Change In Lattice Constant Research Articles

(Invited) Exploring and Manipulating Luminescence Centers in Lead Halide Perovskites through Hydrostatic Pressure

The application of hydrostatic pressure to materials induces changes in lattice constants, vibrational spectra, and electron-phonon interactions, thereby altering the optoelectronic properties of semiconductors. Consequently, hydrostatic pressure has become a widely utilized tool for investigating and enhancing the optical properties of organic/inorganic halide perovskites and their nanostructures. In this presentation, I will discuss the impact of hydrostatic pressure on two distinct types of luminescence centers in perovskite materials. The first type of luminescence centers originates from excitons within perovskite nanocrystals. I will discuss how hydrostatic pressure serves as a valuable tool for elucidating the origin of luminescence in low-dimensional perovskites, including 2D CsPb2Br5, 0D Cs4PbBr6, and 2D Ruddlesden-Popper Hybrid Perovskite Crystals. The second type of luminescence centers exhibit broadband optical emission from self-trapped excitons in low-dimensional perovskites. I will present our findings on the observation of strong exciton-phonon coupling and demonstrate how hydrostatic pressure can be employed to modulate electron-phonon coupling, offering a means to finely tune the broad emission spectrum.

Synthesis and Characterization Studies of Pure and Co Doped ZnO Nano Thin Films

General Background: Thin-film technology is pivotal in advancing modern optoelectronic devices due to its ability to tailor material properties at the nanoscale. Specific Background: Zinc oxide (ZnO) and cobalt oxide (CoO) are prominent materials in this field, with doping techniques such as cobalt incorporation offering potential enhancements in film properties. Knowledge Gap: The impact of cobalt doping on the structural and optical properties of ZnO thin films remains inadequately explored, particularly concerning varying doping concentrations. Aims: Aims to elucidate the effects of cobalt doping on ZnO thin films' structural and optical characteristics, with concentrations of Zn0.6Co0.4O and Zn0.8Co0.2O compared to pure ZnO and CoO films. Results: Thin films were prepared via spin coating on glass substrates at 3000 rpm for 30 seconds. X-ray diffraction analysis revealed that pure ZnO films possess a hexagonal polycrystalline structure, whereas CoO films exhibit a cubic structure. Doping with cobalt resulted in a decrease in peak intensity at the (111) orientation and an increase in grain size. Optical measurements indicated enhanced transmittance and modified absorbance spectra with increased cobalt content, showing maximum absorbance at short wavelengths and diminished values in the visible region. Reflectivity increased with photon energy before decreasing at higher energies. Novelty: Cobalt doping significantly alters both structural and optical properties of ZnO films, with distinct changes in lattice constants, grain size, and optical behavior, suggesting a potential route for optimizing ZnO-based materials. Implications: Cobalt-doped ZnO films hold promise for improved performance in optoelectronic applications, such as solar cells and sensors, necessitating further research into diverse doping strategies to fully harness their technological potential. Highlights: Cobalt doping alters ZnO thin films' structural and optical properties. Spin coating method used at 3000 rpm for 30 seconds. Enhanced optical properties suitable for optoelectronic applications. Keywords: Thin films, Zinc oxide, Cobalt doping, Optical properties, X-ray diffraction

Theoretical design and experimental verification of high-entropy carbide ablative resistant coating

