C-axis Direction Research Articles

Thermal conductivity of WC: Microstructural design driven by first-principles simulations

The relationships between the microstructure and the thermal conductivity of binderless WC have been quantified, considering crystal orientation, isotopic abundance, porosity, and grain size. A significantly higher conductivity is predicted in the out-of-plane (c-axis) direction vs. the in-plane (a-axis) direction, using first principles simulations. Isotopic enrichment of the tungsten sublattice is predicted to increase conductivity, e.g., by a factor of 4–5 in the absence of boundary scattering. The results suggest that for an isotopically pure single crystal a thermal conductivity exceeding 1000 W m−1 K−1 may be achievable normal to the basal plane. The conductivity of samples with various porosities could be well fit by a minimum surface area (exponential) model, with a porosity exponent of b = 4.4. Experiment and simulation show a strong grain size dependence to conductivity below 1 µm, with a saturation beyond ∼10 µm. The experimental plateau values for κ were ∼45 % lower than those of the simulations due to deviations from perfect stoichiometry. We also find a higher scattering coefficient in the experiments, likely due to effects of grain size distribution and elongation. Our study clarifies the physical origin of disagreeing literature reports as being predominantly due to grain boundary scattering and enables microstructural design for thermally demanding environments.

(1H-Benzodiazol-2-ylmethyl)diethylamine

In the crystal of the title compound, C12H17N3, the molecules are linked by N—H...N hydrogen bonds, generating a C(4) chain extending along the c-axis direction. One of the ethyl groups is disordered over two sets of sites with a refined occupancy ratio of 0.582 (15):0.418 (15).

Benzilic acid: a monoclinic polymorph

The title compound, C14H12O3, is an α-hydroxycarboxylic acid whose orthorhombic polymorph has been reported earlier [Qiu et al. (2007). Inorg. Chim. Acta, 360, 1819–1824]. The asymmetric unit contains two complete molecules. Classical hydrogen bonds, as well as C—H...O contacts, connect the molecules to infinite chains along the crystallographic c-axis direction.

Theoretical insights into the ultrahigh piezoelectric performance of (BiFeO3)n/(BaTiO3)n (n = 1–5) superlattices

Perovskite structures have attracted extensive attention in microelectromechanical systems and nanoelectromechanical systems devices due to the high piezoelectric response and the low dielectric constant. The piezoelectric and dielectric properties of the tetragonal BiFeO3/BaTiO3 superlattices grown along the c-axis direction are investigated using density functional theory (DFT) and density functional perturbation theory. The calculated results demonstrate that the (BiFeO3)n/(BaTiO3)n (n = 1–5) superlattices exhibit a profoundly increased piezoelectric response compared to their bulk structures. The (BiFeO3)2/(BaTiO3)2 possesses the highest piezoelectric d33 of 697 pC/N among known lead-free perovskite systems. Furthermore, the (BiFeO3)2/(BaTiO3)2 superlattice possesses a low dielectric ɛ33, and its d33 at 2% tensile strain is 16 times larger than that of an unstrained equilibrium structure. This demonstrates that biaxial tensile strain significantly enhances the piezoelectric response. Combining the special quasirandom structure method with DFT, the structures of a 0.5BiFeO3–0.5BaTiO3 solid solution are predicted, and its calculated d33 is 58 pC/N, which is much smaller than that of a (BiFeO3)2/(BaTiO3)2 superlattice. The results suggest that the (BiFeO3)2/(BaTiO3)2 superlattice might be a potential candidate for nonvolatile random access memory, transducers and actuators, and nanoscale electronic devices.

Crystal structure of (μ2-7-{[bis(pyridin-2-ylmethyl)amino-1κ3 N,N′,N′′]methyl}-5-chloroquinolin-8-olato-2κN;1:2κ2 O)trichlorido-1κCl,2κ2 Cl-dizinc(II)

The title compound, [Zn2(C22H18ClN4O)Cl3], is a dinuclear zinc(II) complex with three chlorido ligands and one pentadentate ligand containing quinolin-8-olato and bis(pyridin-2-ylmethyl)amine groups. One of the two ZnII atom adopts a tetrahedral geometry and coordinates two chlorido ligands with chelate coordination of the N and O atoms of the quinolin-8-olato group in the ligand. The other ZnII atom adopts a distorted trigonal–bipyramidal geometry, and coordinates one chlorido-O atom of the quinolin-8-olato group and three N atoms of the bis(pyridin-2-ylmethyl)amine unit. In the crystal, two molecules are associated through a pair of intermolecular C—H...Cl hydrogen bonds, forming a dimer with an R 2 2(12) ring motif. Another intermolecular C—H...Cl hydrogen bond forms a spiral C(8) chain running parallel to the [010] direction. The dimers are linked by these two intermolecular C—H...Cl hydrogen bonds, generating a ribbon sheet structure in ac plane. Two other intermolecular C—H...Cl hydrogen bonds form a C(7) chain along the c-axis direction and another C(7) chain generated by a d-glide plane. The molecules are cross-linked through the four intermolecular C—H...Cl hydrogen bonds to form a three-dimensional network.

