C-axis Lattice Research Articles

Study on structure and electrochemical properties of the (LaGdMg)Ni 3.35- xCo xAl 0.15 ( x=0-2.0) hydrogen storage alloys

Hydrogen storage alloys (LaGdMg)Ni 3.35- x Co x Al 0.15 ( x=0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0) were prepared by induction melting followed by annealing treatment in argon atmosphere. The effects of partly replacing Ni by Co element in (LaGdMg)Ni 3.35Al 0.15 on the phase structure and electrochemical properties of (LaGdMg)Ni 3.35- x Co x Al 0.15 alloys were investigated. Structure analysis showed that the alloys consisted of Ce 2Ni 7-type (Gd 2Co 7-type), CaCu 5-type, Pr 5Co 19-type, PuNi 3-type phase structure. The addition of Co element obviously reduced the contents of CaCu 5-type phase and increased the contents of Ce 2Ni 7-type phase. However, Pr 5Co 19-type and CaCu 5-type phase obviously increased with the high content of Co. Rietveld analysis showed that the c-axis lattice parameters and cell volumes of the component phases increased with increasing Co content. The electrochemical measurements showed that as the Co content increased, the maximum discharge capacity and the cyclic stability of the annealed alloys both first increased then decreased. The (LaGdMg)Ni 3.05Co 0.3Al 0.15 alloy electrode exhibited the maximum discharge capacity (392.92 mAh/g), and the (LaGdMg)Ni 1.85Co 1.0Al 0.15 alloy electrode showed the best cyclic stability ( S 100=96.1%).

Room Temperature Ferromagnetism in Ni Doped ZnO Powders

Zn1−xNixO (x=0.1%, 0.4%, 0.7%, 1.0%) powders were prepared using the sol-gel technique, and the structural, optical, and magnetic properties of the samples were investigated. X-ray diffraction measurements show that all samples have a wurtzite structure and that the c-axis lattice constant decreases as the Ni content increases. X-ray photoelectron spectroscopy studies reveal that the doped Ni ions are in divalent states in all samples. Optical absorption spectra show that the band energy of Zn1−xNixO powders decreases with increasing Ni concentration. Photoluminescence measurements of Zn1−xNixO show a broad peak at a wavelength centered at about 467 nm which indicates the presence of a fraction of defects in ZnO. The room temperature ferromagnetism observed in all samples is intrinsic in nature.

Phase diagram ofCeFeAs1−xPxOobtained from electrical resistivity, magnetization, and specific heat measurements

We performed a systematic study on the properties of CeFeAs$_{1-x}$P$_{x}$O ($0\leq x\leq 1$) by electrical resistivity, magnetization and specific heat measurements. The c-axis lattice constant decreases significantly with increasing P content, suggesting a remarkable chemical pressure. The Fe-3d electrons show the enhanced metallic behavior upon P-doping and undergo a magnetic quantum phase transition around $x \approx 0.4$. Meanwhile, the Ce-4f electrons develop a ferromagnetic order near the same doping level. The ferromagnetic order is vanishingly small around $x=0.9$. The data suggest a heavy-fermion-like behavior as $x\geq 0.95$. No superconductivity is observed down to 2 K. Our results show the ferromagnetic ordered state as an intermediate phase intruding between the antiferromagnetic bad metal and the nonmagnetic heavy fermion metal and support the cerium-containing iron pnictides as a unique layered Kondo lattice system.

Nitrogen-doped ZnO prepared by capillaritron reactive ion beam sputtering deposition

Nitrogen-doped ZnO thin films have been prepared by reactive ion beam sputtering deposition utilizing a capillaritron ion source. X-ray diffraction (XRD) analysis of the as-deposited film exhibits a single strong ZnO (002) diffraction peak centred at 34.40°. Post-growth annealing causes increase of grain size and decrease of c-axis lattice constant. Micro-Raman spectroscopy analysis of the as-deposited film shows strong nitrogen-related local vibration mode at 275, 582, 640 and 720 cm −1, whereas the E 2 mode of ZnO at 436 cm −1 can barely be identified. Annealing at 500–800 °C causes decrease of 275, 582, 640 and 720 cm −1 and increase of 436 cm −1 intensity, indicating out-diffusion of nitrogen and improvement of ZnO crystalline quality. Unlike un-doped ZnO, the surface roughness of nitrogen-doped ZnO deteriorates after annealing, which is also attributed to the out-diffusion of nitrogen. A nitrogen concentration of ∼10 21/cm 3 was observed while type conversion from n-type to p-type was not achieved, which is likely due to the formation of Zn I–N O or (N 2) O that act as donor/double donors.