Composition design of high-entropy carbides is a topic of great scientific interest for the hot-end parts in the aerospace field. A novel theoretical method through an inverse composition design route, i.e. initially ensuring the oxide scale with excellent anti-ablation stability, is proposed to improve the ablation resistance of the high-entropy carbide coatings. In this work, the (Hf0.36Zr0.24Ti0.1Sc0.1Y0.1La0.1)C1-δ (HEC) coatings were prepared by the inverse design concept and verified by the ablation resistance experiment. The linear ablation rate of the HEC coatings is −1.45 ​μm/s, only 4.78 % of the pristine HfC coatings after the oxyacetylene ablation at 4.18 ​MW/m2. The HEC possesses higher toughness with a higher Pugh's ratio of 1.55 in comparison with HfC (1.30). The in-situ formed dense (Hf0.36Zr0.24Ti0.1Sc0.1Y0.1La0.1)O2-δ oxide scale during ablation benefits to improve the anti-ablation performance attributed to its high structural adaptability with a lattice constant change not exceeding 0.19 % at 2000–2300 ​°C. The current investigation demonstrates the effectiveness of the inverse theoretical design, providing a novel optimization approach for ablation protection of high-entropy carbide coatings.

The competition between charge density wave and superconductivity in the kagome metal CsV3(Sb1-xPbx)5 single crystals

Kagome metal AV3Sb5 (A = K, Rb, Cs) has garnered considerable attention for its abundant exotic quantum phenomena and the special competitive relationship between charge density wave (CDW) and superconductivity (SC). In this work we explore the unconventional coupling between charge density wave and superconductivity by means of the substitutional doping of Pb atoms with Sb atoms. Microscopic characterization and performance tests provide valuable phase diagram information for CsV3(Sb1-xPbx)5. Upon Pb doping, a pronounced competition between the charge density wave transition temperature TCDW and the superconducting temperature Tc was observed. Analysis of changes in Debye temperature ΘD and lattice constant, it is shown that Pb doping affects atomic bonding and may strengthen the coupling between adjacent kagome layers to some extent. Furthermore, the small angular shift of the diffraction peak and increased full width at half maximum (FWHM) indicate that the doping causes crystal defects and distorts the crystal structure, which will change the charge density in the local area and may affect the superconductivity. This result further reveals a close correlation between the superconductivity of the system and the electronic structure of the kagome plane in the CsV3Sb5 system. Overall, our results suggest that the observed competition arises from the complex interplay of multiple factors.

A gradient B2-BCT transition introduced by deformation in eutectic high-entropy alloy

Eutectic high-entropy alloys (EHEAs) exhibit promising characteristics for achieving both high strength and good ductility owing to their diverse deformation mechanisms. In this study, a gradient B2-body-centerd tetragonal (BCT) phase transformation was observed in a deformed Al19Fe20Co20Ni41 (at. %) EHEA. The X-ray diffraction patterns indicate a gradual transformation from the B2 phase to the BCT phase, a process significantly influenced by the applied strain during tensile testing. The gradient transition region between B2 phase and BCT phase occurs within a single grain, where the lattice constant changes step by step. Such a mechanism holds huge potential for enhancing the mechanical properties of EHEAs.

Periodic DFT study of structural transformations of cocrystal CL-20/MMI under high pressure.

The crystal and molecular structure, electronic properties, optical parameters, and elastic properties of a 1:2 hexanitrohexaazaisowurtzitane (CL-20)/2-mercapto-1-methylimidazole (MMI) cocrystal under 0 ~ 100 GPa hydrostatic pressure were calculated. The results show that the cocrystal CL-20/MMI undergoes three structural transitions at 72 GPa, 95 GPa, and 97 GPa, respectively, and the structural transition occurs in the part of the MMI compound. Structural mutations formed new bonds S1-S2, C2-C7, and N1C5 at 72GPa, 95 GPa, and 97 GPa, respectively. Similarly, the formation of new bonds is confirmed on the basis of an analysis of the changes in lattice constants, cell volumes, and partial densities of states (PDOS) for S1, S2, C2, C7, N1, and C3 at the corresponding pressures. The optical parameters show that the pressure makes the peaks of various optical parameters of CL-20/MMI larger, and the optical activity is enhanced. The optical parameters also confirm the structural mutation of CL-20/MMI under the corresponding pressure. CL-20/MMI was calculated by using the first-principles norm-conservative pseudopotential based on density functional theory (DFT) in the CASTEP software package. For the optimization results, the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method is selected to optimize the geometry of the cocrystal in the range of 0-100 GPa. GGA/PBE (Perdew-Burke-Ernzerhof) was selected to relax the cocrystal CL-20/MMI fully without constraints at atmospheric pressure. The sampling scheme in the Brillouin zone [10] is the Monkhorst-Pack scheme, and the number of k-point grids was 2 × 2 × 2. By contrast, this study will use the LDA method to calculate.

