Soft Phonon Research Articles

Rotational phase transitions in antifluorite-type osmate and iridate compounds

We present temperature-dependent single-crystal diffraction results on seven antifluorite-type A2MeX6 compounds with Me = Os or Ir: K2OsCl6, A 2OsBr6 with A = K, Rb, Cs and NH4, and K2IrX 6 with X = Cl and Br. The structural transitions in this family arise from MeX 6 octahedron rotations that generate a rich variety of symmetries depending on the rotation axis and stacking schemes. In order to search for local distortions in the high-symmetry phase we perform refinements of anharmonic atomic displacement parameters with comprehensive data sets. Even at temperatures close to the onset of structural distortions, these refinements only yield a small improvement indicating only small anharmonic effects. The phase transitions in these antifluorites are essentially of displacive character. However, some harmonic displacement parameters are very large reflecting soft phonon modes with the softening covering large parts of the Brillouin zone. The occurrence of the rotational transitions in the antifluorite-type family can be remarkably well analyzed in terms of a tolerance factor of ionic radii.

Realizing High Thermoelectric Performance in GeTe‐Based Supersaturated Solid Solutions

AbstractAs a highly promising thermoelectric material in the mid‐temperature region, pristine GeTe shows deteriorated thermoelectric properties due to the low Seebeck coefficient and the intrinsic high thermal conductivity. Herein, it is discovered that the introduction of strongly correlated d, f electrons by alloying rare‐earth atoms at Ge sites will increase the band effective mass and promote band convergency, which can largely improve the electric transport property. Moreover, the heavy atoms (Pb, Bi) doping induces optical phonon softening and the avoided crossing between acoustic and optical phonon branches, decelerating both optical and acoustic phonon velocities. The modulated composition wave from the fluctuated multi‐component distribution introduces a complicated hierarchical structure including point defects and nanoprecipitates, which provides all‐scale scattering sources for heat‐carrying phonons and results in low lattice thermal conductivity. Consequently, a high zT of 2.4 at 800 K can be obtained in the Ge0.86Pb0.09Bi0.03Ce0.005Te sample due to the combination of strong correlation for d, f electrons and the full‐spectrum scattering for phonons. This work provides an effective strategy to increase the thermoelectric performance of IV–VI compounds by combining the modulated band structure and the softening phonon dispersion.

Computational predictions of optoelectronic energy materials Cs2SiBr6 Cs2GeBr6 & Cs2SnBr6 for phenomenal photovoltaic applications; a first principles study

Exploring prospective materials for energy storage and production is one of the largest confrontation of the century. Solar optoelectronic energy is one of the utmost exceptional renewable resources now a days, due to their small Environmental impact and wide availability. The perovskite type double halides are promising materials for optoelectronic and solar applications. In this article, the optoelectronic characteristics of newly perovskite double halides Cs2SiBr6 Cs2GeBr6 and Cs2SnBr6 are addressed. The calculated band gaps 2.70 eV (Cs2SiBr6), 1.80 eV (Cs2GeBr6), 2.90 eV (Cs2SnBr6) with raising performance for optoelectronic applications. The optical properties are explored by large dielectric constants, high optical absorption coefficients, small optical loss etc. Moreover, the mechanical properties for the studied materials have been performed to confirm the mechanical stability, comactability and reliability of the materials for photovoltaic applications. The thermodynamic and vibrational stability and compatibility has been verified due to negative values of enthalpy, cohesive energies and no soft phonon mode of vibrations. These optoelectronic energy related calculations confirmed that the perovskites type double halides may Cs2SiBr6, Cs2GeBr6 and Cs2SnBr6 may be novel, stimulating potential materials and can be synthesized experimentally for next generation photovoltaic devices and related applications.

Superconductivity in Quasi-One-Dimensional Ferromagnet CrSbSe3 under High Pressure.

