Computer Simulation Studies Of Electron-Phonon And Phonon-Phonon Couplings Of Inorganic Functional Materials

The relationship between lattice vibration and physical properties of material is a focus of condensed-matter physics research,and also is a key factor to consider when performing the functional material design.In crystal,atoms vibrate around their equilibrium positions--it is the natural interpretation of temperature.Here exists many couplings between phonons and other excitations,and most important couplings are electron-phonon and phonon-phonon couplings.They play a key role in determining the physical properties of material related with electronic and phononic transport,such as,metallic conductivity,superconductivity and thermal conductivity et.al.In this paper,we selected high-pressure hydrides and skutterudite FeSb₃ as the investigative objects of the electron-phonon and phonon-phonon couplings,and performed systematically researches of transport properties from density-functional theory?DFT?calculations.The details are follows:1.We predicted two stable superconducting hydrides AsH₈ and SbH₄?Tc > 100K?,and found the chemical trends of the compressed hydrides.We systematically explored the high pressure phase structure of hydrides by using global optimization structure searching in combination with first-principles calculations,and obtained the high-pressure phase diagram.At high pressure,thermodynamic stability of pnictogen hydrides have a reversal?P-H < As-H < Sb-H?.Except for the molecular solid phases stabilized by weak van der Waals interactions around ambient pressure,the P hydrides are found to be unstable against decomposition into the elements,and pressure doesn't have a noticeable trend on their stability,even 400 GPa.We predicted two stable H-rich hydrides?AsH₈ and SbH₄?among the Asand Sbhydrides.Recently,We found a experimental report thant they observated a high Tc?>100 K?superconductivity in P hydrides above 200 GPa preliminarily.However,based on our structure earching results,P hydrides formed in the experiments may be metastable and could be stabilized by kinetic processes at high pressures.We performed the electron-phonon coupling?EPC?calculations for the predicted stable compounds.The superconducting Tc value of AsH₈ and SbH₄ are more than 100 K.Especially,SbH₄ is energetically very stable and adopts a highly symmetrical hexagonal structure,and the stabilization pressure?> 150GPa?is well within reach of experiments.We noted a similarly theoretical work in SbH₄ by Ma et al.Using a structure searching method based on the genetic algorithm,they obtained the same results as that we have found in our work.By using the data mining of the known binary hydrides,we found electronegativity difference between the constituent elements at ambient pressure,related with structural features,chemical properties,energetic stability and superconducting of hydrides.Our work provides a useful roadmap for discovering more stable hydride solids and exploration of their superconducting properties.2.The pure skutterudite of FeSb₃ with no filler has an intrinsic ultralow l?,and our results updated in the understanding of physical mechanism for the reduction of l? in the filled skutterudites.By solving first-principles Boltzmann-Peierls transport equation of phonons,we systematically explored the thermal transport properties of the skutterudite FeSb₃,and found it has an ultralow l??1.14W/mK@300K?,one order of magnitude lower than that of CoSb₃.And it is even lower than l? of most fully filled skutterudites.This is in contrast to the widely used approach where filling is used to reduce l? in skutterudites.We found the origin of ultralow l? is attributed to the overall softening of phonon spectrum,especially the optical phonons with a remarkable decrease in frequency associated with the weaker Sb-Sbbonds in FeSb₃?This is consistent with the fact that Young's modulus of FeSb₃ is smaller than that of CoSb₃?.These softening phonons can be seen as “rattler” to increase the phase space of three-phonon anharmonic scattering processes,resulting in significantly reduced phonon lifetimes.Our results offer insight into the still-debated mechanism responsible for the l? reduction upon filling in skutterudites and also a routine for lowering the l? of skutterudite-type thermoelectric materials.3.Electronegative Sn filled skutterudite SnFe₄Sb₁₂ has an intrinsic ultralow l?,and our results provided an new routine to improve the thermoelectric properties of skutterudites.We selected electronegative Sn as the filler,and studied structural features,chemical properties,lattice dynamics and thermal transport.We found Sn is off-center when entering to the void of FeSb₃?In contrast,based on the results of severl experiments,Sn is still center when entering to the void of CoSb₃?,and weakly bonds with neighbor Sb.Surprisingly,it exists prominent collective rattling vibrations between Sn and Sb.Sn's off-center behavior will induce the distortion of skeleton.By analyzing the potential surface and dynamics feature of Sn,we found that Sn has the “Goldstone” phonon mode.It is the reason why SnFe₄Sb₁₂ has such an ultralow l??0.69W/mK@300K?.In addition,we designed the double-filling to tune the low phonon frequency of Sn.We offer a new perspective how to manually control the LA+TO-LO coupling,and also a technical route to synthesize the high-performance thermoelectric material experimentally.