Unit Cell Of Phase Research Articles

Secondary phase evolution of high-entropy ceramics under heavy-ion irradiation in high-temperature coupling-induced environment

The formation of new phases in high-entropy ceramics can be induced by both heavy-ion irradiation and annealing, meaning that these transitions can simultaneously occur under high-temperature irradiation. Consequently, the radiation resistance of high-entropy ceramics can involve complex mechanisms upon this coupled environment. In this study, the irradiation resistance of the high-entropy (ZrHfTiSn)O2 ceramic was investigated under heavy-ion irradiation at different temperatures. The results show that after irradiation, the secondary triclinic phase exists in all the ion-injected ranges. The triclinic phase can withstand irradiation, leading to dislocations that trigger a local increase in entropy and result in layered amorphization. Amorphization is accompanied by volume expansion that compresses adjacent regions, causing angular differences in the triclinic phase unit cell, and recrystallization of the amorphous region at 500 °C can release this compression. This study provides experimental data and theoretical support for further improving the radiation resistance of related materials.

In-situ synchrotron X-ray diffraction investigation of martensite decomposition in Laser Powder Bed Fusion (L-PBF) processed Ti–6Al–4V

Laser powder bed fusion (L-PBF) processed Ti–6Al–4V components present two substantial challenges: the presence of tensile residual stress and brittleness of the as-fabricated component due to the formation of the martensite (α′) phase. Industrially, post-heat treatment is performed to improve the quasi-static mechanical properties of the Ti–6Al–4V component through martensite (α′) decomposition and residual stress relaxation. However, detailed insights into the martensite decomposition and stress relaxation mechanisms are still lacking. To address this, in-situ high-energy synchrotron x-ray diffraction (HEXRD) was employed to simultaneously track: (I) martensite (α′) decomposition into a mixture of hexagonal α and body-centered cubic β phases, (II) residual stress relaxation process during heating. Rietveld refinement of 2D diffraction patterns recorded over a temperature range spanning from 25 to 1000°C revealed a three-stage evolution in the α′ unit cell parameters. Unit cell parameter analysis combined with the phase fraction analysis based on the diffraction patterns and the chemical composition profiles predicted by Thermocalc simulation gave detailed insight into the martensite (α′) decomposition process. In Stage 1 (until 375°C), the α′ phase unit cell exhibited linear expansion without indications of any phase transformation or stress relaxation. Upon commencement of stage II at 375°C, an increased expansion rate of the martensite (α′) unit cell was observed, attributed to the enhanced outward diffusion of vanadium (V), succeeded by β phase nucleation at 575°C. Simultaneously, during stage II, residual stress relaxation was observed at both macro and micro scale, culminating in a stress-free state achieved at approximately 750–800°C. Lastly, α-to-β phase transformation was observed in stage 3. The thermal evolution of the hexagonal unit cell dimensions was compared between martensitic and mill-annealed α+β microstructures. This comparison unraveled the link between the initial microstructure and the thermal expansion behavior of the hexagonal α′ and α phase.

Synthesis of lithium doped magnesium ferrites and their vibrational and magnetic properties: Correlation of experimental and density functional theory

Recently, there has been an increased focus on investigating the possible use of ferrite nanoparticles as magnetic memory devices. In this study, we report on the structural, microstructural, Raman, density functional theory and magnetic characteristics of Mg1-xLixFe2O4 (x = 0, 0.15, 0.3, 0.45, 0.5 and 0.75). Samples were synthesized by solution combustion synthesis method. Samples were subjected to characterize X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman Spectra. XRD reveals that all synthesized samples show a single phase formation and unit cell volume, crystallite size varied with an increase in Lithium-ion concentration. SEM micrographs confirmed the porous nature increases with the rise of Li concentration. Particles are agglomeration and spherical type of nature was confirmed by TEM images. Further, Raman Spectra confirms the spinel cubic structure of MgFe2O4 nanoparticles belongs to the Fd3m space group. Despite the fact that the full unit cell has 56 atoms (Z = 8), the smallest Bravais cell consists of only 14 atoms (Z = 2). Consequently, in order to understand this effect, the electronic structure of pure and doped models was investigated through the Band Structure and Density of States (D.O.S.) profiles. First the crystalline structure of pure and Li-doped MgFe2O4 was analyzed based on the optimized lattice parameters and M − O (M = Mg, Li and Fe) bond distances. The band-gap was computed to be 2.54 eV being a direct transition between two points, in an excellent agreement with the experimental measurements. To investigate the temperature dependent magnetic properties, the M − H loops of Li doped MgFe2O4 nanoparticles traced at below and above room temperature over the applied magnetic field ±60 kOe. The M − H loops recorded at above and below room temperature demonstrated that the ferromagnetic nature of the sample. Further, the magnetic properties was investigated using temperature dependent FC and ZFC magnetization measurements. Above room temperature, the ZFC and FC curves show irreversibility and split at certain temperatures, as well as a broad maximum in ZFC curves at blocking temperature (TB). Saturation magnetization (Ms) values are higher at 15 K compared to 300 K, attributed to reduced thermal fluctuation. Our results suggest that synthesized materials are useful for room temperature magnetic memory device application.

