Hexagonal Single Crystals Research Articles

Magnetic and dielectric properties of orthorhombic and hexagonal multiferroics Tb1 − x Y x MnO3

The transition from a stable orthorhombic structure to a hexagonal structure has been revealed in Tb1−xYxMnO3 multiferroics at x = 0.2–0.4. It has been shown that almost single-phase crystals with an orthorhombic or hexagonal structure can be obtained by choosing the growth conditions. It has been found that the magnetic and dielectric properties of orthorhombic single crystals with x = 0.2–0.3 are similar to the properties of pure TbMnO3 and are characterized by a strong anisotropy of the magnetic susceptibility at low temperatures and by the presence of a number of magnetic phase transitions, including those to the ferroelectric state. New spontaneous (T ≤ 15 K) and magnetic-field induced (H | C6) phase transitions accompanied by the appearance of an uncompensated rare-earth magnetic moment ∼1 μB/mole have been observed in hexagonal single crystals with x = 0.3–0.5.

Crystallographically engineered, hydrothermally crystallized hydroxyapatite films: an in vitro study of bioactivity

The aim of this study was to evaluate the bioactivity of hydroxyapatite films composed of hexagonal single crystals that display {1010} and {0001} crystallographic faces. The effect of engineered [0001] crystallographic orientation was investigated in parallel. Films were deposited by triethyl phosphate/ethylenediamine-tetraacetic acid doubly regulated hydrothermal crystallization on Ti6Al4V substrates (10, 14, 24 h). Bioactivity was investigated by analysis of MC3T3-E1 pre-osteoblast spreading using scanning electron microscopy and quantitative analysis of cell metabolic activity (Alamar Blue) (0-28 days). Scanning electron microscopy and X-ray diffraction were used to evaluate the ability of films to support the differentiation of MC3T3-E1 pre-osteoblasts into matrix-secreting, mineralizing osteoblasts. Results demonstrated that all films enabled MC3T3-E1 cells to spread, grow, and differentiate into matrix-secreting osteoblasts, which deposited biomineral that could not be removed after extraction of organic material. Differences in [0001] HA crystallographic orientation were not, however, found to significantly affect bioactivity. Based on these results, it is concluded that these hydrothermal hydroxyapatite films are non-toxic, bioactive, osteoconductive, and biomineral bonding. The lack of a relationship between reported hydroxyapatite crystallographic face specific protein adsorption and bulk HA bioactivity are discussed in terms of crystallographic texture, surface roughness, assay robustness, and competitive protein adsorption.

Coherent acoustic phonons in hexagonal HoMnO3 probed by femtosecond spectroscopy

We have observed the coherent acoustic phonons in the hexagonal manganite HoMnO3 single crystals using wavelength-dependent femtosecond pump-probe spectroscopy. The oscillatory component of the photoinduced transient reflectivity change (ΔR/R) at various pump-probe wavelengths is consistent with the model of strain pulse propagation. Furthermore, the oscillation period of the oscillation is strong temperature- and wavelength-dependent. In particular, the period of all oscillations in ΔR/R for various wavelengths are very similar in the vicinity of the Néel temperature (TN), which associates with the appearance of the antiferromagnetic (AFM) long-range ordering.

Microstructural, magnetic and crystal field investigations of nanocrystalline Dy 3+ doped zinc oxide

Dy 3+ doped zinc oxide was prepared by co-precipitation method. The as-prepared samples were annealed at different temperatures to obtain the samples with different particle sizes. The crystallographic phases of all the samples were confirmed by X-ray diffraction (XRD) patterns. Rietveld analysis of the XRD pattern of the sample annealed at 80 °C showed that most of the Dy 3+ ions were substituted in the Zn 2+ site of the hexagonal ZnO lattice. But in case of samples annealed at higher temperatures, a fraction of Dy 3+ ions comes out from the ZnO lattice and this fraction increases with the increase of annealing temperature. The sizes of nanoparticles and the lattice strains of all the samples were obtained from the Hall–Williamson plot. High resolution transmission electron microscopy showed that ZnO nanoparticles are more or less spherical. Magnetic susceptibilities ( χ) of some selected samples measured in the temperature range of 300–14 K indicate that the samples are paramagnetic. Values of χ were successfully fitted by Curie–Weiss law. A good theoretical simulation of χ of the sample annealed at 80 °C has been achieved using the one-electron crystal field interaction of the Dy 3+ ions with its diamagnetic neighbors in the hexagonal single crystal.