Nearly a decade has passed since the discovery of superconductivity in CrAs, but until now, the discovered structure types of chromium-based superconductors are still scanty. It is urgent to expand this family to decipher the interplay between magnetism and superconductivity penetratingly. Here, we report the observation of superconductivity in ferromagnet CrSbSe3 with a quasi-one-dimensional structure under high pressure. Under compression, CrSbSe3 undergoes an insulator-to-metal transition and sequential isostructural phase transitions accompanied by volume collapse. Superconductivity emerges at 32.8 GPa concomitant with metallization in CrSbSe3. A maximum superconducting transition temperature Tc of 7.7 K is achieved at 57.9 GPa benefiting from both the phonon softening and the enhanced p-d hybridization between Se and Cr in CrSbSe3. The discovery of superconductivity in CrSbSe3 expands the existing chromium-based superconductor family and sheds light on the search for concealed superconductivity in low-dimensional van der Waals materials.

Lattice dynamics and enhanced electron–phonon coupling of itinerant magnet Fe1-xCoxSi (x ≥ 0.3): A 57Fe Mössbauer spectroscopy study

The magnetic phase transition in itinerant magnets involves many fundamental physical problems. Here we report a systematic investigation on the 57Fe Mössbauer spectroscopy of B20-type Fe1-xCoxSi (0.3 ≤ x ≤ 0.8) compounds in combination with macroscopic magnetism and resistance measurements. We observed non-Fermi liquid behavior at low temperatures in the region of phase transition (close to x = 0.7), as well as significant phonon softening. In addition, with the increase of Co content, the effect of electron–phonon coupling on the hyperfine field is gradually comparable to that of spin fluctuations. Our observations indicate that electron doping promotes electron–electron correlations and facilitates electron–phonon interactions at the cost of suppression of magnetic ordering and magnetic fluctuations, suggesting that interactions between phonons and other quasiparticles play a crucial role in the magnetic phase transitions of itinerant helical magnets.

Configurational Disorder, Strong Anharmonicity, and Coupled Host Dynamics Lead to Superionic Transport in Li3YCl6 (LYC).

In superionic crystals, liquid-like ionic diffusivities often come hand-in-hand with ultra-low thermal conductivity and soft vibrational dynamics. However, generalized relationships between ion transport and vibrational dynamics remain elusive due to the diversity of superionic materials and complex underlying mechanisms. Here, the links between vibrational dynamics and ion transport in close-packed lithium halide ion conductor Li3YCl6 (LYC) are examined using a suite of atomistic first-principles methods. It is shown that configurational disorder, lattice anharmonicity, and coupled host-mobile ion vibrational dynamics together induce a transition to the superionic state. Statistical correlations between ionic hops and activation of the distribution of vibrational modes are found. However, typical phenomena associated with superionic conductors such as selective breakdown of zone-boundary soft phonons, or long wavelength transverse acoustic modes as in the 'phonon-liquid-electron crystal' concept, are not present. Instead, anharmonic zone-boundary modes aiding Li diffusion are found to broaden and soften selectively but persist across the superionic transition. These anharmonic modes couple Li ion motion with the vibrations of the flexible close-packed anion framework, which remains stable and facilitates ionic hopping. The results provide insights into how configurational disorder and soft-yet-resilient vibrational modes enable ionic hopping, particularly in 3D close-packedcrystals.

A Theoretical Study of Doping Evolution of Phonons in High-Temperature Cuprate Superconductors

Hole-doped high-temperature copper oxide-based superconductors (cuprates) exhibit complex phase diagrams where electronic orders like a charge density wave (CDW) and superconductivity (SC) appear at low temperatures. The origins of these electronic orders are still open questions due to their complex interplay and correlated nature. These electronic orders can modify the phonons in the system, which has also been experimentally found in several cuprates as a softening in the phonon frequency at the CDW vector. Recent experiments have revealed that the softening in phonons in cuprates due to CDW shows intriguing behavior with increasing hole doping. Hole doping can also change the underlying Fermi surface. Therefore, it is an interesting question whether the doping-induced change in the Fermi surface can affect the softening of phonons, which in turn can reveal the nature of the electronic orders present in the system. In this work, we investigate this question by studying the softening of phonons in the presence of CDW and SC within a perturbative approach developed in an earlier work. We compare the results obtained within the working model to some experiments.

