Soft Phonon Research Articles

Softening of the optical phonon by reduced interatomic bonding strength without depolarization.

Softening of the transverse optical (TO) phonon, which could trigger ferroelectric phase transition, can usually be achieved by enhancing the long-range Coulomb interaction over the short-range bonding force1, for example, by increasing the Born effective charges2. However, it suffers from depolarization effects3,4 as the induced ferroelectricity is suppressed on size reduction of the host materials towards high-density nanoscale electronics. Here, we present an alternative route to drive the TO phonon softening by showing that the abnormal soft TO phonon in rocksalt-structured ultrawide-bandgap BeO (ref. 5) is mainly induced by a substantial reduction in the short-range bonding interaction due to the Be-O bond stretching caused by an electron cloud-overlap-induced Coulomb repulsion between two adjacent oxygen ions that are arranged octahedrally around an extremely small Be ion. We further demonstrate the emergence of robust ferroelectricity in strain-induced perovskite BaZrO3 and ultrathin HfO2 and ZrO2 films6,7 grown epitaxially on lattice-mismatched SiO2/Si substrate arising from the softening of the TO phonon driven by a reduction in the short-range bonding strength of biaxial strain-induced stretching bonds. These findings shed light on developing a unified theory for ferroelectricity enhancement in ultrathin films free from depolarization fields by tailoring chemical bonds using ionic radius differences, strains, doping and lattice distortions.

Chemically tuning stability and superconductivity of Pd–H compounds: A routine to high temperature superconductivity under modest pressures

Motivated by searching room-temperature superconductors that could be realized near ambient conditions, palladium hydrides were chosen as the research subject considering that they can stably exist under ambient conditions, and Li as an electron donor for its outstanding performance in chemically tuning stability. A novel cubic phase structure of Li2PdH6 with a remarkably high estimated Tc of ∼165 K at 90 GPa was found using particle swarm optimization algorithm calculations. The superconducting behavior persists down to 10 GPa with a high Tc of 106.382 K. Even though the parent binary Pd–H system is not a good superconductor, the introduction of extra electrons breaks up the H2 molecules, inducing the increase of atomic hydrogen compared with parent hydride, which is necessary for outstanding superconducting behavior. The existence of relatively soft phonons associated with the H atoms in phonon dispersion curves is responsible for its high-Tc. Our results indicated that the doping of Li to binary hydrides, especially to binary hydrides with low-Tc that exist under ambient pressure, can produce robust phonon-mediated superconductivity. This may be a strategy to design and optimize room-temperature superconductors that can be synthesized under modest pressure. The findings may pave the way for realizing new high-Tc superconductors in experiments under lower pressure than recently documented superconducting hydrides.

Comparing the compression behavior of the antiperovskites CePt3Si, CePt3B, and YPt3B from combined X-ray diffraction experiments and density functional theory

In this study we report the synthesis three antiperovskites YPt3B, CePt3B, and CePt3Si as well a focused investigation of YPt3B, which all have the same P4mm symmetry with a large c∕a ratio at ambient conditions. The compounds are investigated under high-pressure using powder X-ray diffraction in diamond anvil cells. A structural transition is found to occur in the YPt3B compound at 23(2) GPa caused by the softening of a zone boundary phonon. Using a combination of symmetry analysis and structural prediction, the high-pressure phase is assigned to a structure described by the P2 (P121) space group. Calculation of the phonon dispersion confirms that this structure is dynamically stable with A = Ce, Y and Z = Si, B. It was found that the three APt3Z compounds behave differently with pressure. The bulk moduli for the three compounds are found to be 185(5) GPa, 155(3) GPa, and 128(2) GPa for YPt3B, CePt3B, and CePt3Si, respectively. We found that the Z atom moves away from the center with pressure for A = Ce along the c axis, whereas for A = Y, the Z atom stays in the approximately same position. This behavior is also reflected in their c∕a ratio which stays approximately constant in YPt3B. Overall we find that the Ce-bearing compounds are more compressible in the ab plane, reducing the tilting of the octahedra. As a result, in YPt3B and CePt3B the transition to the structure described by the P2 space group is likely to occur at higher pressures.