Micrometer‐Sized Organic Single Crystals, Anisotropic Transport, and Field‐Effect Transistors of a Fused‐Ring Thienoacene

Hexagonal micrometer-sized organic single crystals of a fused-ring thienoacene are grown controllably by physical vapor transport over a large area. A “two-dimensional organic-ribbon mask” technique is developed to fabricate organic field-effect transistors based on an individual microcrystal. The accomplished transistors exhibit mobility as high as 1.8 cm2V−1s−1 with on/off ratios of typically >107.

Anisotropy of the magnetic properties and the electronic structure of transition-metal diborides

The temperature dependences of the magnetic susceptibility χ and its anisotropy Δχ=χ∥−χ⊥ have been measured for hexagonal single crystals of transition-metal diborides MB2 (M=Sc,Ti,V,Zr,Hf) in the temperature interval 4.2–300K. It is found that the anisotropy Δχ is weakly temperature-dependent, a nonmonotonic function of the filling of the hybridized p−d conduction band, and largest for group-IV transition metals. First-principles calculations of the electronic structure of diborides and the values of the paramagnetic contributions (spin and Van Vleck) to their susceptibility show that the behavior of the magnetic anisotropy is due to the competition between Van Vleck paramagnetism and orbital diamagnetism of the conduction electrons.

Thickness-dependent bending modulus of hexagonal boron nitride nanosheets

Bending modulus of exfoliation-made single-crystalline hexagonal boronnitride nanosheets (BNNSs) with thicknesses of 25–300 nm and sizes of 1.2–3.0 µm were measured using three-point bending tests in an atomic force microscope. BNNSs suspended onan SiO2 trench were clamped by a metal film via microfabrication based on electronbeam lithography. Calculated by the plate theory of a doubly clamped plateunder a concentrated load, the bending modulus of BNNSs was found toincrease with the decrease of sheet thickness and approach the theoreticalC33 value of a hexagonal BN single crystal in thinner sheets(thickness<50 nm). The thickness-dependent bending modulus was suggested to be due to the layerdistribution of stacking faults which were also thought to be responsible for thelayer-by-layer BNNS exfoliation.

Preparation of ultrafine boride powders by metallothermic reduction method

Ultrafine TiB2, ZrB2, and ReB2 powders were prepared by the metallothermic reduction method using Mg. Obtained TiB2, ZrB2, and ReB2 powders were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, dynamic light scattering, and scratch test. The morphology of the powders was a laminar hexagonal single crystal. The mean particle sizes of TiB2 and ZrB2 increased with increasing reaction temperature, and that of ReB2 was 0.11 μm at 1093 K. The obtained ReB2 powder can scratch the surface of polycrystalline synthetic diamonds (HK = 7000 kg/mm2). The obtained powders will be useful for developing of new superhard alloys and pigments.

Jahn-Teller effect on exciton states in hexagonal boron nitride single crystal

Optical properties near the band edge of hexagonal boron nitride were studied at 8 K. The photoluminescence spectrum shows two series of bands, namely, sharp (S) and diffuse (D), which are also distinguished by their fast (0.6 ns) for S and slow (5 ns) for D radiative decay time. Each series is composed of four bands with large Stokes shifts that are attributed to self-trapped excitons by the strong exciton-phonon interaction. The respective four luminescence bands of the two series originate from the four free-exciton levels in which the doubly degenerated dark and bright exciton levels theoretically predicted are resolved with the Jahn-Teller distortion in the excited states.

Growth of ZnO crystal by self-flux method using Zn solvent

Zinc oxide (ZnO) single crystal was grown, for the first time, by the self-flux method using a high-purity zinc solvent saturated with oxygen. ZnO crystals were successfully grown in the solvent by moving a growing crystal slowly, at about 17 mm/day along a temperature gradient (1.5 °C/mm) in a furnace, from 1050 to 500 °C. Hexagonal single crystals 1–3 mm in length and 0.5–1 mm in thickness were obtained. Energy-dispersive X-ray diffraction, secondary ion mass spectrometry, and photoluminescence confirmed that the purity and crystallinity of the ZnO crystals grown in this work were high.