Theoretical prediction of two-dimensional CrSi2N4 as a potential anode material for Na-ion batteries

Efficient anode materials are critical for high-performance rechargeable lithium-ion batteries (LIBs) and sodium-ion batteries. This paper systematically investigates the potential of the CrSi2N4 monolayer as anode material for LIBs and sodium-ion batteries by first-principles density functional theory calculations. It was found that CrSi2N4 exhibits outstanding performance in sodium-ion batteries, with a low diffusion energy barrier of 0.10 eV and a high theoretical specific capacity of 490 mAh g−1. Meanwhile, the average open circuit voltage is 0.47 V, comparable to the typical anode materials. In addition, a small lattice constant change of 0.3%–3.1% ensures the cycling stability of CrSi2N4 in sodium-ion batteries. This work suggests a promising candidate anode material for sodium-ion batteries.

Stretchable, stable, and structural color reversible enabled by mesoscopic regulation and design for flexible photonic crystal mechanical sensors

This paper reports a flexible mechanical sensor developed by combining an opal photonic crystal and a flexible medium through UV curing technology. Under the action of external force, the deformation of the flexible medium causes the change of the photonic crystal lattice constant, which leads to the displacement of the reflection peaks, thus realizing the detection of the deformation of the film. The flexible medium is mixed by photoinitiators and polymer monomers, and its mechanical properties are directly related to the mass fraction of photoinitiators and polymer monomers. The results show that the film can reach a maximum strain of about 180% when the mass fraction of photoinitiator is 3% and the volume ratio of the polymer functional monomers is 8:2. In addition, for the responsiveness test of the sensor under tensile, the results show that at 0%, 26%, and 78% of tensile strain, its structural color is blue-shifted sequentially, and its spectral peaks are 668 nm, 535 nm, and 488 nm, respectively. Finally, the sensor is applied to the detection of acoustic waves in air, and it is found that its spectral response peaks can be obtained for acoustic waves of different frequencies (250–650 Hz), and a spectral peak at 500 Hz. This flexible photon sensor has the potential to match the acoustic resistance of water. It will greatly expand the response research of ocean exploration in the low frequency range.

Doping of metal into transition metal dichalcogenides PdTe2

Rh-doped PdTe2 single crystals were successfully prepared using the solid-state reaction. The change of lattice constants and T c with increasing Rh content were studied and investigated. It was found that there is a very slight increase of lattice constant c with increasing Rh content. The values of T c were above 2 K when Rh content is equal to 0.05 and 0.1. The largest T c was~2.7 K for Rh0.05PdTe2 single crystal. T c does not increase with increasing x from 0.05 to 0.1. M-T measurement of Rh0.05PdTe2 single crystal with applied magnetic field H perpendicular to the c axis and along and c axis gives evidence of type II superconducting behavior. This study shows Rh doping is a promising method to increase T c and investigate SC in PdTe2.