Phonon transport manipulation in TiSe2 via reversible charge density wave melting

Titanium diselenide (TiSe2) is a layered material that under a critical temperature of Tc ≈ 200 K features a periodic modulation of the electron density, known as charge density wave (CDW), which finds applications in quantum information and emerging electronic devices. Here, we present first-principles calculations showing the suppression of the CDW via photoexcitation and consequent stabilization of the undistorted high-temperature phase, in agreement with experimental observations. Interestingly, the unfolded CDW melting is accompanied by a sizable reduction in the thermal conductivity, κ, of up to 25% and a large entropy increase of ~10 J K−1 kg−1. The significant κ variation is almost entirely originated from photoinduced changes in the phonon–phonon scattering processes involving a high-symmetry soft phonon mode. Our results open new possibilities in the design of devices for thermal management and phonon-based logic, and suggest original applications in the of context solid-state cooling.

Thermal insulation enhanced by the dopant-induced phonon softening discovered in thermoelectric Heusler compounds

Suppression of phonon transports is a key requirement to achieve high thermoelectric performance, which has been often realized by doping an element to enhance the phonon scattering. However, detailed origins behind the dopant-induced low lattice thermal conductivity (κph) have not yet been comprehensively understood. Here, we demonstrate a κph reduction mechanism induced by the phonon softening in the element-doped Fe2VAl thermoelectric Heusler compounds. Inelastic X-ray scattering experiments revealed that overall optical phonons are softened with increasing Ti-doping concentration in marked contrast to the Ta-doped Fe2VAl. By means of photoelectron spectroscopy, we found that the observed phonon softening is originating from the rigid-band shift of the electronic density of states and the consequent deviation of the pseudo-gap position from the Fermi level, which causes the energetic instability of the electronic system. This instability is accompanied by the reduction of the filled bonding states, giving rise to the phonon softening. This mechanism of κph reduction caused by the close correlation between electronic and phononic systems will shed new light on achieving low-κph and thereby developing unconventional high-performance thermoelectric materials.

Enhanced Thermoelectric Performance of p-type AgSbTe2 via Cu Doping.

Recently, the p-type semiconductor AgSbTe2 has received a great deal of attention due to its promising thermoelectric performance in intermediate temperatures (300-700 K). However, its performance is limited by the suboptimal carrier concentration and the presence of Ag2Te impurities. Herein, we synthesized AgSb1-xCuxTe2 (x = 0, 0.02, 0.04, and 0.06) and investigated the effect of Cu doping on the thermoelectric properties of AgSbTe2. Our results indicate that Cu doping suppresses the Ag2Te impurities, raises the carrier concentration, and results in an improved power factor (PF). The calculation reveals that Cu doping downshifts the Fermi energy level, reduces the energy band gap and the difference among several valence band maximums, and thereby explains the improvement of PF. In addition, Cu doping reduces the thermal conductivity, possibly attributed to the inhibition of Ag2Te impurities and the phonon softening of the AgSb1-xCuxTe2. Overall, Cu doping improves the ZT of AgSb1-xCuxTe2. Among all samples, AgSb0.96Cu0.04Te2 has a maximum ZT of ∼1.45 at 498 K and an average ZT of ∼1.11 from 298 to 573 K.