Unconventional Crystal Structure of the High-Pressure Superconductor La_{3}Ni_{2}O_{7}.

The discovery of high-temperature superconductivity in La_{3}Ni_{2}O_{7} at pressures above 14GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO_{6} octahedral bilayers stacked along the c-axis direction was consistently posited in initial studies on La_{3}Ni_{2}O_{7}. Here, we reassess this structure in optical floating zone-grown La_{3}Ni_{2}O_{7} single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO_{6} octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La_{3}Ni_{2}O_{7}, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.

Temperature-controlled synthesis of novel boron nanofibers by laser ablation technique

The synthesis of boron nanofibers (BNFs) at different temperatures using the double-pulsed laser ablation (DPLA) technique were reported. Q-switched Nd: YAG laser with 532 and 1064 nm dual beam wavelengths of laser used to ablate a solid boron target. A vapor–solid process at a furnace temperature and pressure supported boron 'nanofibers' growth with the active metal catalysts of 1 % Ni and 1 % Co in a quartz tube furnace in flowing argon (Ar) gas. The crystalline phase purity of the synthesized BNFs at 950, 1050, and 1150 °C were determined by X-ray diffraction (XRD), and the results from XRD confirm that each BNF is preferentially grown in the c-axis (100) direction of alpha boron (α-B). Electron microscopy revealed that each BNF exhibit a length of 1–5 μm and a diameter width of 5–100 nm, while the elemental chemical compositional nature of the BNFs is identified from energy dispersive X–ray spectroscopy (EDX). Image analysis shows the average diameter of each BNF synthesized to be 23.10, 19.13, and 16.09 nm. The lattice distance of each BNF is found to be 0.40, 0.39, and 0.38 nm, respectively. Fingerprint Raman spectroscopy revealed the fundamental vibrational modes of BNFs at Eg and A1g.

Coupling between 2-pyridyl-selenyl chloride and phenyl-seleno-cyanate: synthesis, crystal structure and non-covalent inter-actions.

A new pyridine-fused seleno-diazo-lium salt, 3-(phenyl-selan-yl)[1,2,4]selena-diazolo[4,5-a]pyridin-4-ylium chloride di-chloro-methane 0.352-solvate, C12H9N2Se2 +·Cl-·0.352CH2Cl2, was obtained from the reaction between 2-pyridyl-selenenyl chloride and phenyl-seleno-cyanate. Single-crystal structural analysis revealed the presence of C-H⋯N, C-H⋯Cl-, C-H⋯Se hydrogen bonds as well as chalcogen-chalcogen (Se⋯Se) and chalcogen-halogen (Se⋯Cl-) inter-actions. Non-covalent inter-actions were explored by DFT calculations followed by topological analysis of the electron density distribution (QTAIM analysis). The structure consists of pairs of seleno-diazo-lium moieties arranged in a head-to-tail fashion surrounding disordered di-chloro-methane mol-ecules. The assemblies are connected by C-H⋯Cl- and C-H⋯N hydrogen bonds, forming layers, which stack along the c-axis direction connected by bifurcated Se⋯Cl-⋯H-C inter-actions.

Anisotropic thermoelectric power in single crystalline Mg1-xAlxB2 compounds

The thermoelectric power of single crystalline Mg1−xAlxB2 compounds is measured and analyzed along the main crystallographic directions (ab-plane and c-axis direction). It is shown that the classical model of thermoelectric power with phonon drag and diffusion terms provides satisfactory explanation of the temperature dependence of the Seebeck parameter S(T). Agreement of S(T) with the study of resistivity is achieved with some variation of the Debye temperature being a model-sensitive parameter. Presence of positive and negative charge carriers is observed and competing contributions of the π and σ bands are discussed.

Crystal structure of propane-1,3-diaminium squarate dihydrate.

