Pressure effects on the electrical transport and anharmonic lattice dynamics of r-GeTe:A first-principles study

Various strategies for thermoelectric material optimization have been widely studied and used for promoting electrical transport and suppressing thermal transport.As a nontraditional method,pressure has shown great potential,as it has been applied to obtain a high thermoelectric figure of merit,but the microscopic mechanisms involved have yet to be fully explored.In this study,we focus on r-GeTe,a lowtemperature phase of GeTe,and investigate the pressure effects on the electronic structure,electrical transport properties and anharmonic lattice dynamics based on density functional theory(DFT),the Boltzmann transport equations(BTEs)and perturbation theory.Electronic relaxation times are obtained based on the electron-phonon interaction and the constant relaxation time approximation.The corresponding electrical transport properties are compared with those obtained from previous experiments.Hydrostatic pressure is shown to increase valley degeneracy,decrease the band effective mass and enhance the electrical transport property.At the same time,the increase in the low-frequency phonon lifetime and phonon group velocity leads to an increase in lattice thermal conductivity under pressure.This study provides insight into r-GeTe under hydrostatic pressure and paves the way for a high-pressure strategy to optimize transport properties.

Rapid laser synthesis of surfactantless tantalum-based nanomaterials as bifunctional catalysts for direct peroxide–peroxide fuel cells

Efficient and durable electrocatalysts are instrumental in enabling next-generation fuel cell technologies.At present,expensive precious metals are used as state-of-the-art catalysts.In this report,cost-effective nanosized tantalum-based alternatives are synthesized for the first time via a green and scalable laser pyrolysis method as bifunctional catalysts for direct peroxide–peroxide fuel cells.This rapid laser pyrolysis strategy allows for the production of nanoparticles at a laboratory scale of grams per hour,compatible with a detailed exploration of the functional properties of as-synthesized nanoparticles.By varying the precursor ratio between ammonia and tantalum ethanolate,five tantalum-based nanomaterials(TaNOC)are prepared with crystalline phases of Ta_(2)O_(5),Ta_(4)N_(5),Ta_(3)N_(5),and TaN in tunable ratios.Electrochemical studies in neutral and alkaline conditions demonstrate that Ta_(4)N_(5) is the active component for both H_(2)O_(2) oxidation and reduction.Kinetic isotope effect studies show that protons are involved at or before the rate-determining step.Long-term stability studies indicate that Ta_(3)N_(5) grants surfactant-free TaNOC-enhanced longevity during electrocatalytic operations.Taken together,bifunctional TaNOC can act as active and robust electrocatalysts for H_(2)O_(2) reduction and oxidation.Laser pyrolysis is envisioned to produce refractory metal nanomaterials with boosted corrosion resistance for energy catalysis.

Effect of group-3 elements doping on promotion of in-plane Seebeck coefficient of n-type Mg_(3)Sb_(2)

Mg_(3)Sb_(2)-based alloys are promising thermoelectric materials with a reasonably low thermal conductivity.However,their electrical transport property is usually limited by the low carrier concentration.Mg_(3)Sb_(2) has a multi-valley conduction band with a six-fold degeneracy,benefiting n-type thermoelectric performance.Recently,n-type Y-doped Mg_(3)Sb_(1.5)Bi_(0.5) and Sc-doped Mg_(3)Sb_(2)-Mg_(3)Bi_(2) alloys show a large figure of merit(ZT).In this paper,the doping effect of group-3 and chalcogen elements on the electronic structures and electrical transport properties of Mg_(3)Sb_(2) was investigated via the first-principles calculations.Chalcogen elements have a slight effect on the electronic structure,and Te-doped Mg_(3)Sb_(2) shows better normalized power factors in both the out-of-plane and in-plane directions,compared to the Sdoped and Se-doped systems.Distinctly different doping effects appear in Mg_(3)Sb_(2) doped with group-3 elements.A increased density of states near the bottom of the conduction band can be induced by Sc or Y.Sc-doped and Y-doped Mg_(3)Sb_(2) show higher normalized power factors along the in-plane direction than those doped with chalcogens.

Negative differential friction coefficients of two-dimensional commensurate contacts dominated by electronic phase transition

Friction force(f)usually increases with the normal load(N)macroscopically,according to the classic law of Da Vinci–Amontons(f=μN),with a positive and finite friction coefficient(μ).Herein near-zero and negative differential friction(ZNDF)coefficients are discovered in two-dimensional(2D)van der Waals(vdW)magnetic CrI_(3)commensurate contacts.It is identified that the ferromagnetic–antiferromagnetic phase transition of the interlayer couplings of the bilayer CrI_(3)can significantly reduce the interfacial sliding energy barriers and thus contribute to ZNDF.Moreover,phase transition between the in-plane(p_(x)and p_(y))and out-of-plane(p_(z))wave-functions dominates the sliding barrier evolutions,which is attributed to the delicate interplays among the interlayer vdW,electrostatic interactions,and the intralayer deformation of the CrI_(3)layers under external load.The present findings may motivate a new concept of slide-spintronics and are expected to play an instrumental role in design of novel magnetic solid lubricants applied in various spintronic nano-devices.

Soft-mode dynamics in the ferroelectric phase transition of GeTe

GeTe that exhibits a strong anharmonicity and a ferroelectric phase transition between the rhombohedral and cubic structures has emerged as one of the leading thermoelectric materials.Herein,combining molecular dynamics simulations and inelastic neutron scattering measurements,the lattice dynamics in GeTe have been investigated to reveal the soft-mode mechanisms across the phase transition.We have constructed a first-principles-based machine-learning interatomic potential,which successfully captures the dynamical ferroelectric phase transition of GeTe by adopting the neural network technique.Although the low-energy acoustic phonons remain relatively unaffected at elevated temperatures,the high-energy optical,and longitudinal acoustic phonons demonstrate strong renormalizations as evidenced from the vibrational phonon spectra,which are attributed to the large anharmonicity accompanying the phase transition.Furthermore,our results reveal a nonmonotonic temperature dependence of the soft-modes beyond the perturbative regime.The insight provided by this work into the soft-modes may pave the way for further phonon engineering of GeTe and the related thermoelectrics.

Highly selective phonon diffusive scattering in superionic layered AgCrSe_(2)

Superionic materials that exhibit coexistence of rigid crystalline lattices and liquid-like fluctuating substructures have emerged as promising thermoelectric materials.The inadequate understanding of the phonon behavior in the superionic state,however,still prevents further revealing of the underlying correlation between the thermally induced liquid-like atomic dynamics and anomalous thermal transport properties.Herein,by adopting a hybrid scheme to directly characterize anharmonic phonon quasiparticles from ab-initio molecular dynamics,we manifest that low-energy transverse phonons dominated by Ag atoms totally collapse,whereas longitudinal optical phonons remain largely intact during the superionic transition.