A simple Urea approach to N-doped α-Mo2C with enhanced superconductivity

Abstract Chemical doping is a critical factor in the development of new superconductors or optimizing the superconducting transition temperature (T c) of the parent superconducting materials. Herein, a new simple urea approach is developed to synthesize the N-doped α-Mo2C. Benefiting from the simple urea method, a broad superconducting dome is found in the Mo2C1-x N x (0 ≤ x ≤ 0.49) compositions. XRD results show that the structure of α-Mo2C remains unchanged and that there is a variation of lattice parameters with nitrogen doping. Resistivity, magnetic susceptibility, and heat capacity measurement results confirm that the superconducting transition temperature (T c) was strongly increased from 2.68 K (x = 0) to 7.05 K (x = 0.49). First-principles calculations and our analysis indicate that increasing nitrogen doping leads to a rise in the density of states at the Fermi level and doping-induced phonon softening, which enhances electron-phonon coupling. This results in an increase in T c and a sharp rise in the upper critical field. Our findings provide a promising strategy for fabricating transition metal carbonitrides and provide a material platform for further study of the superconductivity of transition metal carbides.

Ultralow Lattice Thermal Conductivity and Large Glass-Like Contribution in Cs3Bi2I6Cl3: Rattling Atoms and p-Band Electrons Driven Dynamic Rotation.

Understanding the origin of ultralow lattice thermal conductivity κL of halide perovskites is of great significance in the energy conversion field. The soft phonon modes and the large anharmonicity corresponding to the dynamic rotation of halogen atoms play an important role in limiting the thermal transport of halide perovskites. The dynamic rotation has long been thought to originate from the electrostatic repulsion of lone pairs around halogen atoms. Here, by studying the layered perovskite Cs3Bi2I6Cl3, it is found that the interaction between the lone pairs contributed by the s bands of halogen atoms is short-range, and the dynamic rotation is really driven by the occupied p-band electrons. It dominates Cs3Bi2I6Cl3 with ultralow κL, < 0.2 W mK-1 at 300 K. Moreover, soft optical phonons are presented ≈1 and 2.2 THz that constitute relatively flat and dense bands due to the rattling Cs and Cl atoms, contributing a large glass-like component to the κL. The results have important implications for understanding the origin of the ultralow κL in halide perovskites and for designing novel perovskites to serve the energy conversionfield.

Structural distortion-induced low-temperature dielectric dispersion in lanthanide titanate pyrochlores

Rare-earth titanate pyrochlores have attracted significant attention for their unique magnetic frustration; however, research on the origin of low-temperature dielectric dispersion and the relationship between dielectric properties and structure lags far behind. Here, by systematically investigating the dielectric properties of representative rare-earth titanates R2Ti2O7 (R = La, Nd, Sm, Er, Yb, and Lu), we demonstrate that R2Ti2O7 with a cubic pyrochlore structure exhibits low-temperature dielectric dispersion behavior, while the other compounds with a monoclinic perovskite-like layered structure possess no dispersion behavior but excellent temperature-stable dielectric property. The dielectric dispersion in cubic pyrochlores arises from the structural distortion. Furthermore, the existence of structural distortion is affirmed by the anomalous phonon softening of A1g Raman mode around the dielectric dispersion temperature, and the origin of the structural distortion is attributed to anharmonic phonon–phonon interactions induced by intrinsic vacant oxygen at Wyckoff 8a sites. In addition, with increasing ionic radius from R = Lu to Sm, the increased lattice parameter leads to varied bond length and bond angle of Ti-O(1)-Ti, which strengthens the local lattice distortion of TiO(1)6 octahedra and thus enhances diffusion degree of dielectric dispersion. On the other hand, the absence of intrinsic vacant oxygen site hardly gives rise to the local structural distortion and thus no dielectric dispersion in monoclinic R2Ti2O7. Our work not only clarifies the mechanism of dielectric dispersion but also gives a comprehensive perspective on the structure–property relationship of rare-earth titanates R2Ti2O7, and thus lays a solid foundation for further work on related materials.

Size-Dependent Spin Crossover and Bond Flexibility in Metal-Organic Framework Nanoparticles.