Crucial role of Mg on phase transformation and stability of Sm–Mg–Ni-based AB2 type hydrogen storage alloy

Rare earth–Mg–Ni-based alloys are promising candidate for high-capacity hydrogen storage materials, and Mg is the key elements to maintain its crystal structure stable. In order to understand the role of Mg element in rare earth–Mg–Ni based alloys, the alloys with the composition of RE1–xMgxNi1.97Co0.03Al0.07 (RE = Sm0.80Y0.20; x = 0.35, 0.44, 0.52) were synthesized by powder sintering treatment. The increasing Mg content promotes the phase transformation from SmNi2 (Fd-3 m group) phase to MgCu4Sn-type (F-43 m group) phase. First-principle calculations and XRD simulations show that Mg element is reasonably approximated as the para-position substitution of rare earth elements at 4c sites when it enters into the unit cell of SmNi2 phase. The Sm0.38Y0.10Mg0.52Ni1.93Co0.03Al0.05 alloy with Mg fully occupying the 4c sites forms a stable structure, and it shows flat hydrogen absorption/desorption platforms, with a hydrogen absorption capacity of 1.12 wt%. Meanwhile, it has more excellent cycling stability. After 20 cycles of hydrogen absorption, the crystal structure of the alloy still maintains MgCu4Sn-type phase, and the capacity retention rate can reach 94.42 %.

Phase equilibria in the ZrO2–HfO2–Nd2O3 system at 1500 °C and 1700 °C

The phase equilibria in the ZrO2–HfO2–Nd2O3 ternary system at 1500 °C and 1700 °C were studied over the whole concentration range by X-ray diffraction and microstructural analyses. Corresponding isothermal sections were constructed from the data obtained. It was found that solid solutions in this system are derived from a tetragonal (T) modification of ZrO2, a monoclinic (M) modification of HfO2, a hexagonal (A) modification of Nd2O3, a cubic phase with a fluorite (F) structure of ZrO2 (HfO2), and an ordered phase with a pyrochlore (Py) structure of Nd2Zr2O7 (Nd2Hf2O7). The phase boundaries and unit cell lattice parameters were determined. The solubility of Nd2O3 in M − HfO2 is pretty low and about less than 1 mol%, as confirmed by XRD and microstructural analyses. The ordered pyrochlore-type (Py) phase of Nd2Zr2O7 (Nd2Hf2O7) forms continuous series of solid solutions at 1500 °C and 1700 °C. The homogeneous region of these continuous series of solid solutions changes insignificantly with increasing temperature. The changes in the construction of the isothermal section of the ZrO2-HfO2-Nd2O3 phase diagram at 1500 °C compared to 1700 °C are associated with the thermal stability of cubic fluorite-type (F) solid solutions. Other phases in the ZrO2-HfO2-Nd2O3 ternary system are not detected at the studied temperatures.

Preparation, phase stability, and magnetization behavior of high entropy hexaferrites

The polycrystalline SrFe12O19 samples deeply substituted up to at.67% by Al3+, Ga3+, In3+, Co3+, and Cr3+ cations with a high configurational mixing entropy were prepared by solid-phase synthesis. Phase purity and unit cell parameters were obtained from XRD and analyzed versus the average ionic radius of the iron sublattice. The crystallite size varied around ∼4.5μm. A comprehensive study of the magnetization was realized in various fields and temperatures. The saturation magnetization was calculated using the Law of Approach to Saturation. The accompanying magnetic parameters were determined. The magnetic crystallographic anisotropy coefficient and the anisotropy field were calculated. All investigated magnetization curves turned out to be nonmonotonic. The magnetic ordering and freezing temperatures were extracted from the ZFC and FC curves. The average size of magnetic clusters varied around ∼350nm. The high values of the configurational mixing entropy and the phenomenon of magnetic dilution were taken into account.

