First-principles Lattice Dynamics Calculations Research Articles

Mechanism of Negative Thermal Expansion in Monoclinic Cu2P2O7 from First Principles.

Negative thermal expansion (NTE) materials generally have high-symmetry space groups, large average atomic volumes, and corner-sharing octahedral and tetrahedral coordination structures. By contrast, monoclinic α-Cu2P2O7, which has a small average atomic volume and edge-sharing structure, has been reported to exhibit NTE, the detailed mechanism of which is unclear. In this study, we investigate the A2B2O7 polymorphs and analyze the NTE behavior of α-Cu2P2O7 using first-principles lattice-dynamics calculations. From the polymorphism investigation in 20 A2B2O7 compounds using 6 representative crystal structures, small A and B cationic radii are found to stabilize the α-Cu2P2O7-type structure. We then analyze the NTE behavior of α-Cu2P2O7 using quasi-harmonic approximation. Our calculated thermal expansion coefficients and anisotropic atomic displacement parameters were in good agreement with those of the experimental reports at low temperatures. From the mode-Grüneisen parameter distribution plotted over the entire first-Brillouin zone, we found that the phonon contributing most significantly to NTE emerges not into the special points but between them. In this phonon mode, the O connecting two PO4 tetrahedra rotates, and the Cu and O vibrate perpendicular to the bottom of the CuO5 pyramidal unit, which folds the ac lattice plane. This vibration behavior can explain the experimentally reported anisotropic NTE behavior of α-Cu2P2O7. Our results demonstrate that the most negative mode-Grüneisen parameter contributing to NTE behavior is not always located on high-symmetry special points, indicating the importance of lattice vibration analyses for the entire first-Brillouin zone.

Phonon mixing in the charge density wave state of ScV6Sn6

Kagomé metals are widely recognized, versatile platforms for exploring topological properties, unconventional electronic correlations, magnetic frustration, and superconductivity. In the RV6Sn6 family of materials (R = Sc, Y, Lu), ScV6Sn6 hosts an unusual charge density wave ground state as well as structural similarities with the AV3Sb5 system (A = K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with first-principles lattice dynamics calculations to reveal phonon mixing processes in the charge density wave state of ScV6Sn6. In the low temperature phase, we find at least four new peaks in the vicinity of the V-containing totally symmetric mode near 240 cm−1 suggesting that the density wave acts to mix modes of P6/mmm and Rbar{3}m symmetry - a result that we quantify by projecting phonons of the high symmetry state onto those of the lower symmetry structure. We also test the stability of the short-range ordered density wave state under compression and propose that both physical and chemical pressure quench the effect. We discuss these findings in terms of symmetry and the structure-property trends that can be unraveled in this system.

Terahertz Spectroscopy and DFT Analysis of Phonon Dynamics of the Layered Van der Waals Semiconductor Nb3 X 8 (X = Cl, I).

We have conducted a terahertz spectroscopic study and a density functional theory analysis of the phonon dynamics of the layered van der Waals semiconductors Nb3Cl8 and Nb3I8. Several infrared-active phonon modes were observed in the terahertz region, and their frequencies were found to be in excellent agreement with our first-principles lattice dynamics calculations. For Nb3Cl8, the observed phonon spectra are consistent with a structural transition at 90 K from the high-temperature P3̅m1 phase to the low-temperature R3̅m phase. Also, our study confirmed that the structural and magnetic transitions were coupled in Nb3Cl8. For Nb3I8, which is nonmagnetic at and below room temperature, no significant temperature or magnetic field dependence was observed in the phonon spectra. Our study provides an intriguing connection between the structural properties and the paramagnetic-nonmagnetic transitions in Nb3Cl8 and Nb3I8.