Monocrystalline silicon materials with the group IVA elements doped: A first-principles analysis

In this paper, the changes of lattice constant, stability and electronic structural properties of Si64−xMx (M=C, Ge, Sn, Pb; x=0, 1, 2, 3, 4) doped system under various doping types and concentrations were studied by using first principles calculation to determine the influence of doped IVA family elements on the properties of monocrystalline silicon materials. The results have shown that, the Ge, Sn and Pb doped systems will produce an expansion effect, and the lattice constant of the C doped system will decrease with the increasing of the number of doped atoms, and will have the greatest impact on the lattice constant of the system. The increase in defect formation energy is Pb¿C¿Sn¿Ge, the Ge doped system is the easiest to synthesize; and as the doping concentration increases, the defect formation energy of each doping system also increases, indicating that high concentration doping systems are more difficult to synthesize. There are no virtual frequencies in the phonon spectra of all doped systems, and the systems are thermally stable. In addition, doping will gradually reduce the bandgap width as the doping concentration increases. The band gap characteristics will also change: when doped with C, the system will become Metalloid; when doped with Ge, the system remains unchanged; when doped with Sn and Pb, the system will become a direct band gap semiconductor. From the perspective of band gap and charge differential density, the effect of different doping elements on enhancing electronic transitions is C¿Pb¿Sn¿Ge. This means that C doping system has the best improvement in conductivity.

Effects of Double Doping Germanium and Indium on the Thermoelectric Properties of Permingeatite

Cu3Sb1–x–yGexInySe4 (0.02 ≤ x ≤ 0.12; 0.04 ≤ y ≤ 0.08) permingeatite compounds doped with Ge and In were prepared using solid-state synthesis. The phases and microstructures were analyzed, and the charge transport and thermoelectric properties were evaluated according to the Ge and In doping content. Most of the samples contained a single phase of permingeatite with a tetragonal structure; however, secondary phases (Cu0.875InSe2, In2Se3, and InSb) were detected in the samples with x = 0.12 and y = 0.08. Both the a-axis and c-axis lattice constants of permingeatite were increased by Ge and In doping, with a = 0.5651–0.5655 nm and c = 1.1249–1.1255 nm, but the change in lattice constant due to the change in doping amount was insignificant. The melting point of the sample double-doped with Ge and In was determined to be 736 K, which was lower than the melting point (741 K) of pure Cu3SbSe4. This lowering of the melting point was attributed to the formation of a solid solution. The electrical conductivity exhibited degenerate semiconductor behavior, decreasing with increasing temperature. As the Ge and In doping content increased, the carrier concentration and electrical conductivity increased; however, when x ≥ 0.12, the electrical conductivity did not increase further. The Seebeck coefficient exhibited positive values and p-type conduction characteristics. In addition, intrinsic transitions did not occur in the measurement temperature range, and the Seebeck coefficient increased as the doping level increased. The power factor exhibited a positive temperature dependence, and Cu3Sb0.86Ge0.08In0.06Se4 exhibited the highest value of 0.89 mWm–1K–2 at 623 K. As the temperature increased, the thermal conductivity tended to decrease because of the decreased lattice thermal conductivity and slightly increased electronic thermal conductivity. All the samples exhibited minimum thermal conductivities of 0.94–1.11 Wm–1K–1 at 523 K. At high temperatures, the double doping of Ge and In improved the thermoelectric performance; thus, the dimensionless figure of merit obtained at 623 K for Cu3Sb0.86Ge0.08In0.06Se4, was 0.47.

Thermoelectric properties of Tsai-type Au–(Al, Ga, In)–Gd approximants

In this work, the thermoelectric properties of Tsai-type Au–(Al, Ga, In)–Gd approximant crystals (ACs) are investigated over a temperature range of 300–773 K. The changes in lattice constants and effective speed of sound suggest a weaker chemical bonding caused by a replacement of Al by heavier Ga or In atoms. Based on electrical conductivity measurements, Au–Al–Gd shows the deepest pseudogap in spectrum conductivity at the Fermi level, followed by Au–Ga–Gd and Au–In–Gd ACs. The lattice thermal conductivity at 300 K is as low as 0.71–0.81 W m−1 K−1 and asymptotically approaches a minimum lattice thermal conductivity of 0.52–0.59 W m−1 K−1 as calculated using the Cahill model. As a result, the highest power factor and dimensionless figure of merit reach ∼120 µW m−1 K−2 and ∼0.008 at ∼700 K for the Au–In–Gd system. Our systematic approach of weakening the chemical bonding nature by heavy atom substitution promises a better thermoelectric performance of Tsai-type ACs.