Dy3+doped (K,Na)NbO3-based multifunctional ceramics for achieving enhanced temperature-stable piezoelectricity and non-contact optical temperature sensing performance

A novel multifunctional KNN-based ceramics with decent piezoelectricity and intriguing photoluminescence are achieved through simultaneous modulation of diffused phase transition behavior and unit cell distortionviaoptimizing the Dy3+content.

Molecular dynamics simulation and non-isothermal crystallization kinetics of polyamide 4 and different bio-based polyamide blends.

The effects of the structural units and blending ratio on the crystallization behavior of blends of polyamide 4 (PA4) with polyamide 56 (PA56) and polyamide 11 (PA11) were studied using molecular dynamics simulations and non-isothermal crystallization kinetics. The simulation results show that the crystallinity of PA4/PA56 blends (B4/56) with a PA56 content of 30-50% was 3.5-10.8% lower than that of B4/56 with a PA56 content of 20%, and the crystallinity of PA4/PA11 blends (B4/11) decreased by 9.5% as PA11 content increased from 20% to 50%. The experimental results show that both B4/56 and B4/11 form PA4- and PA56-rich (PA11-rich) phases through crystallization-induced phase separation. The interplanar spacing of the PA4-rich phase of B4/56 changed relative to that of PA4, indicating that some PA56 entered the PA4-rich phase unit cell. As the PA56 content increased from 20% to 50%, the crystallinity of B4/56 decreased by 11.2%, and the crystallization-induced phase separation grew distinct. The B4/56 with a higher PA4 content crystallized more easily. As the PA11 content increased from 20% to 50%, the crystallinity of B4/11 decreased by 12.5%, and PA11 barely participated in the crystallization of the PA4-rich phase. The blending ratio had no significant effect on the crystallization rate and crystal-growth degree of B4/11, and the non-isothermal crystallization activation energy of B4/11 was significantly higher than that of B4/56, indicating that the crystallization ability of the B4/11 blend system is worse. This study provides a theoretical basis for the design and performance regulation of PA4-based polyamide blends.

What lies beneath? Investigations of atomic force microscopy-based nano-machining to reveal sub-surface ferroelectric domain configurations in ultrathin films

Multiferroic materials, encompassing simultaneous ferroelectric and ferromagnetic polarization states, are enticing multi-state materials for memory scaling beyond existing technologies. Aurivillius phase B6TFMO (Bi6TixFeyMnzO18) is a unique room temperature multiferroic material that could ideally be suited to future production of revolutionary memory devices. As miniaturization of electronic devices continues, it is crucial to characterize ferroelectric domain configurations at very small (sub-10 nm) thickness. Direct liquid injection chemical vapor deposition allows for frontier development of ultrathin films at fundamental (close to unit cell) dimensions. However, layer-by-layer growth of ultrathin complex oxides is subject to the formation of surface contaminants and 2D islands and pits, which can obscure visualization of domain patterns using piezoresponse force microscopy (PFM). Herein, we apply force from a sufficiently stiff diamond cantilever while scanning over ultrathin films to perform atomic force microscopy (AFM)-based nano-machining of the surface layers. Subsequent lateral PFM imaging of sub-surface layers uncovers 45° orientated striped twin domains, entirely distinct from the randomly configured piezoresponse observed for the pristine film surface. Furthermore, our investigations indicate that these sub-surface domain structures persist along the in-plane directions throughout the film depth down to thicknesses of less than half of an Aurivillius phase unit cell (˂ 2.5 nm). Thus, AFM-based nano-machining in conjunction with PFM allows demonstration of stable in-plane ferroelectric domains at thicknesses lower than previously determined for multiferroic B6TFMO. These findings demonstrate the technological potential of Aurivillius phase B6TFMO for future miniaturized memory storage devices. Next-generation devices based on ultrathin multiferroic tunnel junctions are projected.