Structure, luminescence and electrical properties of ZnO thin films annealed in H 2 and H 2O ambient: A comparative study

Undoped ZnO films were grown on a c-plane sapphire by plasma-assisted molecular-beam epitaxy technique, and subsequently annealed at 200–500 °C with steps of 100 °C in water vapour and hydrogen ambient, respectively. It is found that the c-axis lattice constant of the ZnO films annealed in hydrogen or water vapour at 200 °C increases sharply, thereafter decreases slowly with increasing annealing temperature ranging from 300 °C to 500 °C. The stress in the as-grown ZnO films was more easily relaxed in water vapour than in hydrogen ambient. Interestingly, the controversial luminescence band at 3.310 eV, which is often observed in photoluminescence (PL) spectra of the ZnO films doped by p-type dopants, was observed in the PL spectra of the annealed undoped ZnO films and the PL intensity increases with increasing annealing temperature, indicating that the 3.310 eV band is not related to p-type doping of ZnO films. The electron concentration of the ZnO films increases sharply with increasing annealing temperature when annealed in hydrogen ambient but decreases slowly when annealed in water vapour. The mechanisms of the effects of annealing ambient on the properties of the ZnO films are discussed.

Enhancement of Ethanol Sensing Properties by Alloying ${\rm TiO}_{2}$ With ZnO Tetrapods

The Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O tetrapods were synthesized using simple thermal oxidation method from Zn and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixture. The tetrapods exhibited single crystalline hexagonal wurtzite structure with the prefer growth direction of [0002] along the legs. The sensors based on Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O tetrapods were fabricated and investigated the ethanol sensing properties. The FE-SEM, HRTEM, SAED, XRD, and RS results suggested that Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O alloy was formed with a slightly decrease of c-axis lattice parameter. The decrease of sensor resistance under ethanol atmosphere was observed and suggested that the Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O tetrapods possessed n-type property of semiconductor similar to ZnO. The Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O tetrapod sensors exhibited higher sensitivity than that of pure ZnO tetrapod sensors for entire ethanol concentration with optimum operating temperature of 300°C. Thus, the enhancement of sensitivity due to alloying TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with the ZnO tetrapods was observed and maybe explained by an increase of adsorbed oxygen ions due to substitution of Ti atom into Zn atom. Also, the slope value of the plot between log(S-1) and log C suggested that adsorbed oxygen ion species at the surface of the Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O tetrapods was O <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sup> which was same as pure ZnO tetrapods. Finally, these results have an important implication for a development of ethanol sensors based on metal oxide semiconductors for alcohol breath analyzer.

Characterization of MgZnO films grown by plasma enhanced metal-organic chemical vapor deposition

MgZnO (magnesium–zinc-oxide) films were grown on (11–20) sapphire substrates and Zn-polar ZnO substrates by plasma enhanced metal-organic chemical vapor deposition (PE-MOCVD) employing microwave-excited plasma. Structural, electrical and optical properties were investigated by X-ray diffraction, atomic force microscope, Hall, transmittance and photoluminescence measurement. The c-axis lattice constant decreases proportionally to an increase in the Mg content of Mg x Zn 1 − x O films. Therefore, this indicates that Mg atoms can be substituted in the Zn sites. Mg contents in films on ZnO substrates increase up to 0.11. In addition, Ga doped ZnO films were grown on (11–20) sapphire substrates. The resistivity of the films on (11–20) sapphire is controlled between 1.2 × 10 − 3 Ω cm to 1 Ω cm by changing the process conditions. The overall results indicate the promising potential of this PE-MOCVD method for related (Zn, Mg)O films formation because of the reactivity of the radicals, such as oxygen radicals (O⁎).