Size reduction offers a synthetic route to tunable phase change behavior. Preparing materials as nanoparticles causes drastic modulations to critical temperatures (Tc), hysteresis widths, and the "sharpness" of first-order versus second-order phase transitions. A microscopic picture of the chemistry underlying this size dependence in phenomena ranging from melting to superconductivity remains debated. As a case study with broad implications, we report that size-dependent spin crossover (SCO) in nanocrystals of the metal-organic framework (MOF) Fe(1,2,3-triazolate)2 arises from metal-linker bonds becoming more labile in smaller particles. In comparison to the bulk material, differential scanning calorimetry indicates a ∼ 30-40% reduction in Tc and ΔH in the smallest particles. Variable-temperature vibrational spectroscopy reveals a diminished long-range structural cooperativity, while X-ray diffraction evidence an over 3-fold increase in the thermal expansion coefficients. This "phonon softening" provides a molecular mechanism for designing size-dependent behavior in framework materials and for understanding phase changes in general.

The physical properties of crystalline iron-rich silicides

The formation enthalpy, electronic structure, lattice dynamics and elasticity of several complex ordered iron-rich silicides (Fe11Si5, Fe13Si3, Fe17Si12 and Fe24Si5) are calculated from the first-principles calculations. It is found that the Debye temperature ΘD of the system decreases with the increase of the ratio of the number of iron atoms to the number of silicon atoms in the same structure. The softening of low-energy acoustic phonons is more obvious with the increase of iron atoms. The elastic constants C11 and C44, Young's modulus E and shear modulus G of the system decrease with the increase of iron atoms. An anomalous phenomenon occurs in the high frequency region of Fe24Si5. The contribution of Fe1 atoms with smaller magnetic moment to the phonon density of states is much larger than that of the Si and other Fe atoms. This indicates that the local metal bond strength between Fe1-Fe1 atoms is stronger.

On the Strong Composition Dependence of the Martensitic Transformation Temperature and Heat in Shape Memory Alloys.

General derivation of the well-known Ren-Otsuka relationship, 1αdTodx=-αβ (where To, x, α and β(>0) are the transformation temperature and composition, as well as the composition and temperature coefficient of the critical shear constant, c', respectively) for shape memory alloys, SMAs, is provided based on the similarity of interatomic potentials in the framework of dimensional analysis. A new dimensionless variable, tox=ToxTmx, describing the phonon softening (where Tm is the melting point) is introduced. The dimensionless values of the heat of transformation, ΔH, and entropy, ΔS, as well as the elastic constants c', c44, and A=c44c' are universal functions of to(x) and have the same constant values at to(0) within sub-classes of host SMAs having the same type of crystal symmetry change during martensitic transformation. The ratio of dtodx and α has the same constant value for all members of a given sub-class, and relative increase in c' with increasing composition should be compensated by the same decrease in to. In the generalized Ren-Otsuka relationship, the anisotropy factor A appears instead of c', and α as well as β are the differences between the corresponding coefficients for the c44 and c' elastic constants. The obtained linear relationship between h and to rationalizes the observed empirical linear relationships between the heat of transformation measured by differential scanning calorimetry (DSC) (QA⟶M) and the martensite start temperature, Ms.

Correlation between optical phonon softening and superconducting Tc in YBa2Cu3Ox within d-wave Eliashberg theory

Abstract We provide a mathematical description, based on d-wave Eliashberg theory, of the strong correlation between the experimentally observed softening of Raman modes associated with in-plane oxygen motions and the corresponding superconducting critical temperature T c , as a function of oxygen doping x, in YBa2Cu3O x . The theoretical model provides a direct link between physical trends of soft optical Ag (in-plane) oxygen modes, the level of oxygen doping x, and the superconducting T c . Different regimes observed in the trend of T c vs doping can be related to corresponding regimes of optical phonon softening in the Raman spectra. These results provide further evidence related to the physical origin of high-temperature superconductivity in rare-earth cuprate oxides and to the significant role of electron–phonon coupling therein.

