We show that the observed experimental stripelike modulation of the surface can be related to surface reconstruction and manifestation of the three-dimensional 2x 2x 2 bulk CDW. Finally, the consequence of realization of the C2/m structure on the electronic properties is discussed. We show that the electronic band structure reconstruction and the accompanying modification of density of states correspond well to the experimental data.

Substantial acceleration of research and more efficient utilization of resources can be achieved in modelling investigated phenomena by identifying the limits of system's accessible states instead of tracing the trajectory of its evolution. The proposed strategy uses the Metropolis-Hastings Monte-Carlo sampling of the configuration space probability distribution coupled with physically-motivated prior probability distribution. We demonstrate this general idea by presenting a high performance method of generating configurations for lattice dynamics and other computational solid state physics calculations corresponding to non-zero temperatures. In contrast to the methods based on molecular dynamics, where only a small fraction of obtained data is used, the proposed scheme is distinguished by a considerably higher, reaching even 80%, acceptance ratio and much lower amount of computation required to obtain adequate sampling of the system in thermal equilibrium at non-zero temperature.

Andrzej Ptok, ∗ Aksel Kobia lka, Ma lgorzata Sternik, Jan Lażewski, Pawe l T. Jochym, Andrzej M. Oleś, 4 Svetoslav Stankov, 6 and Przemys law Piekarz † Institute of Nuclear Physics, Polish Academy of Sciences, W. E. Radzikowskiego 152, PL-31342 Kraków, Poland Institute of Physics, Maria Curie-Sk lodowska University, Plac Marii Sk lodowskiej-Curie 1, PL-20031 Lublin, Poland Institute of Theoretical Physics, Jagiellonian University, Profesora Stanis lawa Lojasiewicza 11, PL-30348 Kraków, Poland Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Laboratory for Applications of Synchrotron Radiation, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, D-76344 Eggenstein-Leopoldshafen, Germany (Dated: June 8, 2021)

Recently discovered heavy fermion CeRh$_{2}$As$_{2}$ compound crystallizes in the nonsymmorphic $P4/nmm$ symmetry, which enables the occurrence of topological protection. Experimental results show that this material exhibits unusual behavior, which is manifested by the appearance of two superconducting phases. In this work, we uncover and discuss a role of Rh$_{2}$As$_{2}$ layers and their impact on the electronic and dynamical properties of the system. The location of Ce atoms between two non-equivalent layers allows for the realization of hidden orbital order. We point out that the electronic band structure around the Fermi level is associated mostly with Ce $4f$ and Rh $4d$ orbitals and suggest the occurrence of the Lifshitz transition induced by the external magnetic field. We discuss also the role played by the $f$--$d$ orbital hybridization in the electronic band structure.

Using the density functional theory, we study the structural and lattice dynamical properties of europium sesquioxide (Eu2O3) in the cubic, trigonal, and monoclinic phases. The obtained lattice parameters and energies of the Raman modes show a good agreement with the available experimental data. The Eu-partial phonon density of states calculated for the cubic structure is compared with the nuclear inelastic scattering data obtained from a 20 nm thick Eu2O3 film deposited on a YSZ substrate. A small shift of the experimental spectrum to higher energies results from a compressive strain induced by the substrate. On the basis of lattice and phonon properties, we analyze the mechanisms of structural transitions between different phases of Eu2O3.

We present systematic ab-initio study on the phonon mode potential as a source of anharmonicity in the crystal. As an example, the transverse optical (TO) mode potential in PbTe has been fitted to density-functional-theory calculated energies of phonons excited with different amplitudes of mode displacements. The corresponding equation of motion has been analytically and numerically solved in 1D and 2D space, respectively. The solution is used for constructing the ensemble of 10,000 systems with potential and kinetic energies selected according to the thermal equilibrium distributions. The velocity auto-correlation function derived from the computed trajectories is then used to calculate the profile of the phonon spectrum for the TO an LA modes at three temperatures of 100, 300, and 600 K. This technique allows for determination of the contribution of non-quadratic potential of the phonon mode to the anharmonicity in the crystal and its effect on the phonon spectrum.