Effects of nano-Al2O3 addition on microstructure, mechanical properties, and wear resistance of Ti(C, N)-based cermets

To enhance high-temperature wear resistance and cutting performances of cermet tools, nano-Al2O3 particles were added into four kinds of Ti(C,N)-based cermets. The microstructure, mechanical properties, wear resistance, and cutting performances were studied. The nano-Al2O3 was supposed to distribute in the binder phase, leading to the change of lattice constants and dispersion strengthening. Due to the distribution of nanoparticles at the grain boundaries, the movement of the grain boundaries was hindered, and the mean grain size decreased from 1.03 μm to 0.89 μm. The hardness and fracture toughness showed the change tendency of rising the peak and then falling with nano-Al2O3 addition. The cermet with 2 wt% nano-Al2O3 displayed the topmost hardness of 1654.4 HV30 and fracture toughness of 9.1 MPa·m1/2, due to the dispersion strengthening and pinning effect of nano particles. During the sliding wear tests at 700 °C, the wear rates of cermet containing 2 wt% nano-Al2O3 decreased by 63% compared to cermet without nano-Al2O3. The superior high-temperature wear resistance was ascribed to the thermal softening resistance of nano-Al2O3, the strengthening and toughening effect of nano-Al2O3 on matrix and the dense tribo-chemical layers. The abrasive wear of cermet with no nano-Al2O3 addition was more significant, while the wear behavior changed to the removal of tribo-chemical layers when nano-Al2O3 was 2 wt%. The tool life of cermet with 2 wt% nano-Al2O3 was increased by 50% in comparison with cermet without nano-Al2O3 during machining 45 steel.

Honeycomb-kagome FSL-graphene: A carbon allotrope as an ultra-high capacity anode material for fast-rechargeable sodium-ion battery

Sodium-ion batteries (SIBs) are promising candidates for large-scale energy storage due to the abundance and low cost of sodium. However, graphite, the primary anode for commercial lithium-ion batteries, cannot be applied to SIBs. Its two-dimensional (2D) counterpart graphene is also inactive toward Na ions because of the delocalized π-electron network. We propose an idea to tackle this problem by introducing kagome topology into the honeycomb lattice, creating localized electronic states for improving the Na storage performance. Herein, we design a form of 2D carbon allotrope (named FSL-graphene), consisting of a kagome and a honeycomb sublattice. It has excellent stability, which is confirmed by the superior cohesive energy, positive phonon modes, high thermal stability, and strong mechanical stability. FSL-graphene exhibits an ultra-high theoretical Na storage capacity of 3347.1 mA h g−1, superior to most previously reported 2D anode materials. In addition, it possesses low diffusion energy barriers (0.19–0.23 eV), low open-circuit voltages (0.59–0.61 V), and small changes in lattice constants (1.3%). Furthermore, the electrolytes with high dielectric constants (e.g., ethylene carbonate) could improve the adsorption and migration of Na on FSL-graphene. This study provides an insight for designing high-performance carbon anode materials for SIBs by focusing on the topological lattices.

Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn2O4.

Spinel compounds AB2X4 consist of both tetrahedral (AX4) and octahedral (BX6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn2O4 has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn2O4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be (Mn0.92+Fe0.13+)A(Mn3+Fe0.93+Mn0.12+)BO4. While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at TS ≈ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn3+ cation. Strong magnetoelastic coupling is unveiled at TN1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to TN2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn3+/Fe3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ∼25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O-B-O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn2O4. Our study indicates that FeMn2O4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.

