Electron Ab Research Articles

Orientation-related shear banding mediated deformation of Sm2Co17-type magnet

The service life and reliability of Sm2Co17-type permanent magnets in high-temperature energy conversion fields are significantly deteriorated due to their mechanical fragility. Herein, the deformation mechanisms of a strongly anisotropic Sm2Co17-type magnet are probed. Nanoindentation results suggest a weak anisotropy of hardness while a strong anisotropy of incipient plasticity related to crystal orientations, namely the nucleation of atomic shear bands working as plastic carriers is the easiest on {101̅0} planes but most difficult on {0001} planes. A statistical model on the critical shear stress of incipient plasticity indicates that the generation of shear band embryos is mainly processed via homogeneous nucleation from bond breakage. Transmission electron microscopy and ab initio calculations confirm that atomic bands along the pyramidal {011̅1} or basal {101̅0} planes primarily accommodate the deformation for loading direction along or perpendicular to the c-axis, respectively. The thickness of the {011̅1} and {101̅0} shear bands are in the range of 2–4 atomic layers. Nevertheless, due to the blocking effects of 1:3R Z-platelets and 2:17R twins on the progress of {011̅1} shear bands, the lengths of {011̅1} shear bands are much shorter than {101̅0} bands. Our work offers insights into the atomic-scale deformation mechanisms of Sm2Co17-type magnets.

Evaluation of computed tomography anode voltage in clinical application: the influence of profiled filters

Monitoring of operational parameters of X-ray diagnostic equipment, including computer tomographs, is mandatory during acceptance and periodic tests in medical organizations. The importance of control of anode voltage of X-ray radiator is described (as one of the main technical parameters in diagnostic tests of patients, which can influence the quality of visualization and safe operation of the equipment). Anode voltage is evaluated by non-invasive methods using universal dosimeters in the user mode of equipment operation. The influence of profiled filters of Somatom Definition AS, SOMATOM go.All (Siemens, Germany) scanners on the accuracy of anode voltage measurement was investigated. The tests were conducted in clinic conditions without partial removal of the gantry (a structural element of the tomograph, inside which the system “X-ray emitter – detector” is located) and connection to the high-voltage circuit of the device. The distance between the sensitive elements of the measuring device and the displacement of the center of the sensing area relative to the position of the central axis of the X-ray beam of computed tomography scanner were taken into account. Normalized curves of kerma values, anode voltage values and their approximating analytical functions were obtained using a Piranha R&F/M 657 universal dosimeter (RTI Electronics AB, Sweden) at its displacement relative to the laser centralizer of computed tomography scanner. Based on the scheme for noninvasive estimation of anode voltage value, a formula of the relative deviation of the measured value of the anode voltage associated with the presence of a profiled filter at the output of the X-ray emitter of the tomograph and the displacement of the center point of the sensitive area of the measuring device relative to the central axis of the X-ray beam of the tomograph is obtained. The validity of the data based on the formula is confirmed experimentally. The results of the study will be helpful to technical specialists who maintain or control the X-ray computed tomography operating parameters.

Origins of Severe Structural Changes during Alloying-Dealloying Reactions in Black Phosphorus.

Li-alloying reactions facilitate the incorporation of a large number of Li atoms into the crystalline structures of electrodes, such as black phosphorus (BP). However, the reactions inevitably induce multistep phase transitions characterized by drastic atomic rearrangements and lattice collapse. Despite many theoretical and experimental studies on alloying mechanisms, long-term debates persist regarding the structures of the intermediate phases, the accurate pathways of phase transitions, the formation of specific configurations, and alloying/dealloying reversibility. Here, through a combination of operando electron diffraction measurements and ab initio simulations at the atomic and electronic scales, we identify key factors that govern the severe structural changes during alloying-dealloying reactions in BP. P-P bonds of three-bond P atoms are continuously broken during lithiation, generating two-bond P atoms with a high ability to accept inserted electrons and Li ions. Consequently, the pristine layered structure in BP is transformed to P7 cages in Li3P7, which then evolve to chain configurations in LiP and finally to isolated P atoms in Li3P. Specifically, the preferential formation of the P7 cage results from its lowest binding energy with three Li ions compared to other cage isomers. Furthermore, only LiP can be reversibly transformed to the crystalline structure of Li3P7 during charge, but it is thermodynamically favorable for Li3P7 and Li3P intermediates to be delithiated to amorphous structures. Our findings offer unique insights into the alloying mechanisms and deepen the fundamental understanding of alloying anode systems.

