Single Crystalline Samples Research Articles

Bulk high-temperature superconductivity in pressurized tetragonal La2PrNi2O7.

The Ruddlesden-Popper (R-P) bilayer nickelate, La3Ni2O7, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa (ref. 1). Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions2-4. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction2,4,5. The presence of a new 1313 polymorph and competing R-P phases obscured proper identification of the phase for HTSC6-9. Thus, achieving bulk HTSC and identifying the phase at play are the most prominent tasks. Here we address these issues in the praseodymium (Pr)-doped La2PrNi2O7 polycrystalline samples. We find that substitutions of Pr for La effectively inhibit the intergrowth of different R-P phases, resulting in a nearly pure bilayer structure. For La2PrNi2O7, pressure-induced orthorhombic to tetragonal structural transition takes place at Pc ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18-20 GPa reach and , which are the highest values, to our knowledge, among known nickelate superconductors. Importantly, bulk HTSC was testified by detecting clear diamagnetic signals below about 75 K with appreciable superconducting shielding volume fractions at a pressure of above 15 GPa. Our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.

Engineering Carrier Density and Effective Mass of Plasmonic TiN Films by Tailoring Nitrogen Vacancies.

The introduction of nitrogen vacancies has been shown to be an effective way to tune the plasmonic properties of refractory titanium nitrides. However, its underlying mechanism remains debated due to the lack of high-quality single-crystalline samples and a deep understanding of electronic properties. Here, a series of epitaxial titanium nitride films with varying nitrogen vacancy concentrations (TiNx) were synthesized. Spectroscopic ellipsometry measurements revealed that the plasmon energy could be tuned from 2.64 eV in stoichiometric TiN to 3.38 eV in substoichiometric TiNx. Our comprehensive analysis of electrical and plasmonic properties showed that both the increased electronic states around the Fermi level and the decreased carrier effective mass due to the modified electronic band structures are responsible for tuning the plasmonic properties of TiNx. Our findings offer a deeper understanding of the tunable plasmonic properties in epitaxial TiNx films and are beneficial for the development of nitride plasmonic devices.

Magnetic structure of the noncentrosymmetric magnet Sr2MnSi2O7 through irreducible representation and magnetic space group analyses.

Magnetic structures of the noncentrosymmetric magnet Sr2MnSi2O7 were examined through neutron diffraction for powder and single-crystalline samples, as well as magnetometry measurements. All allowed magnetic structures for space group P421m with the magnetic wavevector qm = (0, 0, ½) were refined via irreducible representation and magnetic space group analyses. The compound was refined to have in-plane magnetic moments within the magnetic space group Cmc21.1'c (No. 36.177) under zero field, which can be altered to P212121.1'c (No.19.28) above μ0H = 0.067 (5) T to align induced weak-ferromagnetic components within one layer on the ab plane. All refined parameters are provided following the recent framework based upon the magnetic space group, which better conveys when exchanging crystallographic information for commensurate magnetic structures.

Atomistic Simulations of the Shock and Spall Behavior of the Refractory High-Entropy Alloy HfNbTaTiZr

AbstractUsing molecular dynamics simulation, we study the effect of a shock wave on the refractory high-entropy alloy HfNbTaTiZr. A single-crystalline sample, shocked along the [001] direction, is considered. The initial compression leads to only weak dislocation activity and a bcc $$\rightarrow$$ → hcp transformation in some regions of the sample. After the shock wave is reflected from the free back surface of the sample, hcp transforms back to bcc, and twins are formed in the bcc phase. The sample spalls under the high tensile pressures developing after wave reflection. In this stage, we observe dislocation activity from the twin boundaries and inside the nanograins generated by twinning. Under the large tensile stresses, some fcc phase appears together with disordered amorphous regions where voids nucleate and lead to spall. The fracture surfaces follow the twin boundaries set up in the compression phase. The spall strength is similar to the one found in simulations of other bcc metals at similar strain rates. Similar simulations for the equiatomic HfNbTaZr HEA show the same qualitative behavior, with twins and reduced dislocation activity, but without phase transformations.