Biological investigation of sonochemically synthesized CZTS nanoparticles

Cu2ZnSnS4 (CZTS) nanoparticles (NPs) are synthesized by sonochemical technique. The composition analysis showed the synthesized NPs to be stoichiometric. The X-ray diffraction confirms the CZTS phase and tetragonal unit cell structure. Three peaks corresponding to CZTS have been observed in Raman spectroscopy positioned at 293.47 cm−1, 373.8 cm−1, and 336.67 cm−1. The electron diffraction in the selected area showed faded rings with spot patterns indicating crystalline nature. The CZTS NPs have a direct optical bandgap of 1.62 eV, being ascertained by optical absorbance spectroscopy. The CZTS NPs antibacterial activity is determined against pathogenic gram-negative (E. coli, P. aeruginosa, P. vulgaris, and E. aerogenes) and gram-positive (S. aureus) bacteria by measuring the inhibition zone. The 2, 2-diphenyl-1-picrylhydrazyl assay is employed in deciding the antioxidant efficacy of CZTS NPs. The cytotoxicity of CZTS NPs is evaluated in Brine shrimp (Artemia salina) eggs hatched in synthetic seawater made from 38 g/l commercial sea salt. The findings are discussed in detail.

First-Principles Investigations on Structural Stability, Elastic Properties and Electronic Structure of Mg32(Al,Zn)49 Phase and MgZn2 Phase

Al–Mg–Zn alloys reinforced by T–Mg32(Al,Zn)49 phase had higher structure stability and strength than Al–Zn–Mg–(Cu) alloys reinforced by MgZn2 phase, but the reasons for these two kind of alloys was not well-known. To reveal the discrepancy between T phase and MgZn2 phase, the lattice parameters, cohesive energy, and electronic structure as well as the elastic properties were investigated based on density functional theory. Four types of T phase unit cell were employed according to symmetry of space group. The calculated lattice constants well-agreed with experimental data. Compared to MgZn2 phase, T phases obtained lower cohesive energy owing to their partial covalent bond, which may result in a higher structure stability. The elastic modulus E of T phase depended on the occupation of Al atom, and the effect of the occupation of Al atom on the structure and properties of T phase was also discussed.

Assessment of the data repeatability of x-ray diffraction study for silicon nitride powders of different dispersion

X-ray diffraction methods are indispensable for studying crystalline materials and provide determination of the phase composition, internal stresses and the parameters of unit cells of crystalline phases, as well as the crystalline size, which is related to the average grain size and affects the width of the diffraction peaks. We present the results of evaluating the repeatability of the X-ray diffraction data obtained for silicon nitride powders Si 3 N 4 of different particle size and different initial phase ratios α-, β-Si 3 N 4 . The relative error of measuring the diffraction peaks was determined using the intensity detector and the absolute error of calculating the unit cell parameters was determined by the Rietveld method. The average particle size of the initial powders was analyzed using scanning electron microscopy. X-ray diffraction studies were performed according to the Bragg – Brentano scheme using CuKα radiation (λ = 1.5406 Å). A series of three experiments was performed for each powder. It is shown that the relative error of intensity measurements with a detector does not exceed 2% for peaks corresponding to 3σ criterion, and the absolute error of the determination of the unit cell phase parameters by the Rietveld method is 0.001 Å. The results obtained can be used to assess the stability of the diffractometer both for the samples based on silicon nitride and materials of different composition and structure, especially for submicron samples. In the latter case, the error of the parameters obtained by X-ray phase analysis can be taken into account without resorting to statistical estimates, as in the method of least squares.

Orthorhombic Fmmm-C80: A new superhard carbon allotrope with direct band gap

A new all-sp3 hybridized orthorhombic superhard carbon phase Fmmm-C80 (space group: Fmmm) with direct band gap and low density is uncovered by using CALYPSO code and first-principles calculations. The unit cell of Fmmm-C80 phase contains 80 carbon atoms, and its thermodynamic stability is confirmed by the phonon and molecular dynamics simulations. The semiconducting nature of the Fmmm-C80 was revealed by the calculated direct band gap of 4.11 eV. Also, Fmmm-C80 is found to be a potential superhard carbon phase for its large Vickers hardness of 51 GPa.