(η8-Cyclooctatetraene)(η5-fluorenyl)titanium: a processable molecular spin qubit with optimized control of the molecule-substrate interface.

Depositing single paramagnetic molecules on surfaces for sensing and quantum computing applications requires subtle topological control. To overcome issues that are often encountered with sandwich metal complexes, we exploit here the low symmetry architecture and suitable vaporability of mixed-sandwich [FluTi(cot)], Flu = fluorenyl, cot = cyclooctatetraene, to drive submonolayer coverage and select an adsorption configuration that preserves the spin of molecules deposited on Au(111). Electron paramagnetic resonance spectroscopy and ab initio quantum computation evidence a d z 2 ground state that protects the spin from phonon-induced relaxation. Additionally, computed and measured spin coherence times exceed 10 μs despite the molecules being rich in hydrogen. A thorough submonolayer investigation by scanning tunneling microscopy, X-ray photoelectron and absorption spectrocopies and X-ray magnetic circular dichroism measurements supported by DFT calculations reveals that the most stable configuration, with the fluorenyl in contact with the metal surface, prevents titanium(iii) oxidation and spin delocalization to the surface. This is a necessary condition for single molecular spin qubit addressing on surfaces.

A Complete Unveiling of the Mechanism and Chirality in Photoisomerization of Arylazopyrazole 3pzH: Combined Electronic Structure Calculations and AIMS Dynamic Simulations.

The arylazopyrazole 3pzH as a novel photoswitch exhibits quantitative switching and high thermal stability. In this work, combined electronic structure calculations and ab initio multiple spawning (AIMS) dynamic simulations were performed to systemically investigate the cis ↔ trans photoisomerization mechanism and the chiral preference after photoexcitation of 3pzH to the first excited singlet state (S1). Unlike most of the azoheteroarene photoswitches reported previously, many twisted and T-shaped cis isomers were found to be stable for 3pzH in the S0 state, owing to the moderate interaction between the hydrogen atom and π electrons of the aromatic ring. Two twisted cis isomers with different chirality ((M)-Z1 and (P)-Z1), the most stable T-shaped cis isomer ((T)-Z2), and the most stable planar trans isomer (E2) were selected as the initial structures to carry out the AIMS nonadiabatic dynamic simulations. Following excitation to the S1 state, all of the cis isomers decayed to conical intersection (CI) regions via the same bicycle pedal mechanism, while the evolution of the trans isomers to their CI regions was achieved via rotation around the N═N bond. More importantly, chiral preferences were found for the twisted cis isomers in the S1 state through the AIMS dynamic simulations due to the steric effect and static electronic repulsion. Notably, chirality was also observed in S1 isomerization starting from the planar E2 isomer because of the dynamic effect. After the nonadiabatic transition to the S0 state, the bicycle pedal mechanism was found to play a crucial role in cis ↔ trans photoisomerization. The simulated photoisomerization productivities were generally consistent with past experimental observations. Our calculations not only uncover the underlying reason for the excellent photoswitching properties of 3pzH but also enrich the knowledge of photoisomerization for azoheteroarene photoswitches, which will surely benefit their rational design.