Magnetic susceptibilities, crystal field Stark energies, and hyperfine behavior of Sm3+ in hexagonal single crystals of Sm(CF3SO3)3⋅9H2O

Single crystals of samarium trifluoromethanesulfonate (SmTFMS) were prepared from the slow evaporation of the aqueous solution of SmTFMS. The crystals are elongated along the symmetry axis c of the hexagonal crystal. At room temperature, the c axis of the crystal sets parallel with the applied magnetic field which indicates that the susceptibility parallel to the c axis (χ∥) is greater than the susceptibility perpendicular to the c axis (χ⊥). χ∥ and χ⊥ were measured from 300 down to 14 K. Magnetic anisotropy (Δχ=χ∥−χ⊥) obtained from the values of χ∥ and χ⊥ was also checked by direct measurements of Δχ and both these results agreed very well. A crossover between χ∥ and χ⊥ has been observed at ∼56 K i.e., below this temperature χ∥&amp;lt;χ⊥. A good theoretical simulation of the observed magnetic data of SmTFMS has been achieved using the one-electron crystal field (CF) theory. Ordering effects in the observed magnetic data were not noticed down to the lowest temperature (∼14 K) attained, indicating the interionic interaction to be of predominantly dipolar type. To substantiate the CF analysis, the Raman and Fourier transform infrared (FTIR) spectra of the single crystal of SmTFMS were recorded in the wave number ranges of 100–1800 and 400–7000 cm−1, respectively. The calculated values of the CF Stark energies of all the excited multiplets are in agreement with those extracted from the Raman and FTIR spectra. Thermal variation of the calculated electronic specific heat shows Schottky anomaly at ∼8 K. The temperature dependences of quadruple splitting and hyperfine heat capacity were also studied.

Synthesis of Large-scale GaN Nanowires by Ammoniating Ga2O3/V Films

Large-scale GaN nanowires were synthesized on Si(111) substrates through ammoniating Ga 2 O 3 /V films. The as-grown products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that the grown GaN nanowires have a smooth and clean surface with diameters ranging from 20 nm to 60 nm and lengths of about several tens of micrometers. The results of HRTEM and selected-area electron diffraction (SAED) show that the nanowires are pure hexagonal GaN single crystal. The photoluminescence (PL) spectrum indicates that the GaN nanowires have good emission property. The growth mechanism is discussed briefly.

Phase transitions and rare-earth magnetism in hexagonal and orthorhombicDyMnO3 single crystals

The floating-zone method with different growth ambienceshas been used to selectively obtain hexagonal or orthorhombicDyMnO3 single crystals. The crystals were characterized by x-ray powder diffraction of groundspecimens and a structure refinement as well as electron diffraction. We reportmagnetic susceptibility, magnetization and specific heat studies of this multiferroiccompound in both the hexagonal and the orthorhombic structure. The hexagonalDyMnO3 shows magneticordering of Mn3+ (S = 2) spins on a triangularMn lattice at TNMn = 57 K characterized by a cusp in the specific heat. This transition is notapparent in the magnetic susceptibility due to the frustration on the Mntriangular lattice and the dominating paramagnetic susceptibility of theDy3+ (S = 9/2) spins.At TNDy = 3 K, a partial antiferromagnetic order of Dy moments has beenobserved. In comparison, the magnetic data for orthorhombicDyMnO3 display three transitions. The data broadly agree with results from earlierneutron diffraction experiments, which allows for the following assignment:a transition from an incommensurate antiferromagnetic ordering ofMn3+ spins atTNMn = 39 K, a lock-intransition at Tlock−in = 16 K and a second antiferromagnetic transition atTNDy = 5 K due to the ordering of Dy moments. Both the hexagonal and the orthorhombic crystalsshow magnetic anisotropy and complex magnetic properties due to 4f–4f and 4f–3dcouplings.

Quadrupole moments of neutron-deficient 20,21Na

The electric-quadrupole coupling constant of the ground states of the proton drip line nucleus 20Na (Iπ=2+, T1/2=447.9 ms) and the neutron-deficient nucleus 21Na (Iπ=3/2+, T1/2=22.49 s) in a hexagonal ZnO single crystal were precisely measured to be |eqQ/h|=690±12 kHz and 939±14 kHz, respectively, using the multi-frequency β-ray detecting nuclear magnetic resonance technique under presence of an electric-quadrupole interaction. An electric-quadrupole coupling constant of 27Na in the ZnO crystal was also measured to be |eqQ/h|=48.4±3.8 kHz. The electric-quadrupole moments were extracted as |Q(Na20)|=10.3±0.8 efm2 and |Q(Na21)|=14.0±1.1 efm2, using the electric-coupling constant of 27Na and the known quadrupole moment of this nucleus as references. The present results are well explained by shell-model calculations in the full sd-shell model space.

Structure and Characterization of Mg-Doped GaN Nanowires

Large-scale Mg-doped GaN nanowires were synthesized by ammoniating Mg∶Ga2O3 thin films at 850 ℃ which were deposited on the Si substrate using the resembling Delta doping method. These GaN nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy and high resolution transmission electron microscopy (HRTEM). The results indicated that nanowires were hexagonal wurzite GaN single crystals. The diameter of nanowires was in the range of 35-50 nm with lengths of up to several tens of micrometers.