Bandgap engineered 1.48 eV GaAs0.95P0.05 solar cell with enhanced efficiency using double BSF layer

One of the most important criteria to design more than 30% efficient III–V compound/Si based dual junction solar cell is that we must design atleast 20% efficient III–V compound material top cell. In this regard, we designed a bandgap engineered GaAs0.95P0.05 single junction solar cell with reduced bandgap of (E g ) = 1.48 eV. Reducing the bandgap from 1.72 eV to 1.48 eV for GaAs0.95P0.05 cell leads to generate higher short circuit current, while having the tradeoff with the open circuit voltage. Due to small change in lattice constant of GaAs0.95P0.05 cell, some recombination is observed near the junction area. Although the minimal degradation is observed in open circuit voltage, the higher short circuit current drives the overall efficiency of the GaAs0.95P0.05 single junction solar cell. The designed solar cell provides an extended internal absorption for longer wavelength of spectrum. The high electron mobility of 8500 cm2 V–S−1 was observed with very high electron to hole mobility ratio of 21.25. The optimization of the cell is done using two back surface field layers (AlInP and AlGaInP) of higher bandgap material. The high short circuit current density of J SC = 25.93 mA cm−2 with V OC = 1.1635 V achieved by the designed cell with the highest efficiency of η = 25%. The solar cell is irradiated under 1-Sun solar irradiation in the AM1.5 G environment providing 1000 W m−2 of power spectral density. The External and Internal Quantum efficiency of more than 95% is achieved by the designed solar cell.

Effect of Europium Substitution on the Structural, Magnetic and Relaxivity Properties of Mn-Zn Ferrite Nanoparticles: A Dual-Mode MRI Contrast-Agent Candidate

Magnetic nanoparticles (MNPs) have been widely applied as magnetic resonance imaging (MRI) contrast agents. MNPs offer significant contrast improvements in MRI through their tunable relaxivities, but to apply them as clinical contrast agents effectively, they should exhibit a high saturation magnetization, good colloidal stability and sufficient biocompatibility. In this work, we present a detailed description of the synthesis and the characterizations of europium-substituted Mn-Zn ferrite (Mn0.6Zn0.4EuxFe2-xO4, x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.15, herein named MZF for x = 0.00 and EuMZF for others). MNPs were synthesized by the coprecipitation method and subsequent hydrothermal treatment, coated with citric acid (CA) or pluronic F127 (PF-127) and finally characterized by X-ray Diffraction (XRD), Inductively Coupled Plasma (ICP), Vibrating Sample Magnetometry (VSM), Fourier-Transform Infrared (FTIR), Dynamic Light Scattering (DLS) and MRI Relaxometry at 3T methods. The XRD studies revealed that all main diffraction peaks are matched with the spinel structure very well, so they are nearly single phase. Furthermore, XRD study showed that, although there are no significant changes in lattice constants, crystallite sizes are affected by europium substitution significantly. Room-temperature magnetometry showed that, in addition to coercivity, both saturation and remnant magnetizations decrease with increasing europium substitution and coating with pluronic F127. FTIR study confirmed the presence of citric acid and poloxamer (pluronic F127) coatings on the surface of the nanoparticles. Relaxometry measurements illustrated that, although the europium-free sample is an excellent negative contrast agent with a high r2 relaxivity, it does not show a positive contrast enhancement as the concentration of nanoparticles increases. By increasing the europium to x = 0.15, r1 relaxivity increased significantly. On the contrary, europium substitution decreased r2 relaxivity due to a reduction in saturation magnetization. The ratio of r2/r1 decreased from 152 for the europium-free sample to 11.2 for x = 0.15, which indicates that Mn0.6Zn0.4Eu0.15Fe1.85O4 is a suitable candidate for dual-mode MRI contrast agent potentially. The samples with citric acid coating had higher r1 and lower r2 relaxivities than those of pluronic F127-coated samples.