P-TRANS: A Monte Carlo ray-tracing software to simulate phonon transport in arbitrary nanostructures

P-TRANS is a software to simulate phonon transport in arbitrary nanostructures based on the Boltzmann transport theory. The core part of the software is a Monte Carlo ray-tracing solver that uses the stochastic method to sample phonon transport in various nanostructures. The solver is written in FORTRAN with well-organized modules and utilizing the OpenMP and MPI parallelization to achieve high performance. The software can handle a typical calculation of nanostructures with a feature size of ∼1000 nm on a normal PC within seconds, which is useful when exploring the wide variety of structures, for instance for structural optimization or high-throughput screening in material informatics. The software takes the bulk phonon properties obtained from the first-principles anharmonic lattice dynamics calculations as inputs. The database of bulk phonon properties is available on the software webpage, which currently contains more than 60 materials and will increase in the future. We also provide a graphical user interface to design the nanostructures and to run the simulation for non-expert users. In this paper, we introduce the theoretical background, the technical details and implementation, and the applications of the software. Program summaryProgram title:P-TRANSCPC Library link to program files:https://doi.org/10.17632/n76jpj3fnz.1Developer's repository link:https://github.com/TEEL-UTokyo/P-TRANSCode Ocean capsule:https://codeocean.com/capsule/9387119Licensing provisions: GPLv3Programming language: FortranSupplementary material:http://www.phonon.t.u-tokyo.ac.jp/p-trans/Nature of problem: Knowing the effective thermal conductivity of nanostructures is important for numerous applications. Phonons are the dominate heat carriers in dielectrics and their transport is influenced by the voids, grain boundaries, and free surfaces in the nanostructure. Here we are focusing on the effective thermal conductivities of nanostructures with complex geometries and boundaries.Solution method: Monte Carlo ray tracing simulations.

Multiphonon Raman Scattering and Strong Electron-Phonon Coupling in 2D Ternary Cu2MoS4 Nanoflakes.

The interaction between photogenerated carriers with lattice vibrations plays a fundamental role in the nonradiative recombination and charge-transfer processes occurring in photocatalysis and photovoltaics. Here, we employ Raman spectroscopy to investigate the electron-phonon interaction in ternary layered Cu2MoS4 nanoflakes. Multiphonon Raman scattering with up to fourth-order longitudinal optical (LO) overtones is observed under above-band gap excitation, indicating a strong electron-phonon coupling (EPC) that could be described by the cascade model. The Huang-Rhys factor was derived to characterize the strength of EPC and was found to be increasing with decreasing temperature. First-principles calculations of lattice dynamics and electron-phonon matrix elements suggest that the strong EPC in Cu2MoS4 is dominated by Fröhlich coupling between electron and the electric fields, which is induced by the localized phonon mode originating from a flat phonon branch. Our findings facilitate the understanding of electron-phonon interaction in 2D ternary Cu2MoS4 and pave the way for developing and optimizing optoelectronic devices.

Dependence of excess vibrational entropies on grain boundary structures in MgO: A first-principles lattice dynamics

First-principles lattice dynamics calculations were performed to reveal an atomic-level origin of excess vibrational entropies at grain boundaries (GBs) in MgO. Fourteen symmetric tilt GBs with various structural units were systematically examined. The excess vibrational entropies were found to vary depending on the individual GBs, and as a result, the relative thermodynamic stability of the GBs studied changed with temperature. The excess GB volumes were less correlated with the excess vibrational entropies. By contrast, classifying ions in terms of their coordination numbers, bond-length changes at GBs were well correlated with atom-projected excess vibrational entropies for both Mg and O ions. Bond-length changes at GBs, which originate from changes in bond strength, are therefore a critical, well-defined descriptor for evaluating excess vibrational entropies and thereby excess free energies in MgO.

Near-field infrared spectroscopy of monolayer MnPS3

Author(s): Neal, SN; Kim, HS; Smith, KA; Haglund, AV; Mandrus, DG; Bechtel, HA; Carr, GL; Haule, K; Vanderbilt, D; Musfeldt, JL | Abstract: We measured the near-field infrared response of MnPS3 in bulk, few-, and single-layer form and compared our findings with traditional far-field vibrational spectroscopies, a symmetry analysis, and first-principles lattice dynamics calculations. Trends in the Bu mode near 450cm-1 are striking, with the disappearance of this structure in the thinnest sheets. Combined with the amplified response of the activated Ag mode and analysis of the Au+Bu features, we find that symmetry is unexpectedly increased in few- and single-sheet MnPS3 due to a restoration of the threefold axes of rotation. Monoclinicity in this system is therefore a consequence of the long-range stacking pattern and temperature rather than local structure.

