High Crystal Symmetry Research Articles

Superior intrinsic thermoelectric performance with zT of 1.8 in single-crystal and melt-quenched highly dense Cu(2-x)Se bulks.

Practical applications of the high temperature thermoelectric materials developed so far are partially obstructed by the costly and complicated fabrication process. In this work, we put forward two additional important properties for thermoelectric materials, high crystal symmetry and congruent melting. We propose that the recently discovered thermoelectric material Cu2-xSe, with figure of merit, zT, over 1.5 at T of ~ 1000 K, should meet these requirements, based on our analysis of its crystal structure and the Cu-Se binary phase diagram. We found that its excellent thermoelectric performance is intrinsic, and less dependent on grain size, while highly dense samples can be easily fabricated by a melt-quenching approach. Our results reveal that the melt-quenched samples and single crystals exhibit almost the same superior thermoelectric performance, with zT as high as 1.7–1.8 at T of ~973 K. Our findings not only provide a cheap and fast fabrication method for highly dense Cu2-xSe bulks with superior thermoelectric performance, paving the way for possible commercialization of Cu2-xSe as an outstanding component in practical thermoelectric modules, but also provide guidance in searching for new classes of thermoelectric systems with high crystal symmetry or further improving the cost performance of other existing congruent-melting thermoelectric materials.

High-performance few-layer Mo-doped ReSe₂ nanosheet photodetectors.

Transition metal dichalcogenides (TMDCs) have recently been the focus of extensive research activity owing to their fascinating physical properties. As a new member of TMDCs, Mo doped ReSe2 (Mo:ReSe2) is an octahedral structure semiconductor being optically biaxial and highly anisotropic, different from most of hexagonal layered TMDCs with optically uniaxial and relatively high crystal symmetry. We investigated the effects of physisorption of gas molecule on the few-layer Mo:ReSe2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe2 photodetectors show a better photoresponsivity (~55.5 A/W) and higher EQE (10893%) in NH3 than in air. By theoretical investigation, we conclude that the physisorption of NH3 molecule on Mo:ReSe2 monolayer can cause the charge transfer between NH3 molecule and Mo:ReSe2 monolayer, increasing the n-type carrier density of Mo:ReSe2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe2 can be used to design multifunctional optoelectronic and sensing devices.

Tuning of net magnetic moment in BiFeO3 multiferroics by co-substitution of Nd and Mn

The structural and magnetic properties of Bi1−xNdxFe1−xMnxO3 ceramics prepared by the tartaric acid modified sol–gel technique have been studied to understand the effect of structural modification on the magnetic Properties of BiFeO3. The co-substitution of Nd and Mn at Bi and Fe sites respectively in BiFeO3 significantly suppress the impurity phases (Bi25FeO40, Bi2Fe4O9 etc.). The Rietveld analysis of X-ray diffraction (XRD) patterns indicates the existence of compositional driven crystal structure transformation from rhombohederal (R3c space group, higher crystal symmetry) to the orthorhombic (Pbnm space group, lower crystal symmetry) with the increase in substitution concentration due to excess chemical pressure (lattice strain). The quantitative crystallographic phase analysis has been carried out by Rietveld analysis of all the XRD patterns. Magnetic measurements reveal that co-substituted BiFeO3 nanoparticles for x=0.050 have enhanced remnant magnetization about 21 times as compared to pure one. The remnant magnetization reaches a maximum value at the morphological phase boundary (x=0.050) and further increase (x>0.050) in substitution concentration results in the reduction of remnant magnetization due to the appearance of complete antiferromagnetic ordering in the orthorhombic structure because of the significant contribution from the crystallographic phase of Pbnm space group (as obtained from the quantitative crystallographic phase contribution by the Rietveld analysis).

Structural Stability and Half Metallic Features of Zn0.5Cr0.5S under Pressure

Spin-polarized first-principle was performed to study the structural stability and the electronic states of Cr doped ZnS with the Cr component of 50% in zincblende (ZB), wurtzite (W) and rocksalt (RS) structures under pressure. The results show that the zincblende and wurtzite structures become unstable under low pressures of about 4.68 and 9.61 GPa, respectively, but the rocksalt structure can be maintained up to an extremely high pressure of about 32.92 GPa. Both zincblende and wurtzite Zn0.5Cr0.5S display half metallic features under pressure, while rocksalt Zn0.5Cr0.5S exhibits metallic feature. The half metallic features can be ascribed to the stronger interactions between S-3p and Cr-3d states and the metallic feature is due to the higher crystal symmetry of rocksalt Zn0.5Cr0.5S. These results can provide helpful guidance for Cr doped ZnS to be used in spintronic devices.