Propane-1,3-diaminium squarate dihydrate, C3H12N2 2+·C4O4 2-·2H2O, results from the proton-transfer reaction of propane-1,3-di-amine with squaric acid and subsequent crystallization from aqueous medium. The title compound crystallizes in the tetra-gonal crystal system (space group P4bm) with Z = 2. The squarate dianion belongs to the point group D 4h and contains a crystallographic fourfold axis. The propane-1,3-diaminium dication exhibits a C 2v -symmetric all-anti conformation and resides on a special position with mm2 site symmetry. The orientation of the propane-1,3-diaminium ions makes the crystal structure polar in the c-axis direction. The solid-state supra-molecular structure features a triperiodic network of strong hydrogen bonds of the N-H⋯O and O-H⋯O types.

In-plane hard magnetic BaFe12O19 thin films grown on a-plane Al2O3 112¯0 substrates

Ferrimagnetic barium hexaferrite (BaFe12O19) thin films of varying thicknesses were grown on a-plane Al2O3 112̅0 substrates at elevated temperatures using pulsed laser deposition. X-ray diffraction measurements, Raman spectroscopy, and atomic force microscopy analyses were used to investigate the BaFe12O19 film growth and structure. The results indicate that the formation of an α-Fe2O3 ‘bridge layer’ promotes the growth of a BaFe12O19 phase showing an in-plane uniaxial magnetic anisotropy with an easy-axis of magnetization along the [ 0001 ] hexagonal c-axis direction. The crystallographic relationship between the existing phases is given by BaFe12O19 101̅0 //α-Fe2O3 112̅0 //Al2O3 112̅0. As the thickness increases, the fraction of the BaFe12O19 phase increases further. This is accompanied by a change in grain morphology turning from a columnar to a more elongated shape, where the short axis of the BaFe12O19 grains points along the [ 0001 ] easy axis direction.

High-Efficiency Purification and Morphology Regulation of CaSO4·2H2O Crystals from Phosphogypsum.

Phosphogypsum is a solid waste with great environmental stockpile pressure produced by the wet production of phosphoric acid. Although there are various methods to purify and utilize phosphogypsum, the means for environmentally friendly, low energy consumption, and high value-added utilization still need to be further explored. Here, CaSO4·2H2O crystal was directly purified and regulated from phosphogypsum by using the anti-solvent method. The antisolvent can be adsorbed in the c-axis direction of the crystal and further inhibit the growth rate in this direction, resulting in a change in the morphology of the crystal. By adjusting the polarity and chain length of the anti-solvent, the morphology of CaSO4·2H2O crystal can change from butterfly-like flake crystals to hexagonal prism-like crystals. When n-propanol with long chain was used as the anti-solvent, the morphology of the CaSO4·2H2O crystal showed a hexagonal prism with a specific surface area of 19.98 m2/g and a Cu2+ loading efficiency of 52.67%. The encouraging results open up new possibilities for the application of phosphogypsum.

Synthesis, crystal structure and Hirshfeld surface of ethyl 2-[2-(methyl-sulfan-yl)-5-oxo-4,4-diphenyl-4,5-di-hydro-1H-imidazol-1-yl]acetate (thio-phenytoin derivative).

The di-hydro-imidazole ring in the title mol-ecule, C20H20N2O3S, is slightly distorted and the lone pair on the tri-coordinate nitro-gen atom is involved in intra-ring π bonding. The methyl-sulfanyl substituent lies nearly in the plane of the five-membered ring while the ester substituent is rotated well out of that plane. In the crystal, C-H⋯O hydrogen bonds form inversion dimers, which are connected along the a- and c-axis directions by additional C-H⋯O hydrogen bonds, forming layers parallel to the ac plane. The major contributors to the Hirshfeld surface are C⋯H/H⋯C, O⋯H/H⋯O and S⋯H/H⋯S contacts at 20.5%, 14.7% and 4.9%, respectively.

Operative slip systems and their critical resolved shear stresses in η-Fe2Al5 investigated by micropillar compression at room temperature

The plastic deformation behavior of single crystals of orthorhombic η-Fe2Al5 has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. Plastic flow is observed even at room temperature by the operation of six slip systems; (001)<010>, (001)<110>, (001)<130>, {22‾3}<110>, {311}<1‾03> and {301}<1‾03>. The CRSS values for the six identified slip systems are very high all in the range of 1.1∼1.5 GPa and do not vary much with specimen size. In the middle of the stereographic projection, the (001)<010>, (001)<110>, (001)<130> and {22‾3}[110] slip systems operate according to the relative Schmid factors with the similar CRSS values in the range of 1.08∼1.23 GPa. In orientations close to [001], the {311}<1‾03> slip system as well as the {301}<1‾03> slip system operate with a much higher CRSS values around 1.5 GPa, producing wavy slip traces due to the occurrence of frequent cross-slip among these slip planes. In orientations close to the [100]-[110]-[010] symmetry line, on the other hand, premature failure occurs without the operation of any slip systems, although, the Schmidt factor-wise, the {311}<1‾03> and {301}<1‾03> slip systems could operate. The selection of slip systems, their CRSS values and the possible dislocation dissociation modes are discussed based on the overlapped atomic volume that occurs during shear along the slip direction on the slip plane, taking into account the partial occupancies of Al atoms in the linear atomic chain along the orthorhombic c-axis direction.