A -site cation size effect on oxygen octahedral rotations in acentric Ruddlesden-Popper alkali rare-earth titanates

We demonstrate inversion symmetry breaking induced by oxygen octahedral rotations in layered perovskite oxides $\mathrm{K}{A}_{R}{\mathrm{TiO}}_{4}$ (${A}_{R}$ = rare earth) using a combined experimental and theoretical approach including synchrotron x-ray diffraction, optical second harmonic generation, and first-principles lattice dynamics calculations. We experimentally find an interesting but counterintuitive phenomenon, i.e., the acentric-to-centric phase transition temperatures for K family are higher than those for previously reported Na family, in contrast to expectations based on the Goldschmidt tolerance factor, where the octahedral rotation instability toward the acentric phases would reduce with an increase in the radius of $A$-site alkali metal ions. Our detailed analysis of first-principles calculations for ${A}_{A}{A}_{R}{\mathrm{TiO}}_{4}$ (${A}_{A}=\mathrm{Na}$, K, Rb) reveals that the alkali metal and rare-earth ions play quite different roles in driving the octahedral rotations. Since rare-earth ions attract oxide ions more strongly than alkali metal ions due to the higher valence of the former in comparison with the latter (${A}_{R}^{3+}$ vs ${A}_{A}^{+}$), the optimization of coordination environment of rare-earth ions is the primary driving force of the octahedral rotations. Alkali metal ions serve to impose ``bond strains'' parallel to the layers, playing a secondary role in the octahedral rotations. Incorporation of large alkali metal ions generates a significant in-plane biaxial bond strain in ${A}_{R}\mathrm{O}$ and ${\mathrm{TiO}}_{2}$ layers through the expanded ${A}_{A}\mathrm{O}$ layers, and thereby facilitates the octahedral rotations because of the otherwise highly underbonding of rare-earth ions. Thus, the effect of $A$-site alkali metal size on the octahedral rotation instability can be explained in terms of the interlayer lattice mismatch. This understanding allows us to propose a geometric descriptor governing the structural instability in ${A}_{A}{A}_{R}{\mathrm{TiO}}_{4}$ layered perovskites. We believe that control over the interlayer lattice mismatch could be a useful strategy to tune the octahedral rotations in layered compounds.

Presence of water at elevated temperatures, structural transition, and thermal expansion behavior in LaPO4:Eu

We report powder neutron diffraction studies in $\mathrm{L}{\mathrm{a}}_{0.95}\mathrm{P}{\mathrm{O}}_{4}:\mathrm{E}{\mathrm{u}}_{0.05}\phantom{\rule{4pt}{0ex}}(\mathrm{LaP}{\mathrm{O}}_{4}:\mathrm{Eu})$ at various temperatures up to 1163 K. From Rietveld refinement of the diffraction data, we identify the presence of water up to 567 K and an irreversible phase transition from hexagonal to monoclinic structure around 900 K involving the coexistence of phases over 700 to 1050 K. We observe from the neutron experiments that the thermal expansion in the hexagonal phase is much smaller than that in the monoclinic phase. We have performed first-principles lattice dynamics calculations in both the phases to identify the anharmonic phonons and thereby understand the thermal expansion behavior. The low-energy translational and librational modes of $\mathrm{P}{\mathrm{O}}_{4}$ tetrahedral units in the hexagonal phase show negative thermal expansion behavior, which is responsible for the overall smaller positive thermal expansion coefficient in the hexagonal phase in comparison to that in the monoclinic phase.

Polar metallic behavior of strained antiperovskites ACNi3(A=Mg,Zn,and Cd) from first principles

Stable structures of metallic antiperovskites $A{\mathrm{CNi}}_{3}\phantom{\rule{4pt}{0ex}}(A=\mathrm{Mg},\text{Zn},\text{and Cd})$ under epitaxial strain are explored using first-principles lattice dynamics calculations. Although the ground states of $A{\mathrm{CNi}}_{3}$ are cubic nonpolar phases without structural distortion, transitions to polar phases are predicted to arise in these compounds under compressive strain while maintaining metallic states. In particular, the polar phase of ${\mathrm{MgCNi}}_{3}$ would be attainable with moderate strain of a few percent. The polar distortions are considered to be generated by the enhancement of the hybridization between C $2p$ and Ni $3d$ states, which is a mechanism analogous to that of the previously discovered polar metal ${\mathrm{CeSiPt}}_{3}$.