First-order transition under a magnetic ordered state in SmPtSi2

We have synthesized polycrystalline and single-crystalline samples of SmPtSi2, and measured their resistivity, specific heat, magnetization, and Seebeck coefficient. The existence of two magnetic phase transitions has been confirmed, one at T H = 8.6 K and the other T L = 5.6 K. A hump-type anomaly in resistivity, a lambda-type anomaly in specific heat, a downward bend in magnetization, and a semiconductor-like increase in the Seebeck coefficient were observed at T H, indicating an antiferromagnetic transition accompanied by a gap opening on the Fermi surface. In contrast, a sharp drop in resistivity, a sharp spike in specific heat, and a drop in magnetization were observed at T L. The Seebeck coefficient showed metallic temperature dependence below T L. With increasing pressure, T H and T L shifted to higher temperatures. The transition at T L was no longer observed at pressures above 1.5 GPa. These findings suggest that the transition at T L is a transition from an antiferromagnetic ordered state with the gap on the Fermi surface to a different antiferromagnetic state.

Crack growth and fracture mechanics of CuCrFeNiCo high-entropy alloy during tension testing

In this work, the crack growth and fracture mechanics of CuCrFeNiCo high-entropy alloy (HEA) during tension process are studied through molecular dynamics simulation method. The single-crystalline, nanocrystalline, and twinned-nanocrystalline CuCrFeNiCo HEA samples with an initial crack are prepared. The influences of boundary conditions, crack length and crystallographic orientation are considered in detail. The results indicate that the phase transition from face-centered cubic (FCC) structure into hexagonal close-packed (HCP) structure and the appearance of Shockley dislocations are the majority in all samples. The dislocations appear most densely in the twinned-nanocrstalline sample and most sparsely in the single-crystalline sample. The growth of the initial crack combined with the formation and expansion of new cracks along the grain boundaries (GBs) is the determining factor in the fracture mechanics of the CuCrFeNiCo HEA samples. The deformation capacity of the samples with free boundary conditions along the y-axis is better and the plastic deformation process is longer than the samples with periodic boundary conditions along the y-axis. The tensile strength values of the CuCrFeNiCo HEA samples change significantly in the range from 2.61 GPa to 7.75 GPa when changing the simulation conditions. The von Mises stress in the grains is markedly lower than that in the GBs.

Spin stiffness and spin excitation gap of van der Waals ferromagnetic Fe3+δGeTe2

Fe3+δGeTe2 (FGT) has proved to be an interesting van der Waals (vdW) ferromagnetic compound with a tunable Curie temperature ( TC ). However, the underlying mechanism for varying TC remains elusive. Here, we systematically investigate and compare low-temperature magnetic properties of single crystalline FGT samples that exhibit TC s ranging from 160 K to 205 K. Spin stiffness (D) and spin excitation gap (Δ) are extracted using Bloch’s theory for crystals with varying Fe content. Compared to Cr-based vdW ferromagnets, FGT compounds have higher spin stiffness values but lower spin wave excitation gaps. We discuss the implication of these relationships in Fe–Fe ion magnetic interactions in FGT unit cells. The itinerancy of magnetic electrons is measured and discussed under the Rhodes–Wohlfarth ratio (RWR) and the Takahashi theory.

Strong influence of magnetic field and non-uniform stress on elastic modulus and transition temperatures of twinned Ni–Fe(Co)–Ga alloy

The magnetization value and electric resistivity of the single-crystalline sample of Ni50Fe19Co4Ga27 shape memory alloy were measured. The elastic modulus was determined by the Dynamic Mechanical Analysis (DMA). The characteristic temperatures of martensitic transformation (MT) of the alloy were estimated from the temperature dependences of magnetization, electric resistivity and elastic modulus. A significant disparity between MT temperatures resulting from DMA and those estimated from magnetic and resistivity measurements was discovered. It was argued that the discrepancy is caused by the non-uniform mechanical stressing of twinned single crystal by the DMA analyzer. Moreover, the DMA measurements revealed a significant decrease of the elastic modulus of twinned martensite under the applied magnetic field of 1.5 kOe. To explain this effect, the temperature-dependent Young’s modulus of twinned crystal lattice was computed. The computations showed that the experimentally observed field-induced change of the elastic modulus is caused by the stress-assisted detwinning of the crystal lattice by the applied magnetic field.

Momentum-Resolved Electronic Structures and Strong Electronic Correlations in Graphene-like Nitride Superconductors.

Although transition-metal nitrides have been widely applied for several decades, experimental investigations of their high-resolution electronic band structures are rare due to the lack of high-quality single-crystalline samples. Here, we report on the first momentum-resolved electronic band structures of titanium nitride (TiN) films, which are remarkable nitride superconductors. The measurements of the crystal structures and electrical transport properties confirmed the high quality of these films. More importantly, from a combination of high-resolution angle-resolved photoelectron spectroscopy and first-principles calculations, the extracted Coulomb interaction strength of TiN films can be as large as 8.5 eV, whereas resonant photoemission spectroscopy yields a value of 6.26 eV. These large values of Coulomb interaction strength indicate that superconducting TiN is a strongly correlated system. Our results uncover the unexpected electronic correlations in transition-metal nitrides, potentially providing a perspective not only to understand their emergent quantum states but also to develop their applications in quantum devices.