Low lattice thermal conductivity induced by interlayer anharmonicity in p-type BaFZnAs compound with high thermoelectric performance

Inspired by the excellent thermoelectric (TE) performance of ZrSiCuAs-type materials, the crystal structure, thermal and electronic transport mechanisms, and TE properties of the BaFZnAs compound are explored using first-principles calculations and Boltzmann transport theory. The BaFZnAs compound is an anisotropic material, with a direct bandgap of 1.20 eV determined by the Heyd-Scuseria-Ernzerhof (HSE06) functional. The [Zn2As2]2- layer within BaFZnAs compound forms carrier transport channel, which is beneficial for high power factor. The strong out-of-plane interlayer anharmonicity of the [Ba2F2]2+ and [Zn2As2]2- layers with weak interactions leads to a low lattice thermal conductivity (1.64 W/mK at 300 K) for the BaFZnAs compound. Based on the carrier relaxation time evaluated in the consideration of multiple carrier scattering rates and electronic transport parameters, the optimal ZTs of 1.44 and 2.06 are achieved for n-type and p-type BaFZnAs compounds at 900 K, associating with the corresponding ZTs of 2.10 (0.72) and 2.64 (0.51) along the a-axis and c-axis directions. Our present work not only offers multifaceted insights into the thermal and electronic transport properties of BaFZnAs compound, but also provides a new inspiration for further TE exploration of ZrSiCuAs-type materials.

Novel complex based on [Co(SCN)4] and piperazine derivate: Synthesis, characterization, theoretical study, thermal features, optical studies and electronic investigations

The crystal structure of the synthesized compound, [Co(SCN)4] (C10H14FN2)2, comprises separate complexes in which the Co2+ cations are fourfold coordinated by four N- bonded thiocyanate motifs to induce distorted CoN4 tetrahedra and two 1-(2-fluorophenyl) piperazinium cations. The entities are further linked by N H···S H-bonds in a 3-D network generated along the c-axis direction establishing chains. Interestingly, single crystal diffraction has occurred, in addition to the X-ray powder diffraction to punctuate the purity of the synthesized complex, adding to the optical behavior study using FT-IR. Intermolecular interaction and stabilization of dimer complexes in the crystal packing are explored using Hirshfeld surface analysis to highlight the role of molecular interaction in the crystal packing. The di-electrical investigations show interesting electrical behavior of the tilted compound. The thermal stability analyses occurred and it appeared that the dissociation of the entities starts at about 500 K. In addition, density functional theory (DFT) calculations were used to study the electronic properties and the optical properties of the studied system.

Preparation of Nb5+ Doped Na3V2(PO4)3 Cathode Material for Sodium Ion Batteries.

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the abundance and low cost of sodium resources. Cathode material plays a crucial role in the performance of sodium ion batteries determining the capacity, cycling stability, and rate capability. Na3V2(PO4)3 (NVP) is a promising cathode material due to its stable three-dimensional NASICON structure, but its discharge capacity is low and its decay is serious with the increase of cycle period. We focused on modifying NVP cathode material by coating carbon and doping Nb5+ ions for synergistic electrochemical properties of carbon-coated NVP@C as a cathode material. X-ray diffraction analysis was performed to confirm the phase purity and crystal structure of the Nb5+ doped NVP material, which exhibited characteristic diffraction peaks that matched well with the NASICON structure. Nb5+-doped NVP@C@Nbx materials were prepared using the sol-gel method and characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Raman and Brunauer -Emmett-Teller (BET) analysis. First-principles calculations were performed based on density functional theory. VASP and PAW methods were chosen for these calculations. GGA in the PBE framework served as the exchange-correlation functional. The results showed the NVP unit cell consisted of six NVP structural motifs, each containing octahedral VO6 and tetrahedral PO4 groups to form a polyanionomer [V2(PO4)3] along with the c-axis direction by PO4 groups, which had Na1(6b) and Na2(18e) sites. And PDOS revealed that after Nb doping, the d orbitals of the Nb atoms also contributed electrons that were concentrated near the Fermi surface. Additionally, the decrease in the effective mass after Nb doping indicated that the electrons could move more freely through the material, implying an enhancement of the electron mobility. The electrochemical properties of the Nb5+ doped NVP@C@Nb cathode material were evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectric spectroscopy (XPS). The results showed that NVP@C@Nb0.15 achieved an initial discharge capacity as high as 114.27 mAhg-1, with a discharge capacity of 106.38 mAhg-1 maintained after 500 cycles at 0.5C, and the retention rate of the NVP@C@Nb0.15 composite reached an impressive 90.22%. NVP@C@Nb0.15 exhibited low resistance and high capacity, enabling it to create more vacancies and modulate crystal structure, ultimately enhancing the electrochemical properties of NVP. The outstanding performance can be attributed to the Nb5+-doped carbon layer, which not only improves electronic conductivity but also shortens the diffusion length of Na+ ions and electrons, as well as reduces volume changes in electrode materials. These preliminary results suggested that the as-obtained NVP@C@Nb0.15 composite was a promising novel cathode electrode material for efficient sodium energy storage.