High symmetric SrRuO3 (001) thin films: Perfectly lattice-matched electrodes for multiferroic BiFeO3

High symmetric tetragonal (c/a < 1 and c/a > 1) and cubic (c/a = 1) SrRuO3 thin films were developed on SrTiO3 substrate by controlling RF-power and substrate temperature. These SrRuO3 symmetries exhibit larger in-plane lattice parameters (3.936–3.98 Å) compared to bulk orthorhombic SrRuO3 (3.93 Å), and are coincidentally similar to the predicted high temperature tetragonal (>820 K) and cubic (>950 K) SrRuO3 phases. Importantly, the lattice parameters of cubic (c/a = 1) SrRuO3 (3.968 Å) films match very well to the pseudocubic BiFeO3 (3.966 Å), opening new perspectives for BiFeO3/SrRuO3/SrTiO3 heterostructures for spintronics. Due to high crystal symmetry and large unit cell expansion (∇V/V = ∼1.1%-4.2%), a non-Fermi-liquid (R ∝ T3/2) and weak anisotropic magnetic behaviors, and reduced TC's are commonly observed which are different from the orthorhombic SrRuO3.

Crystal asymmetry and low-angle grain boundary governed persistent photoinduced magnetization in small bandwidth manganites

Persistent photo-induced magnetization (PPM) in low bandwidth manganite Pr1−xCaxMnO3 (PCMO) thin film is reported in the low hole doped regime (x = 0.1). X-ray diffraction, x-ray photoelectron spectroscopy, and magnetic measurements in dark and under illumination on the as-grown, vacuum, and oxygen annealed thin films of PCMO showed that the PPM effect is strongly dependent on the crystal symmetry, low angle grain boundaries, and the Mn valence states in the material. Compared with the as-grown and vacuum annealed film, the oxygen annealed film shows much less low-angle grain boundaries, higher crystal symmetry, and long range ferromagnetic (FM) ordering and in this sample PPM is not significant. These results clearly indicate that in this large band gap material PCMO, photo-induced FM interaction mainly arises due to improved domain-wall movement of the short range FM clusters. Significant PPM can be obtained for films with higher amount of grain boundaries, oxygen vacancies, and strain related defects.

Structural transitions and enhanced ferroelectricity in Ca and Mn co-doped BiFeO3 thin films

Ca and Mn co-doped BiFeO3 (BFO) thin films were fabricated on (111) Pt/Ti/SiO2/Si substrates via a simple solution approach. Enhanced ferroelectric properties were obtained in these Ca and Mn co-doped BFO films, e.g., large remnant polarization value (Pr ∼ 89 μC/cm2) and large remnant polarization to saturated polarization ratio (Pr/Ps ∼ 0.84) for Bi0.90Ca0.10Fe0.90Mn0.10O3 film. X-ray diffraction and Raman spectra of these films showed that B-site Mn doping causes substantial structural transition toward orthorhombic phase and A-site Ca doping facilitates an evolution to tetragonal phase with higher crystal symmetry. The structural transitions are in favor of enhanced ferroelectric properties in the BFO films. The approach of A and B site co-doping is proved to be effective in enhancing ferroelectric performance in multiferroic BFO films.

SrAl2O4:Eu2+,Dy3+ nanobelts: Synthesis by combustion and properties of long-persistent phosphorescence

Abstract

BaAl2O4:Eu2+, Dy3+ Nanotube Synthesis by Heating Conversion of Homogeneous Coprecipitates and Afterglow Characteristics

Hexagonal BaAl2O4:Eu2+, Dy3+ polynary complex nanotubes with long-lasting phosphorescence were obtained through a facile coprecipitation approach followed by a postcalcining reaction in a weak reducing atmosphere. In the case of low annealing temperature, anion vacancies and surface stress can induce lattice contraction due to poor crystallininty; moreover, Eu2+ ions can occupy two different crystallographic Ba2+ sites due to low symmetry, resulting in an appearance of double emission peaks. For the sample annealed at higher temperature, however, Eu2+ ions only occupy substitutedly the Ba2+ sites with lowest energy due to high crystallinity; moreover, as compared to the sample annealed at low temperature, its emission band redshifts as the results of both high crystal symmetry around Eu2+ ions and large average optical path. Additionally, Eu2+ and Dy3+ ions substitute incompletely for Ba2+ sites in nanostructures, leading to the decrease of effective electron trap densities and depths, and therefore tubular nanostructures show fast afterglow decay rate in comparison with the bulk counterpart.