Glass-ceramics based on Li1.5Al0.5Ge1.5(PO4)3 for advanced all-solid-state batteries

Glass-ceramic electrolytes of the Li1.5Al0.5Ge1.5(PO4)3-xB2O3 series (0≤x≤0.15 wt%) were obtained by glass crystallization under the same conditions of heat treatment. The influence of B2O3 addition on the phase composition and unit cell parameters of lithium-aluminium-germanium-phosphate glass-ceramics was investigated by XRD. The obtained solid electrolytes have the NASICON-type structure with space group R-3c. Glass-ceramics is single-phase at 0≤x≤0.15. Transport properties of glass-ceramics electrolytes were investigated by impedance spectroscopy. The conductivity of Li1.5Al0.5Ge1.5(PO4)3 was found to be lower compared to the B2O3-added solid electrolytes. Namely, the composition with 0.05 wt% has the highest lithium-ion conductivity, which is 5.5·10−4 S/cm at 25 °C. Also the thermal compatibility of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte with Li4T5O12 anode material and the influence of Li3BO3 addition on their compatibility were investigated by DSC method.

Microstructure and hydrogen storage properties of Ti10+xV80-xFe6Zr4 (x=0~15) alloys

The microstructure and hydrogen storage properties of Ti10+xV80-xFe6Zr4 (x = 0, 5, 10, 15) alloys have been studied. XRD and SEM analyses show that all alloys consist of a BCC main phase and a small fraction of C14 Laves secondary phase, in which the latter precipitates along the grain boundary of the former becoming network structure. With increasing Ti content in the alloy, the lattice parameter and cell volume of the BCC main phase of the alloy increase. The chemical composition of each phase is analysed by EDS, from which the lattice parameters of BCC phase have a good linear relationship with their average atomic radii. All bulk alloys have good activation behaviors and hydriding kinetics. With the increase of Ti content, the incubation time for activation decreases first and then increases under an initial hydrogen pressure of 4 MPa at 298 K. The incubation time of Ti15V75Fe6Zr4 alloy is only 12 s. It is one of the shortest incubation time in V-based solid solution alloys as far as we know, which may be related to the existence of C14 Laves phase. All the alloys have relatively high hydrogen absorption capacities of above 3 wt%, which increase first and then decrease as the Ti content increases, achieving the maximum capacity of 3.61 wt% at x = 10 at 298 K. With increasing x, the equilibrium plateau pressure of dehydrogenation of the samples at 353 K decreases owing to the expansion of unit cell of main phase, which is far below 0.1 MPa for x = 10 and 15. The maximum desorption capacity of 1.94 wt% (desorbed to 0.001 MPa) is obtained at x = 5, compared to that of 1.6 wt% (desorbed to 0.1 MPa) achieved at x = 0.

Wideband and Low-Profile Integrated Dual-Circularly-Polarized Transmit-Arrays Enabled by Antenna-Filter-Antenna Phase Shifting Cells

In this article, the theory, design, and demonstration of wideband and low-profile dual-circularly polarized (dual-CP) transmit arrays (TAs) are reported, which support independent beamforming for left-/right-handed CP waves. This is achieved by utilizing an antenna–filter–antenna configuration for the TA unit cells with a profile of only about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.11\lambda _{0}$ </tex-math></inline-formula> , which consists of receiving and transmitting stacked patches connected by two microstrip delay lines. By utilizing the phase delay lines and unit cell rotation, the dynamic phase and the Berry phase can be simultaneously exploited for providing continuous dual-CP transmissive phase compensation insensitive to the angle of incidence. The diffraction analysis of a linear gradient array of the proposed cells confirms its wideband performance of abnormal refraction. In addition, broadband CP planar feed arrays were designed and verified for functioning as low-profile feeds with symmetrical patterns for the TAs. As proof-of-concept examples, two <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -band dual-CP TAs integrated with the planar feeds were synthesized, fabricated, and measured, possessing a total profile of about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.1\lambda _{0}$ </tex-math></inline-formula> . The two prototypes offer either symmetrical or asymmetrical dual-CP beams over a wide bandwidth. Their measured peak gain is higher than 22 dBic, while the joint <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11} &lt; -15$ </tex-math></inline-formula> dB, axial ratio < 2 dB, beam squinting <3°, and 2–dB gain bandwidth is wider than 12%. The design methodology and the proposed dual-CP TAs are useful in communication systems for space and wireless applications.