Structural Changes due to Heating and Stress Loading of Metastable Quenched Martensite Structures in Ti–(10–20) mass% Nb Alloys

The metastable-phase characteristics of Ti–Nb alloys can be exploited to improve their functional properties such as damping. In this study, we investigated the structural changes in the metastable quenched martensite structure of Ti–Nb alloys subjected to heating and tensile strain. We examined the differential scanning calorimetry (DSC) heating curves in the reduction state, X-ray diffraction (XRD) profiles under loading/unloading, and material properties such as Young’s modulus and internal friction upon heating. In the DSC heating curve of the 10%-cold-rolled Ti15Nb specimen, an exothermic peak was observed, and for Ti18Nb and Ti20Nb, the exothermic peak exhibited broadening. We speculate that the underlying reason is the biphasic formation resulting from specimen deformation. From the XRD measurements, we found that the lattice tended to shrink upon stress application and recover upon unloading. Significant changes in Young’s modulus and internal friction were observed in the α′′ structures of Ti18Nb and Ti20Nb during initial heating up to 373 K. We posit that the material properties changed owing to structural changes, such as lattice-constant changes, biphasic formation, and crystal orientation changes, resulting from heating or plastic deformation.

Pressure-induced first-order antiferromagnetic to ferromagnetic transition in MnN

MnN systems have been widely used to investigate Mn incorporated into Ш–V compounds, which combines the spin and electron charge as promising diluted magnetic semiconductors (DMS). However, few experiments have demonstrated the magnetic transformation of MnN with lattice constant changes, although it has been mentioned many times in previous theoretical calculations. In this study, we synthesize MnN single crystal under high pressure and temperature. Furthermore, we perform high-pressure resistance, magnetic resistance and synchrotron radiation on MnN single crystal/polycrystalline and observe the first-order phase transition accompanied with antiferromagnetic (AFM) to ferromagnetic (FM) magnetic structure transition under 34 GPa. The experimental result and first-principles calculations reveal the same magnetic and structural transition of MnN in the compression process from equilibrium volume 18.2 to16 Å3. An analysis of electronic structure and magnetic interactions in terms of the Heisenberg Hamiltonian indicates a strong FM coupling in the [001] plane due to the double exchange mechanism of Mn eg orbitals. With the reduced Mn-N bond length under high pressure, the enhanced FM double exchange interactions could induce the magnetic transition from AFM to FM and subsequently lead to a structural transition from a face-centered tetragonal structure to a cubic structure. These findings provide the foundations for GaMnN or MnN layer DMS.

Observation of interfacial strain relaxation and electron beam damage thresholds in Al0.3In0.7N/GaN heterostructures by transmission electron microscopy

The structural quality of indium-rich Al0.3In0.7N grown by metal modulated epitaxy (MME), previously demonstrating x-ray diffraction (XRD) figures of merit ∼11 times better than the previous literature is investigated to explain the origin of such a large quality improvement. Four-dimensional scanning transmission electron microscope was used to map the lattice parameter near the AlInN/GaN heterojunction and indicate a 5.4% lattice constant change, suggesting 75% relaxation within ∼2 nm from the interface. Cross-sectional TEM Moiré fringes are observed at the AlInN/GaN heterointerface, indicating that there are misfit dislocations between AlInN and GaN which, while rare, have been observed for other highly mismatched In-rich III-Nitrides. The TEM measurements show regions of contrast indicating larger scale variations in strain, but defect contrast associated with dislocations and/or intrinsic basal stacking faults was minimal, indicating a good quality AlInN film and confirming prior XRD results. Significant electron beam induced damage can occur and depended strongly on operational conditions. The damage threshold current density was estimated using time-dependent TEM to be ∼5.7 A/cm2, significantly lower than from prior studies of InGaN. Damage also strongly depends on the thickness of the TEM foil examined and occurred at thicknesses greater than found for InGaN. The present study suggests that the MME technique is an excellent candidate for growing high-quality indium-rich AlInN films as compared to the traditional molecular beam epitaxy or metal organic chemical vapor deposition techniques.