Grain size effect on the phase transformation temperature of nanostructured CuFe2O4

We report a large decrease in tetragonal to cubic phase transformation temperature when grain size of bulk CuFe2O4 is reduced by mechanical ball milling. The change in phase transformation temperature was inferred from in situ high temperature conductivity and x-ray diffraction measurements. The decrease in conductivity with grain size suggests that ball milling has not induced any oxygen vacancy while the role of cation distribution in the observed decrease in phase transformation temperature is ruled out from in-field F57e Mössbauer and extended x-ray absorption fine structure measurements. The reduction in the phase transformation temperature is attributed to the stability of structures with higher crystal symmetry at lower grain sizes due to negative pressure effect.

Attempt to improve sensitivity in measuring diamagnetic anisotropy by increasing duration of rotational oscillation in microgravity

Technical basis to detect small diamagnetic anisotropy ΔχDIA is established in a method using microgravity μG. Sensitivity of Δχ can be improved by increasing length of measurable period of rotational oscillation of magnetically stable axis with respect to field direction. In order to achieve the above condition, position of the diamagnetic sample should be stabilized in static field area, which was realized by introducing a magnetic circuit with high field homogeneity. Function of a mobile sample stage was effective in minimizing translational motion of sample. Accordingly, magnitude of measurable anisotropy may be decreased to a level of 10-10 emu/g. ΔχDIA of materials with high crystal symmetry, namely hexagonal ice, ZnO, SiC and BaTiO3, can be obtained by the above improvement. Magnetically active property due to magnetic anisotropy may be recognized for almost all the solid materials when sensitivity reaches the above level.

Fabrication and phase modulation in organic single-crystalline configurationally locked, phenolic polyene OH1 waveguides

A novel and promising technique for the fabrication of electro-optically active single crystalline organic waveguides from 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile (OH1) is presented. OH1 is an interesting material for photonic applications due to the large electro-optic coefficients (r333 = 109+/-4 pm/V at 632.8 nm) combined with a relatively high crystal symmetry (orthorhombic with point group mm2). Due to the very favorable growth characteristics, large-area (> 150 mm(2)) single crystalline thin films with very good optical quality and thickness between 0.05-10 microm have been grown on amorphous glass substrates. We have developed and optimized optical lithography and reactive ion etching processes for the fabrication of wire optical waveguides with dimensions of w x h = 3.4 x 3.5 microm(2) and above. The technique is capable of producing low loss integrated optical waveguides having propagation losses of 2 dB/cm with a high refractive index contrast between core-cladding and core-substrate of delta n = 1.23 and 0.72, respectively at 980 nm. Electro-optic phase modulation in these waveguides has been demonstrated at 632.8 nm and 852 nm. Calculations show that with an optimized electrode configuration the half-wave voltage x length product V(pi) x L can be reduced from 8.4 Vcm, as obtained in our device, to 0.3 Vcm in the optimized case. This allows for the fabrication of sub-1 V half-wave voltage, organic electro-optic modulators with highly stable chromophore orientation.

Preparation of La1 − x − yCaxSryCrO3 with High-Density Structural Phase Transition and Electrical Conduction Properties

Combining simultaneous substitution of Ca and Sr for the La site in and pulverization of calcined particles with planet-type yttria-stabilized ball mill , ceramics with sintering density of more than 93% were successfully prepared. Not only sintering properties but also structural phase transition behavior, such as temperature, variation of enthalpy, and entropy, of could be controlled by alkaline earth composition. The variation of the transition behavior on Sr content could be fairly explained with the model, assuming that the phase with high crystal symmetry has high thermodynamic stability, i.e., low standard Gibbs energy of formation. Because not only has high sintering properties but also no structural phase transition between room temperature to solid oxide fuel cell (SOFC) operation and equal electrical conductivity to that of , which was reported to be a promising material for SOFC interconnectors, it can be expected that is a superior candidate to as interconnector material for SOFCs.

Role of metal additives in mechanochemical phase transformation of zirconia

Reasons for a phase transition in zirconia subjected to intensive mechanical treatment in planetary mill have been considered. If steel balls and vials are used for milling the comminution is accompanied by oxidation of wear metal particles and successive mechanochemical reaction with ZrO2. Aluminum has been shown to behave similarly and, being deliberately added to zirconia powder, to form solid solutions. Foreign metal cations introduced into the lattice stabilize a modification with higher crystal symmetry thus increasing the threshold size above which the monoclinic modification is stable. An increase of surface energy contribution to the Gibbs’ energy of zirconia plays an important role in phase transformation at the initial stages of mechanical treatment, while henceforth thermodynamic stability is more and more determined by the stabilizing effect of the impurity cations. Under the conditions that rule out contamination of ZrO2 with wear material or other metal additives, dynamic equilibrium sets in between the direct transition to the tetragonal phase and the reverse transition to the monoclinic form.