Superlattice Delineated Fermi Surface Nesting and Electron-Phonon Coupling in CaC6

The superconductivity of CaC6 as a function of pressure and Ca isotopic composition was revisited using DFT calculations on a 2c–double hexagonal superlattice. The introduction of superlattices was motivated by previous synchrotron absorption and Raman spectroscopy results on other superconductors that showed evidence of superlattice vibrations at low (THz) frequencies. For CaC6, superlattices have previously been invoked to explain the ARPES data. A superlattice along the hexagonal c-axis direction is also illustrative of atomic orbital symmetry and periodicity, including bonding and antibonding s-orbital character implied by cosine-modulated electronic bands. Inspection of the cosine band revealed that the cosine function has a small (meV) energy difference between the bonding and antibonding regions, relative to a midpoint non-bonding energy. Fermi surface nesting was apparent in an appropriately folded Fermi surface using a superlattice construct. Nesting relationships identified phonon vectors for the conservation of energy and for phase coherency between coupled electrons at opposite sides of the Fermi surface. A detailed analysis of this Fermi surface nesting provided accurate estimates of the superconducting gaps for CaC6 with the change in applied pressure. The recognition of superlattices within a rhombohedral or hexagonal representation provides consistent mechanistic insight on superconductivity and electron−phonon coupling in CaC6.

Crystal orientation control of a-plane AlN films on r-plane sapphire fabricated by sputtering and high-temperature annealing

Face-to-face annealed and sputter-deposited aluminum nitride (FFA Sp-AlN) has potential in deep-ultraviolet light-emitting devices. Herein, the effects of the substrate off-cut angle (θsub) from an r-plane sapphire toward the c-axis projection direction and sputtering temperature (Tsp) on the crystallinity and surface morphology of a-plane AlN films are investigated. Increasing θsub in the minus-off direction, which occurs when the substrate surface approaches the sapphire c-plane, and lowering Tsp suppress the mixing of anomalous non-a-direction oriented domains. This reduced mixing enhances the surface flatness and crystallinity of a-plane FFA Sp-AlN. Moreover, the c-axis direction of the a-plane AlN film is inverted depending on the substrate off-cut angle. Ab initio calculations indicate that the interface stability between the r-plane sapphire substrate and the a-plane AlN film can explain the dependence of the crystallinity and c-axis orientation of FFA Sp-AlN on the surface off-cut of the r-plane sapphire.

Is Ba3In2O6 a high-Tc superconductor?

It has been suggested that Ba3In2O6 might be a high-Tc superconductor. Experimental investigation of the properties of Ba3In2O6 was long inhibited by its instability in air. Recently epitaxial Ba3In2O6 with a protective capping layer was demonstrated, which finally allows its electronic characterization. The optical bandgap of Ba3In2O6 is determined to be 2.99 eV in-the (001) plane and 2.83 eV along the c-axis direction by spectroscopic ellipsometry. First-principles calculations were carried out, yielding a result in good agreement with the experimental value. Various dopants were explored to induce (super-)conductivity in this otherwise insulating material. Neither A- nor B-site doping proved successful. The underlying reason is predominately the formation of oxygen interstitials as revealed by scanning transmission electron microscopy and first-principles calculations. Additional efforts to induce superconductivity were investigated, including surface alkali doping, optical pumping, and hydrogen reduction. To probe liquid-ion gating, Ba3In2O6 was successfully grown epitaxially on an epitaxial SrRuO3 bottom electrode. So far none of these efforts induced superconductivity in Ba3In2O6, leaving the answer to the initial question of whether Ba3In2O6 is a high-Tc superconductor to be ‘no’ thus far.