Structure and properties of a quasi-one-dimensional compound BaTiS3 under pressure

ABSTRACT We report the studies of structure and properties of a quasi-one-dimensional (quasi-1D) compound BaTiS 3 under pressure. The poly-crystalline samples are synthesized by solid-state reaction method and single-crystalline samples are produced by chemical vapor transport method. The XRD measurement under ambient pressure confirms the hexagonal structure, which consists of face-sharing octahedral TiS 6 chains along c-axis and displays a quasi-1D feature. When applying pressure, the synchrotron X-ray diffraction experiments show that the sample undergoes a phase transition from hexagonal phase to orthorhombic phase at around 10 GPa due to the zig–zag deformation of TiS 6 chains, while the quasi-1D structure is reserved. At ambient pressure, BaTiS 3 exhibits an insulating behavior with a band gap about 0.273 eV. When applying pressure, the sample undergoes a crossover from insulator to metal due to the enhancement of inter-chain electron hopping.

Recrystallization and heterovalent substitution effects on mechanical and electrical parameters of Ag6+x(P1−xGex)S5I–based ceramics

Herein, we present the results of a study of nanopowders of Ag6.25P0.75Ge0.25S5I, Ag6.5P0.5Ge0.5S5I solid solutions, and ceramic materials based on them. Nanopowders were obtained by dry grinding in a ball mill and characterized by XRD and SEM. Solid solutions based ceramic materials obtained by annealing of pressed disks were studied by SEM, EDS, EIS, and microhardness measurement. The SEM method revealed a change in the size of crystallites during recrystallization. The frequency dependence of the electrical conductivity is typical for superionic conductors. For ceramics of the composition Ag6.5P0.5Ge0.5S5I, a slight increase in ionic electrical conductivity of 3.62 × 10–2 S/cm (30 min) and 3.84 × 10–2 S/cm (60 min) is observed with a decrease in crystallite size. For the entire series of solid solutions of Ag6+x(P1−xGex)S5I, a comparative analysis of the parameters of ceramics made from micro- and nanocrystalline powders (with different grain sizes) with single crystalline samples was performed.

Unveiling the structural phase transition and magnetic structure of ternary boride PrIr3B2

We present a comprehensive study of PrIr3B2, which includes a detailed investigation of its crystal and magnetic structure using neutron diffraction. AC and DC magnetization and heat capacity data reveal antiferromagnetic ordering at T N = 10 K. The heat capacity measurements further exhibit a broad peak near 270 K which is related to a structural transition from P6/mmm to C2/m seen in low temperature x-ray diffraction and neutron diffraction. High intensity neutron diffraction data confirm the long-range ordering of Pr3+ spins, with no apparent magnetic moment on either of the Iridium sites. Two possible magnetic structures with either k 1 = [1,0,0] or k 2 = [½,½,0] fit nearly equally well the neutron diffraction data. However, based on previous magnetization studies on a single crystalline sample it is argued that the second solution with k 2 corresponds to the appropriate magnetic structure of PrIr3B2 below 10 K. In this magnetic structure, the Pr3+ moments are oriented at ∼45° to both the a and b axes, with the c-axis being the hard axis of magnetization. Overall, our results provide new insights into the magnetic and structural properties of PrIr3B2.

Unusual spectral features in square-net based nonsymmorphic Kondo lattice system, CeCuX2 (X = As/Sb)

We study the electronic structure of a nonsymmorphic Kondo lattice system, CeCuX2 (X = As/Sb), a promising class of correlated topological materials important for advanced technology. While both the materials show Kondo behavior in their transport properties, CeCuSb2 is antiferromagnetic and no magnetic order is observed in CeCuAs2. We studied high-quality single-crystalline samples employing hard x-ray photoemission spectroscopy. The sample cleaving exposes the square-net structured pnictogen layers. The CeCuSb2 valence band indicates a highly metallic phase. The spectral intensity at the Fermi level in CeCuAs2 is weak, revealing close to semi-metallic behavior of the system. The Ce 3d spectra exhibit multiple features; the intensity of the features changes with the change in surface sensitivity of the technique, suggesting significant differences in the surface and bulk electronic structure. The bulk spectra of the Kondo system do not exhibit the typical f0-feature often observed in such materials. Instead, a distinct feature is observed at the lower binding energy side of the well-screened peak; the signature of this feature is manifested in the spectra from high-quality single-crystalline samples. This is outstanding and calls for physics beyond existing theories of correlated systems.