Analysis of Structural Phase Transition Behavior of La1-x-yCaxSryCrO3

The structural phase transition behavior of La1-x-yCaxSryCrO3, which can be regarded as promising material for interconnector of SOFC due to high sintering property, has been investigated by differential scanning calorimetry. Temperature and variation of enthalpy at the structural phase transition decreased with increase of Sr content among the specimens with the same alkaline earth content. It was also revealed from X- ray diffraction that crystal symmetry of orthorhombic phase became lower whereas that of rhombohedral one higher with Sr substitution. Assuming that the standard Gibbs free energy of formation of La1-x-yCaxSryCrO3 with higher crystal symmetry is lower, the variation of the phase transition behavior on Sr substitution can be qualitatively explained by using thermodynamics.

Alignment of nonmagnetic solid grains that essentially require high magnetic field; alignment of hexagonal ice Ih

Minimum field intensity required to achieve alignment of solid grains, which posses small diamagnetic anisotropy (Δχ)DIA, is discussed based on observed values of hexagonal ice Ih; the field intensity is about 6 Tesla at T = 273K for ice Ih crystal with 1.0 μm in diameter. Grain alignment essentially require strong magnetic field above several Tesla when (Δχ)DIA is in the level of 10−10 emu/g. (Δχ)DIA arises from preferential orientation of individual chemical bond with respect to a magnetic principle axis of the solid body. Hence (Δχ)DIA is negligibly small for materials with high crystal symmetry, such as crystals having a rutile, wurtzite or perovskite structure. Alignment should be studied intensively in high magnetic-field for these types of materials. Whereas alignment occurs below B = 2T for most of the unmeasured diamagnetic crystals with lower symmetry, since their (Δχ)DIA values are expected to exceed 1 × 10−9 emu/g according to the above-mentioned model; this field is produced by an ordinary electromagnet.

Spontaneous Symmetry-Breaking Vortex Lattice Transitions in Pure Niobium

We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals of pure niobium with the magnetic field applied parallel to a fourfold symmetry axis, so as to induce frustration between the cubic crystal symmetry and hexagonal VL coordination expected in an isotropic situation. We observe new VL structures and phase transitions; all the VL phases observed (including those with an exactly square unit cell) spontaneously break some crystal symmetry. One phase even has the lowest possible symmetry of a two-dimensional Bravais lattice. This is quite unlike the situation in high-Tc or borocarbide superconductors, where VL structures orient along particular directions of high crystal symmetry. The causes of this behavior are discussed.

Equilibrium magnetization in the vicinity of the Bragg glass to vortex glass transition inV3Si

We present a study of the quasiequilibrium magnetization of single crystalline ${\mathrm{V}}_{3}\mathrm{Si}$ spheres of different pinning strength in the vicinity of Bragg glass to disordered vortex solid transition. We report a first-order-like magnetization step below the onset of the peak effect. Its occurrence is hysteretic with respect to the direction of field scan. The step is observed only for fields oriented away from the high crystal symmetry directions [110] and [100]. Accompanying the magnetization step is an abrupt reorientation of the vortex lattice towards a crystal symmetry direction. Coupling between the vortex lattice and the underlying crystal lattice is suggested as a possible mechanism for the reorientation transition.

Morphology control of stolzite microcrystals with high hierarchy in solution.

Crystal gazing: A mild solution route has been designed for the controlled synthesis of lead tungstate microcrystals with high crystal symmetry, in which the cationic surfactant, pH value, and temperature play key roles. Helix- (see TEM image), spindle-, chainlike, and dendritic structures can be produced by varying the reaction conditions.

Ballistic hot-electron transport in nanoscale semiconductor heterostructures: Exact self-energy of a three-dimensional periodic tight-binding Hamiltonian

As the length scale for semiconductor heterostructures approaches the regime of the lattice constant, our current theory for calculating ballistic hot-electron transport becomes inapplicable. In this case, a method such as the Green's function formalism should be used to calculate ballistic electron transmission functions from the exact, periodic lattice potential. We present a method for directly calculating the exact surface Green's function for three-dimensional periodic leads which is necessary for such a scheme. Except in cases of high crystal symmetry, the method is limited by the difficulty to solve a nonsymmetric matrix Riccati equation.