Crystal structure of intermetallic compound Y5Co19 and its hydride phases

Y–Co intermetallic compounds and their hydrides were investigated as magnetic materials, but the hydrogenation of these alloys was not observed. We established the existence of Y5Co19 in the phase diagrams of the Y–Co system. Y5Co19, an intermetallic compound, was synthesized and its crystal structure was determined by the Rietveld refinement of X-ray diffraction (XRD) data. The structural model of Y5Co19 is a Ce5Co19-type model with lattice parameters a = 0.4994 (1) nm and c = 4.800 (1) nm. The crystal structure of the original alloy and its hydride phase is closely related to the hydrogen absorption–desorption property. The hydrogen capacity of Y5Co19 was 0.63 H/M, while two plateaus were observed in the P–C isotherm. We discovered two hydride phases: Y5Co19H3.8 (phase I) and Y5Co19H14.9 (phase II), and the structural model of these phases (I and II) was Ce5Co19-type, which was the same as the original alloy. The unit cell of phase I showed an expansion of 3.9% only along the c-axis from the original alloy, whereas that of phase II indicated an expansion along the a- and c-axes. Additionally, a close isotropic expansion was observed in phase II.

Effect of titanium substitution and temperature variation on structure and magnetic state of barium hexaferrites

A number of solid solutions based on BaFe12−xTixO19 M-type barium hexaferrite doped with titanium cations up to x = 2.00 were obtained using conventional ceramic technology. The phase composition, crystal structure and unit cell parameters were refined by the Rietveld method using powder X-ray diffraction data up to T = 900 K. It was found that all the compositions have a magnetoplumbite structure satisfactorily described by P63/mmc space group (No. 194). With increasing temperature and doping concentration, the unit cell parameters increase almost monotonically. The minimum volume of V ~ 696.72 Å3 was determined for the composition with x = 1.00 at T = 100 K, while the maximum value of V ~ 714.00 Å3 is observed for the composition with x = 2.00 at T = 900 K. The mechanism of occupation nonequivalent crystallographic positions with titanium cations is established. The spin-glass component of the magnetic phase state is fixed. The Tdif temperature of the difference between the ZFC-FC curves decreases with an increase in the concentration of titanium cations and the magnetic field from ~237.2 K to ~ 44.5 K, while the Tinf inflection temperature of the ZFC curve increases from ~21.0 K to ~23.8 K. With an increase in the doping concentration, both the Dav average and Dmax maximum clusters grow up to ~ 100 nm. As the magnetic field increases above the critical value, the spin-glass component disappears. For compositions with x > 1.00, the magnetization is not saturated in fields up to 6 T. Along with the formation of the spin-glass component, doping with titanium cations for barium hexaferrite lowers the TC Curie temperature down to T ~600 K. The Ms spontaneous and Mr remanent magnetizations, as well as the Bc coercivity, decrease with increasing doping concentration almost monotonically, while the latter has an inflection point at x = 1.00. The minimum values of spontaneous and remanent magnetization, as well as coercivity, are observed for the composition with x = 2.00 and amount to Ms ~17.7 emu/g, Mr ~1.9 emu/g, and Bc ~3.9 × 10−3 T, respectively. An interpretation of the magnetic state of the doped BaFe12−xTixO19 barium hexaferrite is given taking into account the mechanism of occupation nonequivalent crystallographic positions with titanium cations.

First-Principles Study of Defect Levels Caused by Transition Metal Atoms in Silicon Nitride for Non-Volatile Memory Applications