The influence of deformation texture on nucleation and growth of cube grains in aluminium

The influence of deformation texture on the initial stages of recrystallization of deformed pure Al single crystals and commercial AA1050 alloy were characterized. To precisely quantify the orientation relationship between nuclei and simple deformation structures, single crystalline samples of stable Br(110) [1-1-2] and semi-stable S’(234) [20-28 11] orientations, were plane strain compressed (PSC) to 40% to develop a dominant homogeneous microstructure composed of two sets of microbands. The samples were then lightly annealed to reach the initial stage of primary recrystallization. The results on single crystals were related to the behaviour of the polycrystalline samples in state after hot-rolling then PSC to the same degree and lightly annealed. SEM/EBSD analyses of single crystalline samples demonstrate that the orientations of recrystallized grains are limited to a certain number of orientation groups. However, the orientations of the as-deformed state are not retained in the orientations of new grains. Deformed samples with Br orientation do not show the formation of cube grains during recrystallization, whereas strong nucleation of cube-oriented grains is observed in homogeneously and heterogeneously deformed areas in the semi-stable S’ orientation, despite the cube-oriented nuclei not present in the as-deformed structure. The recrystallization texture components of the AA1050 alloy were related to the standard β-fibre texture formed during the previous cold deformation. Cube grains formed intensively in PSC polycrystalline samples, primarily in association with as-deformed areas with near four variants of S(124) [21-1] and S’ orientations and were characterized by local misorientations around the <111> axes.

Redetermination of the crystal structure of yttrium chromium tetra-boride, YCrB4, from single-crystal X-ray diffraction data.

The structural parameters of yttrium chromium tetra-boride YCrB4 were refined based on single-crystal X-ray diffraction data. YCrB4 is ortho-rhom-bic, having a space group of type Pbam (No. 55) and with lattice parameters of a = 5.9425 (2), b = 11.4831 (4), c = 3.4643 (1) Å. The Y and Cr atoms are located at Wyckoff 4h sites (x, y, 0) and B atoms at the Wyckoff 4g sites (x, y, 1/2). The first structural investigation of YCrB4 was performed using a single crystalline sample [Kuz'ma, (1970 ▸). Kristallografiya. 15, 372-374]. The present study successfully refined all the positional and atomic displacement parameters of the Y, Cr, and B atoms.

Effect of crystal morphology of ultrahigh-nickel cathode materials on high temperature electrochemical stability of lithium ion batteries

Higher nickel content endows Ni-rich cathode materials LiNixCoyMn1−x−yO2 (x > 0.6) with higher specific capacity and high energy density, which is regarded as the most promising cathode materials for Li-ion batteries. However, the deterioration of structural stability hinders its practical application, especially under harsh working conditions such as high-temperature cycling. Given these circumstances, it becomes particularly critical to clarify the impact of the crystal morphology on the structure and high-temperature performance as for the ultrahigh-nickel cathodes. Herein, we conducted a comprehensive comparison in terms of microstructure, high-temperature long-cycle phase evolution, and high-temperature electrochemical stability, revealing the differences and the working mechanisms among polycrystalline (PC), single-crystalline (SC) and Al doped SC ultrahigh-nickel materials. The results show that the PC sample suffers a severe irreversible phase transition along with the appearance of microcracks, resulting a serious decay of both average voltage and the energy density. While the Al doped SC sample exhibits superior cycling stability with intact layered structure. In-situ XRD and intraparticle structural evolution characterization reveal that Al doping can significantly alleviate the irreversible phase transition, thus inhibiting microcracks generation and enabling enhanced structure. Specifically, it exhibits excellent cycling performance in pouch-type full-cell with a high capacity retention of 91.8% after 500 cycles at 55 °C. This work promotes the fundamental understanding on the correlation between the crystalline morphology and high-temperature electrochemical stability and provides a guide for optimization the Ni-rich cathode materials.