Enhancing oxidation resistance of MoAlB based on its anisotropic oxidation mechanism: A theoretical calculation guided experimental investigation

MoAlB is considered the most promising material for oxidation resistance among the MAB phase family. However, the anisotropic oxidation mechanism at the atomic scale has not been fully investigated, which may limit its practical applications. In this work the anisotropic oxidation mechanism of MoAlB was first explored through systematic theoretical calculations, based on which an effective strategy to enhance its oxidation resistance was identified and successfully implemented in the experimental work. The theoretical calculation revealed that the anisotropic oxidation of MoAlB was primarily related to the difference in free energy of surface oxidation reactions due to the degrees of electron interaction localization of O-Al bonding. A series of candidates of various transition metal-doped MoAlB materials were then designed to improve the weaker surface of MoAlB for oxidation resistance enhancement. Among these candidates, Zr-doped MoAlB was selected to further evaluate the effects of doping on the anisotropic oxidation through the electronic structure and Ab initio molecular dynamics (AIMD). Finally, Zr-doped MoAlB was experimentally synthesized to validate the theoretical design. Non-isothermal oxidation kinetics analysis indicated that Zr doping can enhance the oxidation resistance of MoAlB by promoting the formation of a protective oxide scale and increasing the formation activation energy of volatile products, which finally reduced the thickness of the oxidation scale on the surface after isothermal oxidation of the bulk material. This work not only contributes to an in-depth understanding of the oxidation mechanisms in MoAlB but also presents a new approach to enhance its oxidation resistance and other Al containing ceramic materials beyond MAB phases.

Comparing eDQE and eNEQ metrics – is there an alternative approach to assessing image quality in digital mammography?

Abstract Introduction: Early detection of breast cancer requires high-quality mammographic images that have been made possible by the introduction of new technologies, such as full-field digital mammography (FFDM). In this new study, we perform extended measurements to calculate effective detective quantum efficiency (eDQE) and introduce effective noise equivalent quanta (eNEQ). Our aim was to show how these two metrics relate to the image quality of two digital mammography systems. Material and methods: Measurements were performed for a Siemens Mammomat Inspiration and a GE Senographe Pristina system. Each was equipped with an automatic exposure control (AEC) for use in a clinical setting. We used a polymethyl methacrylate (PMMA) phantom at thicknesses of 20, 30, 40 and 70 mm to cover the range of scatter conditions expected in mammography, with and without an anti-scatter grid. The Siemens system had an a-Se detector, and the GE system had an indirect-conversion detector. Measurements of Kerma were performed with Piranha Black 657 meter (RTI Electronics AB). The majority of our calculations were automated, using a modified version of our software. Results: For the two mammographic systems evaluated, we characterized physical quality parameters, such as effective modulation transfer function (eMTF), effective normalized noise power spectrum (eNNPS), eDQE and eNEQ for a wide range of exposures, phantom thicknesses, with and without an anti-scatter grid. Results are presented as a function of spatial frequency. A contrast-detail analysis was performed with a CDMAM 3.4 phantom with dedicated software (CDMAM analysis 1.5.5, NCCPM) and a set of different PMMA phantoms. Conclusions: We successfully demonstrated that the eNEQ metric can be used as a new option to evaluate image quality for images taken with and without a grid and with phantoms of different thicknesses for the Siemens and GE systems. These results were consistent with the results obtained from CDMAM.

Nanoarhitectonics of Inorganic-Organic Silica-Benzil Composites: Synthesis, Nanocrystal Morphology and Micro-Raman Analysis.

The synthesis of nanosized organic benzil (C6H5CO)2 crystals within the mesoporous SiO2 host matrix was investigated via X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and ab initio lattice dynamics analysis. Combining these methods, we have proved that the main structural properties of benzil nanocrystals embedded into SiO2 host membranes with pore diameters of 6.0, 7.8, 9.4, and 13.0 nm are preserved compared to a bulk benzil crystal. Space confinement has an insignificant impact on the lattice vibrational properties of benzil crystals implanted into the host matrices. The ab initio lattice dynamics calculation of the phonon spectrum in the Brillouin zone center shows the mechanical and dynamical stability of benzil lattice, revealing very low optical frequency of 11 cm-1 at point Γ.