Superconductivity in unconventional metals

Based on first-principles calculations, we demonstrate that 1H/2H-phase transition metal dichalcogenides MX2 (M = Nb, Ta; X = S, Se, Te) are unconventional metals, which have an empty-site band of A1′@1e\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${A}_{1}^{{\\prime} }@1e$$\\end{document} elementary band representation at the Fermi level. The computed phonon dispersions indicate the stability of the system at high temperatures, while the presence of the soft phonon mode suggests a phase transition to the charge density wave state at low temperatures. Based on the Bardeen-Cooper-Schrieffer theory and computed electron-phonon coupling, our calculations show that the superconductivity (SC) in NbSe2 is mainly attributed to the soft phonon mode due to the half filling of the empty-site band. Accordingly, the SC has been predicted in unconventional metals TaNS monolayer and 2H-TaN2 bulk with computed TC = 10 K and 26 K respectively. These results demonstrate that the unconventional metals with partial filling of the empty-site band offer an attractive platform to search for superconductors.

Modeling critical thermoelectric transports driven by band broadening and phonon softening

Critical phenomena are one of the most captivating areas of modern physics, whereas the relevant experimental and theoretical studies are still very challenging. Particularly, the underlying mechanism behind the anomalous thermoelectric properties during critical phase transitions remains elusive, i.e., the current theoretical models for critical electrical transports are either qualitative or solely focused on a specific transport parameter. Herein, we develop a quantitative theory to model the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. It is found that the band-broadening effect contributes an additional term to Seebeck coefficient, while the carrier—soft TO phonon interactions greatly affects both electrical resistivity and Seebeck coefficient. The universality and validity of our model are well confirmed by experimental data. Furthermore, the features of critical phase transitions are effectively tuned. For example, alloying S in Cu2Se can reduce the phase transition temperature but increase the phase transition parameter b. The maximum thermoelectric figure of merit zT is pushed to a high value of 1.3 at the critical point (377 K), which is at least twice as large as those of normal static phases. This work not only provides a clear picture of the critical electrical transports but also presents new guidelines for future studies in this exciting area.

Low-frequency electron–phonon interaction and superconductivity in L12 -type trialuminides Al3V

Superconductivity enhancement is often associated with phonon softening by doping or external pressure. In this paper, we report L12 -type trialuminides Al3V as a typical example belonging to this scenario. The electronic, phononic and superconducting properties have been studied by using the first-principle method. Under uniaxial compression, the superconducting transition temperature Tc of Al3V can be increased from ≈1.5 K to its maximum at ≈7.3 K when the lattice approaches the structural instability. A detailed investigation of Al3V reveals that the (V: d)-(Al: p) hybrid bands are strongly coupled to the phonon modes with lower frequency. The uniaxial compression induces a significant softening of the phonon mode with non-zero phonon line-width, and yet keeps the electron density of states near the Fermi level unchanged. This leads to a stronger electron–phonon coupling (EPC) and therefore a higher Tc . On the contrary, the L12 structure is more stable against the hydrostatic pressure and the phonon energy actually grows up when the hydrostatic pressure is increased. As a result, EPC becomes weaker and Tc can be reduced to ⩽ 0.3 K under high hydrostatic pressure. Our results present an interesting example that superconductivity can enhanced by tuning the softened phonon via pressure or doping.

Superconducting ternary hydrides in Ca-U-H under high pressure

The research on hydrogen-rich ternary compounds attract tremendous attention for it paves new route to room-temperature superconductivity at lower pressures. Here, we study the crystal structures, electronic structures, and superconducting properties of the ternary Ca-U-H system, combining crystal structure predictions with ab-initio calculations under high pressure. We found four dynamically stable structures with hydrogen clathrate cages: CaUH12-Cmmm, CaUH12-Fd-3m, Ca2UH18-P-3m1, and CaU3H32-Pm-3m. Among them, the Ca2UH18-P-3m1 and CaU3H32-Pm-3m are likely to be synthesized below 1 megabar. The f electrons in U atoms make dominant contribution to the electronic density of states around the Fermi energy. The electron-phonon interaction calculations reveal that phonon softening in the mid-frequency region can enhance the electron–phonon coupling significantly. The T c value of Ca2UH18-P-3m1 is estimated to be 57.5–65.8 K at 100 GPa. Our studies demonstrate that introducing actinides into alkaline-earth metal hydrides provides possibility in designing novel superconducting ternary hydrides.