Ultralow Thermal Conductivity in Vacancy-Ordered Halide Perovskite Cs3Bi2Br9 with Strong Anharmonicity and Wave-Like Tunneling of Low-Energy Phonons.

Halide perovskites are of great interest due to their exceptional optical and optoelectronic properties. However, thermal conductivity of many halide perovskites remains unexplored. In this study, an ultralow lattice thermal conductivity κL (0.24W m-1 K-1 at 300 K) is reported and its weak temperature dependence (≈T-0.27) in an all-inorganic vacancy-ordered halide perovskite, Cs3Bi2Br9. The intrinsically ultralow κL can be attributed to the soft low-lying phonon modes with strong anharmonicity, which have been revealed by combining experimental heat capacity and Raman spectroscopy measurements, and first-principles calculations. It is shown that the highly anharmonic phonons originate from the Bi 6s2 lone pair expression with antibonding states of Bi 6s and Br 4p orbitals driven by the dynamic BiBr6 octahedral distortion. Theoretical calculations reveal that these low-energy phonons are mostly contributed by large Br motions induced dynamic distortion of BiBr6 octahedra and large Cs rattling motions, verified by the synchrotron X-ray pair distribution function analysis. In addition, the weak temperature dependence of κL can be traced to the wave-like tunneling of phonons, induced by the low-lying phonon modes. This work reveals the strong anharmonicity and wave-like tunneling of low-energy phonons for designing efficient vacancy-ordered halide perovskites with intrinsically low κL.

Insights into enhanced thermoelectric performance of the n-type Mg3Sb2-based materials by amphoteric Al doping

Doping has the potential to alter the levels of anharmonicity in compounds by attenuating bonding strength. In this study, we explore the efficacy of amphoteric Al doping for stimulating anharmonicity in n-type Mg3.25AlxSb1.48Bi0.48Te0.04 to attain enhanced phonon scattering and thermoelectric performance. First-principles calculations and experimental data reveal the occupation of both Sb and Mg2 sites by amphoteric Al atoms in the anionic framework of Mg3.25AlxSb1.48Bi0.48Te0.04. A marginal variation in both carrier concentration and mobility sustains the high power factor without affecting the Seebeck coefficient, implying amphoteric doping induced charge compensation. While phonon velocity, Grüneisen parameter, and crystal orbital Hamilton population calculations results indicate that phonon softening and bond weakening are realized via Al doping, leading to an enhanced lattice anharmonicity and a reduced lattice thermal conductivity. A remarkable enhancement ∼16% in the peak figure of merit ZTpeak and the average ZTavg, was attained for the x = 0.015 sample, when compared with the un-doped sample. Hence, the amphoteric doping can serve as an effective means to optimize ZT values by decoupling the intertwined thermoelectric transport properties.

Coherent Twin Boundary Induced Phonon Softening in Boron Arsenide

Coherent Twin Boundary Induced Phonon Softening in Boron Arsenide

Two-Dimensional-Like Phonons in Three-Dimensional-Structured Rhombohedral GeSe-Based Compounds with Excellent Thermoelectric Performance.

The coupling of charge and phonon transport in solids is a long-standing issue for thermoelectric performance enhancement. Herein, two new narrow-gap semiconductors with the same chemical formula of GeSe0.65Te0.35 (GST) are rationally designed and synthesized: one with a layered hexagonal structure (H-GST) and the other with a non-layered rhombohedral structure (R-GST). Thanks to the three-dimensional (3D) network structure, R-GST possesses a significantly larger weighted mobility than H-GST. Surprisingly, 3D-structured R-GST displays an extremely low lattice thermal conductivity of ∼0.5 W m-1 K-1 at 523 K, which is comparable to that of layered H-GST. The two-dimensional (2D)-like phonon transport in R-GST stems from the unique off-centering Ge atoms that induce ferroelectric instability, yielding soft polar phonons, as demonstrated by the Boson peak detected by the low-temperature specific heat and calculated phonon spectra. Furthermore, 1 mol % doping of Sb is utilized to successfully suppress the undesired phase transition of R-GST toward H-GST at elevated temperatures. Consequently, a peak ZT of 1.1 at 623 K is attained in the rhombohedral Ge0.99Sb0.01Se0.65Te0.35 sample, which is 1 order of magnitude larger than that of GeSe. This work demonstrates the feasibility of exploring high-performance thermoelectric materials with decoupled charge and phonon transport in off-centering compounds.