INVESTIGATION OF THE ENTROPY AND SPECIFIC HEAT CAPACITY OF GaN USING INCOMPLETE GAMMA FUNCTIONS

The aim of this paper is to provide validity and reliable analytical relation for the thermodynamic functions calculated in terms of the Debye temperature using incomplete gamma functions, and examines the entropy and specific heat capacity of hexagonal single crystals of GaN in the 0–1800 K temperature range. The obtained results have been compared with the corresponding experimental and theoretical results. Our results are in excellent agreement with the theoretical results over the entire temperature range. It has also shown that at low temperature, our results are in very good agreement with the experimental results, however, at high temperature, our results are lower than other experimental results.

Phase Equilibria in the System Al2O3-CaO-CoO and Gibbs Energy of Formation of Ca3CoAl4O10

The floating-zone method with different growth ambiences has been used to selectively obtain hexagonal or orthorhombic DyMnO3 single crystals. The crystals were characterized by x-ray powder diffraction of ground specimens and a structure refinement as well as electron diffraction. We report magnetic susceptibility, magnetization and specific heat studies of this multiferroic compound in both the hexagonal and the orthorhombic structure. The hexagonal DyMnO3 shows magnetic ordering of Mn3+ (S = 2) spins on a triangular Mn lattice at T-N(Mn) = 57 K characterized by a cusp in the specific heat. This transition is not apparent in the magnetic susceptibility due to the frustration on the Mn triangular lattice and the dominating paramagnetic susceptibility of the Dy3+ (S = 9/2) spins. At T-N(Dy) = 3 K, a partial antiferromagnetic order of Dy moments has been observed. In comparison, the magnetic data for orthorhombic DyMnO3 display three transitions. The data broadly agree with results from earlier neutron diffraction experiments, which allows for the following assignment: a transition from an incommensurate antiferromagnetic ordering of Mn3+ spins at T-N(Mn) = 39 K, a lock-in transition at Tlock-in = 16 K and a second antiferromagnetic transition at T-N(Dy) = 5 K due to the ordering of Dy moments. Both the hexagonal and the orthorhombic crystals show magnetic anisotropy and complex magnetic properties due to 4f-4f and 4f-3d couplings.

Simulated morphological landscape of polymer single crystals by phase field model

The novel phase field model with the "polymer characteristic" was established based on a nonconserved spatiotemporal Ginzburg-Landau equation (TDGL model A). Especially, we relate the diffusion equation with the crystal growth faces of polymer single crystals. Namely, the diffusion equations are discretized according to the diffusion coefficient of every lattice site in various crystal growth faces and the shape of lattice is selected based on the real proportion of the unit cell dimensions. Spatiotemporal growth of syndiotactic polypropylene single crystals during isothermal crystallization has been investigated theoretically based on this phase field model. Two dimensional numerical calculations are performed to elucidate the faceted single crystal growth including square, rectangular, lozenge-shaped, and hexagonal single crystals. Our simulated patterns are in good agreement with the experimental morphologies, and the physical origin of polymer single crystal growth is discussed.

Origin of basal plane bending in hexagonal silicon carbide single crystals

4H-SiC crystals grown by the physical vapor transport method were investigated using high-resolution X-ray diffraction, defect selective etching, and transmission electron microscopy. The (0 0 0 8) diffraction peak position shifted about 2.5° along the 3 in crystal diameter indicating the bending of the basal plane. Transmission electron microscopy revealed that the basal plane bending is due to plastic-deformation-induced basal plane dislocations with at least a partial edge character. The dislocations had a non-zero net Burgers vector with the net extra half-plane pointing toward the seed side of the crystal, and thus, caused the basal plane to bend concave toward the growth direction. The plastic deformation is suggested to be due to thermoelastic shear stresses imposed on the crystal during the physical vapor transport (PVT) growth.

Mesoscale conductivity and scaling function in aggregates of cubic, trigonal, hexagonal, and tetragonal crystals

The framework of stochastic mechanics is used to obtain scale-dependent bounds on the thermal conductivity of random polycrystals. This is done with the help of a scaling function that enables one to define the mesoscale that separates the effective, macroscopic conductivity from the realization-dependent microscale conductivity. We demonstrate that the scaling function depends upon the single-crystal anisotropy measure $(k)$ and the mesoscale $(\ensuremath{\delta})$ for aggregates made up of cubic, trigonal, hexagonal, and tetragonal single crystals. The proposed methodology unifies the treatment of a variety of crystals over different length scales. Finally, we develop a methodology to construct a material selection diagram in the $(k\text{\ensuremath{-}}\ensuremath{\delta})$ space.