High throughput screening of semiconductors with low lattice thermal transport induced by long-range interactions.

Semiconductors with long-range interactions (LRI) due to resonant bonding exhibit delocalized electronic states and low lattice thermal conductivity, contributing to the efficiency of heat-to-electricity conversion. Here, we build a descriptor for high-throughput screening of LRI materials from the second-order interaction force constants. We identify 75 semiconducting candidates from the binary compounds in the MatHub-3d database that contain LRI. By analyzing the bonding properties of LRI atoms, we classify LRI in materials into two categories: type I and type II. In the structural unit of type I LRI, the atoms have strong bond connections, while a weak bond exists between the two groups in the structural unit of type II LRI. We have identified atypical type I LRI formed by Sb-Sb and Mg-Mg pairs in the emerging thermoelectric material Mg3Sb2, resulting in the softening of TA1 phonons and large anharmonicity. For type II LRI, the LRI of Ge-Ge and Se-Se pairs in R3m-GeSe can cross different layers. Moreover, we observe a combination of type II LRI and rattling effect in BaSe2 to restrict thermal transport. This work is of great significance for understanding the relationship between LRI and thermal transport properties, and for designing new LRI-induced low lattice thermal conductivity materials.

Emergence of promising n-type thermoelectric material through conductive network and strong phonon softening

The synergistic effect of conductive network coupled with strong optical phonon softening and avoided crossing phenomena helps to achieve high thermoelectric performance in a quasi-ternary system.

Enhanced thermoelectric properties of In-filled Co4Sb12 by dispersion of reduced graphene oxide.

Uniform dispersion of nanosized secondary phases in bulk thermoelectric materials has proven to be an effective strategy to reduce the lattice part of thermal conductivity and improve the thermoelectric efficiency. In this work, reduced graphene oxide (rGO) was uniformly dispersed in the In0.5Co4Sb12 bulk material by ultrasonication. The formation of impurity phases of InSb and CoSb2 in the In-filled Co4Sb12 is inevitable, as observed from XRD and EPMA analyses. The Raman spectra of the nanocomposites showed broad peaks suggesting phonon softening and additional peaks corresponding to rGO. Electron transport was not affected by rGO addition, resulting in little change in the electrical resistivity and Seebeck coefficient. The lattice thermal conductivity of the bulk material was significantly reduced by the addition of a small amount of rGO, primarily attributed to the interface scattering of phonons. Hence, the highest zT of ∼1.53 at 773 K was achieved for the In0.5Co4Sb12/0.25 vol% rGO composite in the temperature range from 723 K to 773 K.

Phonon softening induced phase transition of CeSiO4: a density functional theory study.

Density functional theory plus Hubbard U (DFT+U) methodology was used to calculate the structures and energetic landscapes of CeSiO4, including its stetindite and scheelite phases from ambient pressure to ∼24 GPa. To ensure accurate simulations of the high-pressure structures, assessments of strain-stress methods and stress-strain methods were conducted in prior, with the former found to have a better agreement with the experimental result. From DFT calculations the equation of states (EOS) of both stetindite and scheelite were further obtained, with the fitted bulk moduli being 182(2) GPa and 190.0(12) GPa, respectively. These results were found to be consistent with the experimental values of 177(5) GPa and 222(40) GPa. Furthermore, the calculated energetics suggest that the stetindite structure is more thermodynamically stable than the scheelite structure at a pressure lower than 8.35 GPa. However, the stetindite → scheelite phase transition was observed experimentally at a much higher pressure of ∼15 GPa. A further phonon spectra investigation by the density functional perturbation theory (DFPT) indicated the Eg1 mode is being softened with pressure and becomes imaginary after 12 GPa, which is a sign of the lattice instability. Consequently, it was concluded that the stetindite → scheelite transition is predominantly initiated by the lattice instability under high-pressure.

Structure searches and superconductor discovery in XB2 (X = Sc, Ti, V, Cr, and Tc).