Effect of co-doping by Pb and La on structural and magnetic properties of Bi2212 superconducting ceramics

Abstract Doping the Bi2212 phase by Pb usually results in an enhancement of the pinning properties and the T c. Using elements other than Bi and/or other sites one may obtain opposite results. In this work we present the effect on structural and magnetic properties of Bi2212 superconducting ceramics when co-doping with La and Pb is used. Bi1.4Pb0.6Sr2Ca1–x La x Cu2O8+δ (0 ≤ x ≤ 0.025) superconducting ceramic samples were prepared using a solid-state reaction method. Structural and magnetic characterizations were made by means of X-ray diffraction, scanning electron microscopy and AC susceptibility measurements. The obtained crystalline structure is mainly tetragonal with a c-axis lattice parameter decreasing with La content. Substitution by La reduces the fraction content of the parasitic phase Bi2201 and the size of the grains. The transition temperature Tc is enhanced quasi linearly with the La content and the superconducting volume fraction has a maximum for x = 0.01.

A simple approach of fabricating thermoelectric γ-Na xCoO 2 and superconductive Na x(H 2O) yCoO 2-δ films using the sol–gel spin-coating method

We report a simple approach of fabricating thermoelectric γ-Na x CoO 2 film with the c-axis orientation using the sol–gel spin-coating method. The inferred sodium content is x = 0.65 according to the correlation between the c-axis lattice constant and x. Temperature dependence of both the resistivity and thermopower resembles that of the γ-Na 0 .68 CoO 2 film grown by the reactive solid-phase epitaxy. The fitted thermopower data show that the bandwidth of γ-Na x CoO 2 is found to be ~ 101 meV, being close to the quasi-particle band (70–100 meV) derived from an angle-resolved photoemission study of γ-Na 0.7CoO 2. These results enable the possibility of low-cost fabrication of γ-Na x CoO 2-based thermoelectric film devices. Furthermore, we have also topotactically transformed the of γ-Na x CoO 2 film to a superconducting Na x (H 2O) y CoO 2- δ film with T c, onset = 4.12 K.

Effects of Mg doping on structural, optical, and electrical properties of CuScO 2(0001) epitaxial films

CuScO 2 thin films with different Mg concentrations were grown on a-plane sapphire substrates by combining the two-step deposition and post-annealing techniques using Cu 2(Sc 1− x Mg x ) 2O y [ X = 0.01, 0.05, 0.10] targets. The effects of the Mg doping in the Sc-site on the structural, optical, and electrical properties of the films were investigated. A Mg-doped CuScO 2[3R](0001) epitaxial film was obtained at a Mg concentration of 1 at%. The slight increase in the a-axis lattice constant and the slight decrease in the c-axis lattice constant of the film were confirmed using two-dimensional X-ray reciprocal space mapping. No significant increase in optical absorption was observed in the film, and the energy gap for direct allowed transition was estimated at 3.7 eV. The film showed an increase in the electrical conductivity and carrier concentration and a decrease in the Hall mobility compared with those of the non-doped epitaxial film. The decrease in the overlap of Cu 3d orbitals due to the increase in the a-axis lattice constant is one cause of the decrease in the Hall mobility of the film. The temperature dependence of the electrical transport properties of the film exhibited semiconducting characteristics, and the activation energy estimated from the temperature dependence of the carrier concentration was 0.55 eV.

Surface of underdoped YBa2Cu3O7- δ as revealed by STM/STS

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7- δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.

The Influence of Gas Flow Rate on the Growth of YBCO Films Prepared by TFA-MOD

As reported by some authors, during the high temperature firing heat treatment step of TFA-MOD process, the gas flow rate on film surfaces, which is directly related to the transfers of water and the product HF gas, is one of the most important parameters influencing YBCO film nucleation and growth. In our experiments, this influence was investigated by a rotating method rather than changing the volume gas flow directly. As the rotating speed of the sample holder increased, relative gas flow rate on film surfaces increased, and quite high values could be reached without consumption of much gas. It was found that microstructure, texture, superconducting properties of YBCO films were evidently dependent on the rotating speed. With increasing the rotating speed, critical current density of YBCO films converted from precursor films in different quality could be enhanced to around 3 MA/cm2, the c-axis lattice parameter of YBCO unit cell was slightly reduced and the texture of the epitaxial films was improved, while the ratio of a-axis/c-axis oriented grains first increased and then decreased again.