First-principles calculations of lattice dynamics and thermodynamic properties for Yb14MnSb11

Systematic first-principles calculations were performed to study the lattice dynamics of Yb14MnSb11 and hence to obtain a wide range of its thermodynamic properties at high temperatures. The calculated results were analyzed in terms of the lattice contribution and the electronic contribution, together with a comparison with a collection of experimental thermochemical data. At 0 K, the electronic density of states showed the typical feature of a p-type semiconductor—a small amount of unoccupied electronic states exclusively made of the major spin by a range of ∼0.6 eV above the Fermi energy. It showed that the Mn atom had a ferromagnetic spin moment of ∼4 μB. As a semiconductor, it was found that the electronic contribution to the heat capacity was substantial, with an electronic heat capacity coefficient of ∼0.0006 J/mole-atom/K2.

Lattice dynamics of the tin sulphides SnS2, SnS and Sn2S3: vibrational spectra and thermal transport.

We present an in-depth first-principles study of the lattice dynamics of the tin sulphides SnS2, Pnma and π-cubic SnS and Sn2S3. An analysis of the harmonic phonon dispersion and vibrational density of states reveals phonon bandgaps between low- and high-frequency modes consisting of Sn and S motion, respectively, and evidences a bond-strength hierarchy in the low-dimensional SnS2, Pnma SnS and Sn2S3 crystals. We model and perform a complete characterisation of the infrared and Raman spectra, including temperature-dependent anharmonic linewidths calculated using many-body perturbation theory. We illustrate how vibrational spectroscopy could be used to identify and characterise phase impurities in tin sulphide samples. The spectral linewidths are used to model the thermal transport, and the calculations indicate that the low-dimensional Sn2S3 has a very low lattice thermal conductivity, potentially giving it superior performance to SnS as a candidate thermoelectric material.

First-principles calculations of lattice dynamics and thermal properties of polar solids

AbstractAlthough the theory of lattice dynamics was established six decades ago, its accurate implementation for polar solids using the direct (or supercell, small displacement, frozen phonon) approach within the framework of density-function-theory-based first-principles calculations had been a challenge until recently. It arises from the fact that the vibration-induced polarization breaks the lattice periodicity, whereas periodic boundary conditions are required by typical first-principles calculations, leading to an artificial macroscopic electric field. The article reviews a mixed-space approach to treating the interactions between lattice vibration and polarization, its applications to accurately predicting the phonon and associated thermal properties, and its implementations in a number of existing phonon codes.

Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

Inelastic neutron scattering measurements on monoclinic zirconia (ZrO_2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373wK. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Gruneisen parameters. These mode Gruneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.

Looking for Auger signatures in III-nitride light emitters: A full-band Monte Carlo perspective

Recent experiments of electron emission spectroscopy (EES) on III-nitride light-emitting diodes (LEDs) have shown a correlation between droop onset and hot electron emission at the cesiated surface of the LED p-cap. The observed hot electrons have been interpreted as a direct signature of Auger recombination in the LED active region, as highly energetic Auger-excited electrons would be collected in long-lived satellite valleys of the conduction band so that they would not decay on their journey to the surface across the highly doped p-contact layer. We discuss this interpretation by using a full-band Monte Carlo model based on first-principles electronic structure and lattice dynamics calculations. The results of our analysis suggest that Auger-excited electrons cannot be unambiguously detected in the LED structures used in the EES experiments. Additional experimental and simulative work are necessary to unravel the complex physics of GaN cesiated surfaces.