Dynamics of Hydrogen Bond Breaking Induced by Outer-Valence Intermolecular Coulombic Decay

The photoexcitation of weakly bound complexes can lead to several decay pathways, depending on the nature of the potential energy surfaces. Upon excitation of a chromophore in a weakly bound complex, ionization of its neighbor upon energy transfer can occur due to a unique relaxation process known as intermolecular Coulombic decay (ICD), a phenomenon of renewed focus owing to its relevance in biological systems. Herein, we report the evidence for outer-valence ICD induced by multiphoton excitation by near-ultraviolet radiation of 4.4 eV photons, hitherto unknown in molecular systems. In the binary complexes of 2,6-difluorophenylacetylene with aliphatic amines, a resonant two-photon excitation localized on the 2,6-difluorophenylacetylene chromophore results in the formation of an amine cation following an outer-valence ICD process. The unique trends in experimentally observed translational energy distribution profiles of the amine cations following hydrogen bond dissociation, analyzed with the help of electronic structure and ab initio molecular dynamics calculations, revealed the presence of a delicate interplay of roaming dynamics, methyl-rotor dynamics, and binding energy.

Indirect Exciton–Phonon Dynamics in MoS2 Revealed by Ultrafast Electron Diffraction

AbstractTransition metal dichalcogenides layered nano‐crystals are emerging as promising candidates for next‐generation optoelectronic and quantum devices. In such systems, the interaction between excitonic states and atomic vibrations is crucial for many fundamental properties, such as carrier mobilities, quantum coherence loss, and heat dissipation. In particular, to fully exploit their valley‐selective excitations, one has to understand the many‐body exciton physics of zone‐edge states. So far, theoretical and experimental studies have mainly focused on the exciton–phonon dynamics in high‐energy direct excitons involving zone‐center phonons. Here, ultrafast electron diffraction and ab initio calculations are used to investigate the many‐body structural dynamics following nearly‐ resonant excitation of low‐energy indirect excitons in MoS2. By exploiting the large momentum carried by scattered electrons, the excitation of in‐plane K‐ and Q‐ phonon modes are identified with 𝑬′ symmetry as key for the stabilization of indirect excitons generated via near‐infrared light at 1.55 eV, and light is shed on the role of phonon anharmonicity and the ensuing structural evolution of the MoS2 crystal lattice. The results highlight the strong selectivity of phononic excitations directly associated with the specific indirect‐ exciton nature of the wavelength‐dependent electronic transitions triggered in the system.

One dimensional wormhole corrosion in metals

Corrosion is a ubiquitous failure mode of materials. Often, the progression of localized corrosion is accompanied by the evolution of porosity in materials previously reported to be either three-dimensional or two-dimensional. However, using new tools and analysis techniques, we have realized that a more localized form of corrosion, which we call 1D wormhole corrosion, has previously been miscategorized in some situations. Using electron tomography, we show multiple examples of this 1D and percolating morphology. To understand the origin of this mechanism in a Ni-Cr alloy corroded by molten salt, we combined energy-filtered four-dimensional scanning transmission electron microscopy and ab initio density functional theory calculations to develop a vacancy mapping method with nanometer-resolution, identifying a remarkably high vacancy concentration in the diffusion-induced grain boundary migration zone, up to 100 times the equilibrium value at the melting point. Deciphering the origins of 1D corrosion is an important step towards designing structural materials with enhanced corrosion resistance.

New modulated structures induced by electron beam irradiation in SrCrO3 single crystal

Three new modulated structures induced by controllable electron beam irradiation (EBI) were obtained from SrCrO3 single crystal. The crystal and electronic structures were investigated using aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy (EELS) and ab initio calculations. The EELS results revealed that EBI induced ordered oxygen vacancies and generated modulated structures SrCrO2.5 (I4/mmm), SrCrO2.8 (C2/m) and SrCrO2.67 (C2/m) from pristine SrCrO3 (Pmm). The corresponding structural models were proposed and modified by ab initio calculations. Additionally, the electronic structures of the optimized modulated structures were further investigated.