Reduction dependence of superconductivity in the end-member T′ cuprates

We have recently achieved superconductivity in T′- RE 2CuO 4 ( RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low- P O2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high- T c superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T′- RE 2CuO 4. For thin films, the amount of remnant O ap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.

Superconductivity in Co doped Cu 0.5Tl 0.5Ba 2(CaM)Cu 2.95Co 0.05O 10− δ (M = Mg, Be) samples

We have investigated the possible source of suppression of superconductivity in Cu 0.5Tl 0.5Ba 2Ca 2Cu 2.95Co 0.05O 10− δ samples. We substituted Mg ( r = 0.65 Å) and Be ( r = 0.31 Å) at Ca ( r = 0.99 Å) sites in Cu 0.5Tl 0.5Ba 2CaMCu 2.95Co 0.05O 10− δ (M = Mg, Be) superconductor to enhance inter-planar coupling and Li in the charge reservoir layer (Cu 0.5Tl 0.25Li 0.25)Ba 2O 4− δ to supply extra carriers to CuO 2 planes. It was observed that zero resistivity critical temperature is decreased after Mg and Be substitution in Cu 0.5Tl 0.5Ba 2CaMCu 2.95Co 0.05O 10− δ samples. However, the c-axis lattice parameter is increased in both samples in spite of smaller ionic radius of Mg and Be as compared to Ca. However, in Cu 0.5Tl 0.5Ba 2CaMCu 2.95Co 0.05O 10− δ (M = Mg, Be) samples we have achieved enhanced superconductivity by Li doping. It was observed from these studies that cobalt doped in the conducting CuO 2 planes somehow localizes the carriers at Co 3+ sites and decrease the density of mobile carriers. But Li in Cu 0.5Tl 0.5Ba 2O 4− δ charge reservoir layer supplies the required number of carriers to the CuO 2 planes; therefore, the critical temperature is increased in Li doped samples.

The effect of growth ambient on the structural and optical properties of Mg xZn 1− xO thin films

Mg x Zn 1− x O ( x = 0.0–0.20) thin films have been deposited by sol–gel technique on glass substrates and the effect of growth ambient (air and oxygen) on the structural, and optical properties have been investigated. The films synthesized in both ambient have hexagonal wurtzite structure. The c-axis lattice constant decreases linearly with the Mg content ( x) up to x = 0.05, and 0.10 respectively for air- and oxygen-treated films, above which up to x = 0.20, the values vary irregularly with x. The change in the optical band gap values and the ultraviolet (UV) peak positions of Mg x Zn 1− x O films show the similar change with x. These results suggest that the formation of solid solution and thus the structural and optical properties of Mg x Zn 1− x O thin films are affected by the growth ambient.

Microstructures and dielectric properties of BaZr0.2Ti0.8O3 ceramics

The crystalline microstructure, surface morphology and dielectric properties of BaZr0.2Ti0.8O3 (BZT) ceramics were investigated. The c-axis lattice parameter (c=0.4107nm) of the BZT ceramics is larger than the a-axis one (a=0.4101nm). It was found that the crystal structure of the ceramics is to be tetragonal phase, and close to cubic phase. The temperature dependence of the relative permittivity showed that BaZr0.2Ti0.8O3 ceramics is to be relaxor ferroelectrics with the diffuseness constant equal to 1.93. As the applied maximum voltage increased from 500V to 1000V and 1500V, the remanent polarization increased from 1.69μC/cm2 to 3.10μC/cm2 and 3.60μC/cm2, furthermore the coercive field increased from 0.88kV/cm to 1.19kV/cm and 1.35kV/cm.