Direct measurement of coherent phonon dynamics in solution-processed stibnite thin films

We report observations of coherent phonon oscillations in solution-processed polycrystalline stibnite $({\mathrm{Sb}}_{2}$${\mathrm{S}}_{3})$ photovoltaic thin films using transient absorption spectroscopy. Detailed optical spectroscopy correlated by extensive first-principles lattice dynamics calculations indicates that the coherent ${B}_{3g}$ longitudinal optical phonon mode with a frequency of 63.74 \ifmmode\pm\else\textpm\fi{} 0.05 ${\mathrm{cm}}^{\ensuremath{-}1}$ (or 1.911 \ifmmode\pm\else\textpm\fi{} 0.002 THz) at room temperature is generated via the impulsive stimulated-Raman-scattering mechanism. These strong electron-phonon interactions indicate a dominant energy-loss channel in these materials that could impose a fundamental limit on their solar energy conversion efficiency.

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the ‘interfacial atomic disorder’ model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

First-principles calculations of ferroelectric properties in AA′BB′O6 double perovskites with different types of cation ordering

First-principles calculations of lattice dynamics and polarization properties have been performed for double perovskite ABiBNbO6 (A – alkali metal Na, Rb, and B – trivalent metal Sc, Lu). Three possible types of A- and B-site cation ordering are studied: layer and columnar ordering of A-site cations with rocksalt ordering of B-site cations, and layer ordering of both cations. The ground state of all compounds is polar with a large value of polarization for all types of cation ordering. For some compounds we obtain spontaneous polarization of 0.7–1.0 C m−2 that is twice the BaTiO3 value. For RbBiScNbO6 the lowest-energy state has layer ordering of both A- and B-site cations, which is rare for compounds with double-perovskite structure.

How to get superhard MnB2: a first-principles study

In 2009, a super-hard MnB2 with ReB2-type structure was predicted as being in the ground state. However, it has not been synthesized successfully in about two years either by high temperature and high pressure (HTHP) method or by the arc-melting method. To obtain the accurate synthesis conditions, the P–T phase boundary between AlB2-type and ReB2-type MnB2 has been studied by first-principles lattice dynamics calculations within quasi-harmonic approximation (QHA). Our results show that the ReB2-type MnB2 can be synthesized only below 1020 K at ambient pressure. Pressure effect makes their transition temperature decrease. If the pressure is higher than 38 GPa, only AlB2-type MnB2 can be obtained. The synthesis temperatures of previous experiments (either HTHP or arc-melting method) are all above 1020 K, so that only AlB2-type MnB2 can be synthesized. Therefore, it is essential to control the temperature accurately to synthesize the ReB2-type MnB2. On the other hand, the pressure should be controlled to be as low as possible. Further analyses show that the thermodynamic stability of MnB2 at high temperature mostly depends on the vibration frequency of Mn atoms. The stronger interactions between Mn and B in the ReB2-type MnB2 induce the vibration frequencies of Mn atoms shift to higher and increase the Gibbs free energy, causing the thermodynamics instability of ReB2-type MnB2 at high temperature. Therefore, there is no ReB2-type MnB2 synthesized at the temperature higher than 1020 K.

First-principles study of lattice dynamics and thermodynamic properties of LiInX 2 (X = S, Se, Te)

First-principles calculations of lattice dynamics and thermodynamic properties of orthorhombic LiInS 2 and LiInSe 2 and chalcopyrite LiInTe 2 have been performed within density functional perturbation theory using norm conserving pseudopotentials. Theoretical values of phonon mode frequencies are in good agreement with the experimental data available for these crystals obtained by methods of Raman spectroscopy and infrared one. In the whole frequency range a significant decrease of the vibrational frequencies is observed going from LiInS 2 to LiInSe 2 and LiInTe 2, which is a consequence of the anion radius increase. The lattice vibrations of In–X (X = S, Se, Te) bonds are mainly located in the low-frequency and mid-frequency ranges, and the Li–X bond vibrations are dominated in the higher frequency range. The mixed covalent–ionic nature of the three compounds is manifested by Born effective charge data. The vibration patterns of orthorhombic LiInS 2 and chalcopyrite LiInTe 2 were discussed in detail. The temperature dependences of thermodynamic quantities (including the internal energy, free energy, heat capacity, entropy and the Debye temperature Θ D) of all the three compounds were also presented in this paper. It is proved that Debye stiffness increases from LiInTe 2 to LiInSe 2 and LiInS 2.