Revealing a distortive polar order buried in the Fermi sea.

Polar metals are challenging to identify spectroscopically because the fingerprints of electric polarization are often obscured by the presence of screening charges. Here, we unravel unambiguous signatures of a distortive polar order buried in the Fermi sea by probing the nonlinear optical response of materials driven by tailored terahertz fields. We apply this strategy to investigate the topological crystalline insulator Pb1-xSnxTe, tracking its soft phonon mode in the time domain and observing the occurrence of inversion symmetry breaking as a function of temperature. By combining measurements across the material's phase diagram with ab initio calculations, we demonstrate the generality of our approach. These results highlight the potential of terahertz driving fields to reveal polar orders coexisting with itinerant electrons, thus opening additional avenues for material discovery.

A comprehensive investigation of Kingery type Σ3 (111) grain boundaries in TiC, TaC, and WC

Carbide materials find extensive application as tool materials due to their exceptional properties, encompassing high hardness, wear resistance, and robust strength. Exhibiting diverse bonding characteristics, carbides constitute a pivotal material class in modern-day technologies. The structural and energetic attributes of grain boundaries play a vital role in understanding the properties of polycrystalline materials, necessitating an examination of the grain boundary regime. In this study, metal carbide grain boundaries, specifically the Kingery type Σ3 (111) grain boundaries in TiC, TaC, and WC, were systematically investigated utilizing density functional theory calculations. The scrutiny comprehends their structural, electronic, mechanical, and thermal properties. Results show that the investigated grain boundaries are energetically stable. The calculated electronic properties further unveil the presence of metallic behavior. Dynamic stability was observed at 0 K for Σ3 (111) grain boundaries constructed from the L10 cubic lattice structure in TiC, while phonon softening was observed for the Σ3 (111) grain boundaries structures in TaC and WC. Thermodynamic properties, including Helmholtz free energy, heat capacity, and entropy, were thoroughly scrutinized. Mechanical properties suggest the ductile behavior with significant metallic nature of considered grain boundary systems. The study extended to thermal properties, examining the lattice thermal conductivity (κL) of the considered grain boundary systems. It was observed that the Σ3 (111) grain boundary in TiC significantly increases the thermal conductivity.

Phonon-mediated spin transport in quantum paraelectric metals

The concept of ferroelectricity is now often extended to include continuous inversion symmetry-breaking transitions in various metals and doped semiconductors. Paraelectric metals near ferroelectric quantum criticality, which we term ‘quantum paraelectric metals,’ possess soft transverse optical phonons which can have Rashba-type coupling to itinerant electrons in the presence of spin-orbit coupling. We find through the Kubo formula calculation that such Rashba electron-phonon coupling has a profound impact on electron spin transport. While the spin Hall effect arising from non-trivial electronic band structures has been studied extensively, we find here the presence of the Rashba electron-phonon coupling can give rise to spin current, including spin Hall current, in response to an inhomogeneous electric field even with a completely trivial band structure. Furthermore, this spin conductivity displays unconventional characteristics, such as quadrupolar symmetry associated with the wave vector of the electric field and a thermal activation behavior characterized by scaling laws dependent on the phonon frequency to temperature ratio. These findings shed light on exotic electronic transport phenomena originating from ferroelectric quantum criticality, highlighting the intricate interplay of charge and spin degrees of freedom.