Persistent photoconductivity and magnetoresistance in YBa2Cu3O6.34 thin film

We have used persistent photoconductivity to smoothly tune the hole-doping in an underdoped cuprate in the critical doping region and studied its transverse magnetoresistance (MR). A series of YBa2Cu3O6+x (0.02 ≤ x ≤ 1) thin films of thickness ∼50 nm were grown by sputtering and the oxygen contents were estimated from the c-axis lattice constant. The film YBa2Cu3O6.34, with composition at the critical doping regime for the insulator-metal-superconductor transition (Tc < 0.13 K), was chosen for studying persistent photoconductivity and MR. The sample was illuminated at 0.8 K using an ultraviolet stroboscope with wavelength range of 220–700 nm for several hours. Resistivity down to 0.13 K and MR in a field up to 9T were measured in intermittent stages. The sample resistance was found to decrease exponentially with the illumination time. The field dependence of MR changed from quadratic to linear behavior after shining light on the sample for ∼ 13 h. This cross-over in the field dependence of transverse MR clearly shows that the topology of the Fermi surface was changed drastically by shining light. Our observations suggest a pseudogap state as a precursor of superconductivity in YBa2Cu3O6+x.

Effect of fluence on the lattice site of implanted Er and implantation induced strain in GaN

A GaN thin film was implanted with 5×1014cm−2 of 60keV stable 166Er, followed by the implantation of 2×1013cm−2 radioactive 167Tm (t1/2=9.3d) and an annealing sequence up to 900°C. The emission channeling (EC) technique was applied to assess the lattice location of Er following the Tm decay from the conversion electrons emitted by 167mEr, which showed that more than 50% of 167mEr occupies substitutional Ga sites. The results are briefly compared to a 167mEr lattice location experiment in a GaN sample not pre-implanted with 166Er. In addition, high-resolution X-ray diffraction (HRXRD) was used to characterize the perpendicular strain in the high-fluence implanted film. The HRXRD experiments showed that the Er implantation resulted in an increase of the c-axis lattice constant of the GaN film around 0.5–0.7%. The presence of significant disorder within the implanted region was corroborated by the fact that the EC patterns for off-normal directions exhibit a pronounced angular broadening of the order of magnitude of 0.5°.

Electron Microscope Study of Strain in InGaN Quantum Wells in GaN Nanowires

AbstractStrains in GaN nanowires with InGaN quantum wells (QW) were measured from transmission electron microscope (TEM) images. The nanowires, all from a single growth run, are single crystals of the wurtzite structure that grow along the &lt;0001&gt; direction, and are approximately 1000 nm long and 60 nm to 130 nm wide with hexagonal cross-sections. The In concentration in the QWs ranges from 12 to 15 at %, as determined by energy dispersive spectroscopy in both the transmission and scanning electron microscopes. Fourier transform (FT) analyses of &lt;0002&gt; and &lt;1100&gt; lattice images of the QW region show a 4 to 10 % increase of the c-axis lattice spacing, across the full specimen width, and essentially no change in the a-axis value. The magnitude of the changes in the c-axis lattice spacing far exceeds values that would be expected by using a linear Vegard's law for GaN – InN with the measured In concentration. Therefore the increases are considered to represent tensile strains in the &lt;0001&gt; direction. Visual representations of the location and extent of the strained regions were produced by constructing inverse FT (IFT) images from selected regions in the FT covering the range of c-axis lattice parameters in and near the QW. The present strain values for InGaN QW in nanowires are larger than any found in the literature to date for other forms of InxGa1-xN (QW)/GaN.

Lattice Parameter, Lattice Disorder and Resistivity of Carbohydrate Doped MgB2 and Their Correlation with the Transition Temperature

The change in the lattice parameters or the lattice disorder is claimed as a cause of the slight reduction in the transition temperature by carbon doping in MgB2. In this work, an extensive investigation on the effects of carbohydrate doping has been carried out. It is found that not only the a-axis but also the c-axis lattice parameter increases with the sintering temperature. A linear relation between the unit cell volume and the critical temperature is observed. Compared with the well known correlation between the lattice strain and the critical temperature, the X-ray peak broadening itself shows a closer correlation with the transition temperature. The residual resistivity and the critical temperature are linearly correlated with each other as well and its implication is further discussed.