In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb2Te4 by in situ high-resolution transmission electron microscopy experiments and ab initio density functional theory calculations, providing a comprehensive real-time and real-space view of the vacancy ordering process. We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb2Te4, which is relevant to multilevel storage applications. The phase evolution paths in Ge-Sb-Te alloys and Sb2Te3 are illustrated using a summary diagram, which serves as a guide for designing phase-change memory devices.

Observation of intermolecular Coulombic decay and shake-up satellites in liquid ammonia.

We report the first nitrogen 1s Auger–Meitner electron spectrum from a liquid ammonia microjet at a temperature of ∼223 K (–50 °C) and compare it with the simultaneously measured spectrum for gas-phase ammonia. The spectra from both phases are interpreted with the assistance of high-level electronic structure and ab initio molecular dynamics calculations. In addition to the regular Auger–Meitner-electron features, we observe electron emission at kinetic energies of 374–388 eV, above the leading Auger–Meitner peak (3a12). Based on the electronic structure calculations, we assign this peak to a shake-up satellite in the gas phase, i.e., Auger–Meitner emission from an intermediate state with additional valence excitation present. The high-energy contribution is significantly enhanced in the liquid phase. We consider various mechanisms contributing to this feature. First, in analogy with other hydrogen-bonded liquids (noticeably water), the high-energy signal may be a signature for an ultrafast proton transfer taking place before the electronic decay (proton transfer mediated charge separation). The ab initio dynamical calculations show, however, that such a process is much slower than electronic decay and is, thus, very unlikely. Next, we consider a non-local version of the Auger–Meitner decay, the Intermolecular Coulombic Decay. The electronic structure calculations support an important contribution of this purely electronic mechanism. Finally, we discuss a non-local enhancement of the shake-up processes.

Optical spectroscopy and unusual temperature shift of f – f zero-phonon luminescence lines: KTaO3:Er

We present a study of KTaO3:Er (≈0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO3. We provide information about energy level structures controlling f – f optical transitions in Er3+, on formation of minor cubic octahedral and major orthorhombic Er3+ centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.

Concerted Double Hydrogen-Bond Breaking by Intermolecular Coulombic Decay in the Formic Acid Dimer.

Hydrogen bonds are ubiquitous in nature and of fundamental importance to the chemical and physical properties of molecular systems in the condensed phase. Nevertheless, our understanding of the structural and dynamical properties of hydrogen-bonded complexes in particular in electronic excited states remains very incomplete. Here, by using formic acid (FA) dimer as a prototype of DNA base pair, we investigate the ultrafast decay process initiated by removal of an electron from the inner-valence shell of the molecule upon electron-beam irradiation. Through fragment-ion and electron coincident momentum measurements and ab initio calculations, we find that de-excitation of an outer-valence electron at the same site can initiate ultrafast energy transfer to the neighboring molecule, which is in turn ionized through the emission of low-energy electrons. Our study reveals a concerted breaking of double hydrogen-bond in the dimer initiated by the ultrafast molecular rotations of two FA+ cations following this nonlocal decay mechanism.

Directed gas phase preparation of ethynylallene (H2CCCHCCH; X1A') via the crossed molecular beam reaction of the methylidyne radical (CH; X2Π) with vinylacetylene (H2CCHCCH; X1A').

The gas-phase bimolecular reaction of the methylidyne (CH; X2Π) radical with vinylacetylene (H2CCHCCH; X1A') was conducted at a collision energy of 20.3 kJ mol-1 under single collision conditions exploiting the crossed molecular beam experimental results merged with ab initio electronic structure calculations and ab initio molecular dynamics (AIMD) simulations. The laboratory data reveal that the bimolecular reaction proceeds barrierlessly via indirect scattering dynamics through long-lived C5H5 reaction intermediate(s) ultimately dissociating to C5H4 isomers along with atomic hydrogen with the latter predominantly originating from the vinylacetylene reactant as confirmed by the isotopic substitution experiments in the D1-methylidyne-vinylacetylene reaction. Combined with ab initio calculations of the potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, the experimental determined reaction energy of -146 ± 26 kJ mol-1 along with the distribution minimum of T(θ) at 90° and isotopic substitution experiments suggest ethynylallene (p1; ΔrG = -230 ± 4 kJ mol-1) as the dominant product. The ethynylallene (p1) may be formed with extensive rovibrational excitation, which would result in a lower maximum translational energy. Further, AIMD simulations reveal that the reaction dynamics leads to p1 (ethynylallene, 75%) plus atomic hydrogen with the dominant initial complex being i1 formed by methylidyne radical addition to the double CC bond in vinylacetylene. Overall, combining the crossed molecular beam experimental results with ab initio electronic structure calculations and ab initio molecular dynamics (AIMD) simulations, ethynylallene (p1) is expected to represent the dominant product in the reaction of the methylidyne (CH; X2Π) radical with vinylacetylene (H2CCHCCH; X1A').