The silicon nitride-based charge trap memory architecture has been considered as one of the promising solutions for high-bit-density three-dimensional NAND-type flash memories [1]. In this architecture, electrons and holes are trapped by point defects distributed in silicon nitride. This phenomenon induces a shift of the threshold voltage of memory transistors and is applied to store data. Recently, multi-level cell technology has been introduced in NAND-type flash memories to increase the bit density, which has needed to enlarge the threshold voltage shift [2]. Thus, the increase in the density of the point defects which can generate deep trap levels in silicon nitride is demanded to realize the high-bit-density flash memories. In the present work, we investigate the energy levels caused by transition metal (Ti, V, or Mn) defects in silicon nitride using first-principles calculations.Amorphous silicon nitride film has been used as the charge trap layer in flash memories. The hexagonal β-Si3N4 crystalline phase is believed to have similar local structures as amorphous silicon nitride [3]. Therefore, to discuss the realistic nature of transition metal atoms in silicon nitride, a 168-atom supercell was formed by expanding a unit cell of β-Si3N4 phase in a 2x2x3 matrix on a-axis, b-axis, and c-axis, respectively. The structure optimization and self-consistent field (SCF) calculations were performed using Advance/PHASE ver. 3.5 software. The generalized-gradient approximation with the PBE functional was used for the exchange-correlation energy of electrons. The kinetic energy cutoffs for wavefunctions were 40, 52, 64, 76, and 88 Ry and those for charge density were 400, 520, 640, 760, and 880 Ry in the calculations. 2x2x4 k-points sampling in the Monkhorst-Pack grid was used to perform SCF calculations.The behavior of transition metal atoms in semiconductors and dielectrics has been widely studied. Shibata et al. explained the behavior of transition metal atoms in silicon nitride using density-functional theory (DFT) calculations in terms of formation energies [4]. However, there are few reports on the energy levels generated by transition metal atoms in silicon nitride. In the present work, the density of states (DOS) of electrons in four supercell structures of Si72N96, Si71N96Mn, Si71N96V, and Si71N96Ti have been investigated. The calculations with the conditions mentioned earlier have been performed on the supercells, where a transition metal atom (Ti, V, or Mn) was substituted for a Si atom.Figure 1(a) shows total DOS calculated for the supercell Si71N96Mn. We can see five different defect levels of energy 1.59, 1.91, 2.03, 3.00, and 3.12 eV below the bottom of the conduction band. From Atomic Layer DOS (ALDOS) calculation results, we have found that all five trap levels are mainly localized at the Mn atom. Some of these trap levels would be able to behave as deep electron trap levels.Total DOS calculated for the supercell Si71N96V is shown in Fig. 1(b). Similar to the Si71N96Mn, five different defect levels appeared. However, three levels close to the bottom of the conduction band in Si71N96V are shallower in energy as compared to those in Si71N96Mn. Electrons trapped by these shallow levels might be easily emitted to the conduction band. The emission of electrons from the defect levels would degrade the retention characteristics of flash memories. We also calculated total DOS for the supercell Si71N96Ti. No defect levels appeared in the forbidden gap of Si71N96Ti.In conclusion, we have performed the DFT calculations to investigate the defect levels generated by transition metal atoms (Ti, V, or Mn) in silicon nitride. The substitution of a Si atom with a Mn atom generated deep defect levels in the forbidden gap. Some of the defect levels can be expected to behave as deep trap levels for electrons. The doping of silicon nitride with Mn atoms would have potential for enhancing the carrier trapping properties of the silicon nitride charge trap layer. Further experimental studies would be needed to validate the effectiveness of doping of silicon nitride with Mn atoms to the enhancement of the charge trapping properties in flash memories.We would like to show our gratitude to K. Niisato, Y. Ota and T. Inushima for sharing their pearls of wisdom with us during the course of this research. This work was partly supported by JSPS KAKENHI Grant Number JP18K04244.[1] S. Inaba, 2018 IEEE Inte. Memory Workshop, Kyoto, 1 (2018).[2] K. Parat et al., 2018 IEEE Inter. Electron Devices Meeting, San Francisco, 2.1.1 (2018).[3] L. E. Hintzsche et al., Physical Review B., 86, 235204 (2012).[4] D. Shibata et al., ECS Transactions, 64(11), 229 (2014). Figure 1

Structure and hydrogenation features of mechanically activated LaNi5-type alloys

The effect of short-term mechanical activation in a ball mill on the thermodynamics of interaction with hydrogen and the structure of intermetallic and hydride phases was studied for a series of AB5 type alloys from the standpoint of their use as filler in metal-polymer membranes. It was found that the activation processing leads, as expected, to a decrease in the size of coherent scattering domains and an increase in the lattice strain, while the phase composition and unit cell parameters remain unaffected. Despite this, there is a significant change in the pressure-composition isotherms, a reduction of the total capacity ad the plateau region. XRD study of the hydride phases evidences smaller volume expansion of the intermetallic crystal lattice at hydrogen absorption for the alloys subjected to mechanical activation. Analysis of the obtained thermodynamic and structural data allows us to consider the mechanically activated LaNi4.8Al0.2 as the most appropriate for low-pressure and low-temperature application in the hydrogen separation composite membranes. La0.7Ce0.3Ni4.5Cu0.5 and LaNi5 with maximum activation treatment of 3 min are more promising for use at elevated pressures.