With extensive structure searches for XB2 (X = Sc, Ti, V, Cr, and Tc) under pressures up to 100 GPa, we uncovered that the crystal structures of these compounds with the lowest enthalpy have the same space group (P6/mmm) as MgB2 at ambient pressure. Among them, ScB2, TiB2 and VB2 are dynamically stable at ambient pressure, but they do not superconduct. CrB2 becomes dynamically stable at 108 GPa and shows superconductivity with a transition temperature (Tc) of 26.0 K. TcB2 is not dynamically stable until 9 GPa. At 20 GPa, it has a Tc of 23.5 K. Further calculations indicate that CrB2 and TcB2 are also thermodynamically stable, suggesting that it is highly likely that they can be synthesized successfully in the laboratory. We found that transition metal atoms (Cr/Tc) dominate soft phonon vibrations and make significant contributions to the electron-phonon coupling (EPC) and superconductivity in CrB2/TcB2, which is in strong contrast to the case of MgB2, where high-frequency B vibrations dominate the EPC and superconductivity. Our work enriches the understanding of superconductivity in transition metal borides.

Imaging Ultrafast Dynamics of Pressure-Driven Phase Transitions in Black Phosphorus and Anomalous Coherent Phonon Softening.

Applying high pressure to effectively modulate the electronic and lattice structures of materials could unravel various physical properties associated with phase transitions. In this work, high-pressure-compatible femtosecond pump-probe microscopy was constructed to study the pressure-dependent ultrafast dynamics in black phosphorus (BP) thin films. We observed pressure-driven evolution of the electronic topological transition and three structural phases as the pressure reached ∼22 GPa, which could be clearly differentiated in the transient absorption images containing spatially resolved ultrafast carrier and coherent phonon dynamics. Surprisingly, an anomalous coherent acoustic phonon mode with pressure softening behavior was observed within the range of ∼3-8 GPa, showing distinct laser power and time dependences. Density functional theory calculations show that this mode, identified as the shear mode along the armchair orientation, gains significant electron-phonon coupling strength from out-of-plane compression that leads to decreased phonon frequency. Our results provide insights into the structure evolution of BP with pressure and hold potential for applications in microelectromechanical devices.

Acoustic phonon softening enhances phonon scattering in Zintl-phase II-I-V compounds

Acoustic phonon softening enhances phonon scattering in Zintl-phase II-I-V compounds

Emergent phenomena in quantum phase transition

Emergent phenomena near the quantum critical point at sufficiently low temperature attracts accumulating attentions with respect to both theories and applications. On the contrary to materials with itinerant electrons, ferroelectric materials are relatively less studied but promising in studying novel quantum orders. In this report, we focus on one clean model system, strontium titanite oxide, to explore the quantum criticality. The stabilization of a quantum paraelectric phase has been verified by the previous experimental observation of the dielectric permittivity saturating at a rather high value to the order of 104 under 4 Kelvin. To understand the underlying mechanism, we apply the quantum generalization of Ginzburg-Landau theory as well as lattice dynamics, i.e., the stiffness of soft phonon mode to rationalize the deviation from the classical paraelectric to ferroelectric phase transitions. Besides, under the effective upper critical dimension, a logarithmic correction of the relationship between relative permittivity and temperature could explain the upturn found in diagram.

Dispersion Engineering by Hybridizing the Back-Folded Soft Mode of Monomode Elastic Metamaterials with Stiff Acoustic Modes.

In many cases, the hybridization of two or more excitation modes in solids has led to new and useful dispersion relations of waves. Well-studied examples are phonon polaritons, plasmon polaritons, particle-plasmon polaritons, cavity polaritons, and magnetic resonances at optical frequencies. In all of these cases, the lowest propagating mode couples to a finite-frequency localized resonance. Herein, the unusual metamaterial phonon dispersion relations arising from the hybridization of an ordinary acoustical phonon mode with a back-folded soft or easy phonon mode of a monomode elastic metamaterial are discussed. Conceptually, the single easy mode can have strictly zero wave velocity. In reality, its wave velocity is very much smaller than that of all other modes. Considering polymeric three-dimensional printed elastic monomode metamaterials at ultrasound frequencies, it is shown theoretically and experimentally that the resulting pronounced avoided crossing, with a frequency splitting comparable to the mid-frequency, leads to backward-wave behavior for the lowest band over a broad frequency range, conceptually at zero loss.