ELECTRICAL CONDUCTIVITY OF SINGLE CRYSTALLINE Ag7PS6

The search and study of new materials suitable for use in the newest solid-state batteries is an important direction of modern solid-state ionics. Among the potential materials, it is worth mentioning compounds of the argyrodite family. The characteristic feature of these phases is the presence of a disordered cationic sublattice, which provides significant cationic conductivity in the solid state. This work presents the results of studies of the electrical conductivity of a single crystalline sample of ltm-Ag7PS6. The single crystal of Ag7PS6 was grown by the method of directional crystallization from the melt. The crystal was identified by means of XRD method. It was found that the grown Ag7PS6 sample crystallizes in a low-temperature modification in a primitive cubic cell. The frequency (1×101–3×105 Hz) and temperature (25-100°C) dependence of the total electrical conductivity was measured on an oriented and specially prepared ltm-Ag7PS6 single crystalline plate (using gold contacts). The total conductivity of Ag7PS6 single crystals is 6.51×10–7 S/cm (t = 25°C). According to the results of temperature studies of the dependence of the total conductivity, the thermal activation nature of the conductivity was established.&#x0D; Keywords: argyrodites; single crystals; phase analysis; electrical conductivity.

Anionic framework descriptors and microstructure affects on optical parameters of Ag7+x(P1-xGex)S6 single crystals

Herein we present a detailed study of single crystalline samples of Ag7+x(P1-xGex)S6 (x = 0, 0.1, 0.25, 0.33, 0.5, 0.75, 1) solid solutions. The Ag7+x(P1-xGex)S6 crystals were grown by directional crystallization from melt and characterized by techniques of XRD, Raman scattering, optical transmission measurements, spectral ellipsometry and electron microscopy. XRD and Raman scattering indicates a formation of solid solutions. The spectral dependences of refractive index are nonlinear with a maximum at the spectral range of ∼645–820 nm. It has been established, that P5+→Ge4+ cationic substitution leads to a monotonic nonlinear increase in the refractive index n from 2.58 (x = 0) to 2.75 (x = 1). The obtained values of refractive index were used to describe the optical parameters by the Wemple-DiDomenico equation. The spectral dependences of lnα at all studied temperatures (77–300 K) have an exponential shape near the absorption edge, indicating that they obey Urbach's rule. The corresponding value of the pseudo-gap Eg* and Urbach energy EU were determined. Increasing the Ge content in solid solutions causes a decrease in Eg* (1.971–1.466 eV) and EU (179.46–21.90 meV) values. Revealed Eg* values suggest that single crystalline Ag7+x(P1-xGex)S6 might be used as perspective solar harvesting materials. The practical application of these materials can be further enhanced with the results of temperature studies of variations of Eg* values. The changes in optical parameters are explained based on the structural and microstructural parameters of the studied solid solutions.

Influence of recrystallization process on ionic conductivity of Ag6.75P0.25Ge0.75S5I based ceramic materials

This paper presents the results of research on ceramic materials based on Ag6.75(P0.25Ge0.75)S5I nanopowders with different grain size. Nanopowders with an average grain size of ∼100 and ∼160 nm, obtained by ball mill grinding, were characterized by XRD, SEM and diffuse reflectance methods. Ag6.75(P0.25Ge0.75)S5I ceramic samples were obtained by annealing pre-pressed disks at a pressure of 400 MPa. The microstructure and chemical composition of the obtained Ag6.75(P0.25Ge0.75)S5I ceramics were determined by SEM and EDS methods. The prepared Ag6.75(P0.25Ge0.75)S5I ceramics are chemically homogeneous with an average crystallite size of ∼1.99 and ∼2.29 μm and a low porosity of ∼6.4%. The electrical parameters of the investigated ceramics were characterized by the electrochemical impedance spectroscopy method. For both ceramics, there is an increase in total electrical conductivity with increasing frequency. It was established that the values of ionic conductivity of nanoceramic samples exceed the corresponding value of the bulk single crystalline sample. The influence of the recrystallization process and grain size on the electrical parameters of Ag6.75(P0.25Ge0.75)S5I ceramics is explained.

Mechanically exfoliated low-layered [Ca2CoO3]0.62[CoO2]: A single-crystalline p-type transparent conducting oxide

Transparent conducting oxides (TCOs) are essential components of optoelectronic devices and various materials have been explored for highly efficient TCOs having a combination of high transmittance and low sheet resistance. Here, we focus on a misfit thermoelectric oxide [Ca2CoO3]0.62[CoO2] and fabricate the transparent low-layered crystals by a mechanical tape-peeling method using the single-crystalline samples. From the transmittance measurement, we find that the thickness of low-layered samples is several orders of hundred nanometers, which is comparable with the estimation from the scanning electron microscopy images. Compared to the previous results on the polycrystalline and c-axis oriented transparent films, the electrical resistivity is reduced owing to the single-crystalline nature. The figure of merit for the transparent conducting materials in the present low-layered samples is then evaluated to be higher than the values in the previous reports. The present results on the low-layered single-crystalline [Ca2CoO3]0.62[CoO2] may offer a unique class of multi-functional transparent thermoelectric oxides.