Assessment of Regional Diagnostic Reference Levels in Dental Radiography in Tamil Nadu.

Aim:The aim of this article is to assess Tamil Nadu adult diagnostic reference levels (DRLs) by collecting radiation dose data from the four different dental modalities.Materials and Methods:The study was carried out using routine adult exposure settings in 131 intraoral, 75 panoramic, 35 cephalometric, and 10 dental cone beam computed tomography (CBCT) X-ray devices. DRLs were assessed for intraoral and extraoral (panoramic, cephalometric, and CBCT) examinations in terms of incident air kerma (Ka, i) and kerma area product (PKA), respectively. Air kerma measurements, for all dental units, were made using calibrated RTI black Piranha 557 dosimeter (RTI Electronics AB, Sweden). The dosimeter was kept at the exit cone of the X-ray tube and on the detector side of the X-ray unit for intraoral and extraoral air kerma measurements, respectively. The obtained air kerma in extraoral modalities is multiplied with the beam area to evaluate PKA.Results:The third quartile values calculated from the median for adult intraoral (mandibular molar), panoramic, cephalometric, and CBCT were 1.5 mGy, 116 mGycm2, 40 mGycm2, and 532 mGycm2, respectively. The proposed DRL in the present study was comparable to those reported in Germany, Greece, the UK, Japan, and Korea.Conclusion:This study revealed the need for dose management and radiation dose optimization, in various dental facilities in the state. It was also found that dental facilities employed with the digital type of detector are not always related to lower exposure.

Precipitate/matrix incompatibilities related to the {111}Al Ω plates in an Al-Cu-Mg-Ag alloy

The atomic structure of Ω plates forming on {111}Al planes in an Al-Cu-Mg-Ag alloy has been investigated by Z-contrast atomic-resolution scanning transmission electron microscopy imaging and ab initio density functional theory calculations. Ω plates with different thicknesses have been studied in two peak-aged conditions: 150 °C for 24 h and 190 °C for 1.5 h. Volumetric and structural incompatibilities as unrelaxed misfit strains and shear components, respectively, between the Ω plates involving orthorhombic θ-phase fragments and Al matrix were found to be in the plates with thicknesses from 0 to 2.5 cθ (a normal direction to {111}Al). Two types of shear components: [−101]Al // [0−10]θ (τI) and [1−21]Al // [100]θ (τII) related to precipitate/matrix structural incompatibilities have been predicted by calculations. The shear components τI and τII have been found to be energetically favorable in the plates with different thicknesses. Comparing τI and τII absolute values in supercells involving the plates with different thicknesses, 2 cθ thick plates have a shear component close to zero. All the plates analyzed have precipitate/matrix volumetric incompatibilities with Al matrix as misfit strains along [111]Al // [001]θ, which distribute non-uniformly across the plate thickness. Large misfit strains concentrate at the broad plate interfaces, i.e. in Ag2Cu and Cui layers, and cause a prohibitively high barrier to thickening of the Ω precipitates.

Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging

The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO2 has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO2-based memristors. Here, the conductive filament system in the amorphous HfO2-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf6O and its crystalline surroundings (monoclinic or tetragonal HfOx). The phase of the HfOx shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO2 is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO2-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.