Lattice Parameter Expansion Research Articles

The Structure, Magnetic and Absorption Properties of Zn-Ti Substituted Barium-Strontium Hexaferrite Prepared by Mechanochemical Process

Synthesis and characterization of (Ba,Sr)0.5Fe12-xZnxTixO19 M-type hexaferrite was conducted by using mechanochemical process. The composition of Zn-Ti ion in (Ba,Sr)0.5Fe12- xZnxTixO19 M-type hexaferrite were varied as x = 0.0, 0.2, 0.6, and 1.0. The sample was characterized by using X-rays diffractometer (XRD), scanning electron microscope (SEM), vibrating sample magnetometer (VSM), and vector network analyzer (VNA). The XRD patterns were further analyzed by Rietveld analysis program. The Rietveld analysis result indicated that the substitution of Zn-Ti ions led to the expansion of the hexagonal lattice parameter and unit cell volume (Vcell), while the atomic density decreased with increasing of the Zn-Ti ion. The VSM measurement exhibited that the substitution of the Zn-Ti ions caused the change in magnetic properties behavior such as intrinsic magnetic coercivity (Hci), remanence (Mr) and saturation (Ms) magnetization. The value of Hc dropped significantly with the substitution of Zn-Ti ions, and then continued to decrease with the addition of Zn-Ti ions content for x = 0.6, and x = 1.0. SEM observation revealed that all the particles showed nearly hexagonal platelet shape, with the particle size distribution at most in the range between 200–600 nm. The variation of the reflection loss versus frequency was measured by using VNA in (Ba,Sr)0.5Fe12-x(ZnTi)xO19 hexaferrite for x = 0.0–1.0. The optimum reflection loss (RL) was found to be –48 dB at 14 GHz in (Ba,Sr)0.5Fe11.6(ZnTi)0.2O19 hexaferrite for x = 0.2.

Poisson effect driven anomalous lattice expansion in metal nanoshells

Surface stress can have profound effects on nanoscale materials and can lead to a contraction of the lattice in nanoparticles to compensate for the under-coordination of the surface atoms. The effect of elastic properties like Poisson's ratio can be accentuated in lower dimensional systems. The current study focuses on hollow metal nanoshells (MNSs), wherein there is interplay between the surface stresses existing in the inner and outer surfaces. Using a two scale computational method and transmission electron microscopy, we not only show a lattice expansion (in the radial direction) due to purely surface stress effects in a metallic system but also discover anomalous lattice expansion in the case of very thin walled MNSs. We argue that this effect, wherein the stress in the outer surface causes expansion in the radial lattice parameter (instead of compression), is a Poisson effect driven phenomenon. Although Ni nanoshells are used as an illustrative system for the studies, we generalize this effect for all metal nanoshells.

Computational investigations of the band structure and thermodynamic properties of calcium-doped BaS using the FP-LAPW approach

In this work, the full-potential linearized augmented plane wave (FP-LAPW) method was used to calculate the structural, electronic, thermal and thermodynamic properties of BaS and CaS compounds and their ternary mixtures, Ba1−xCaxS. The local-density approximation (LDA), the Wu-Cohen generalized gradient approximation (WC-GGA) and the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) were used as the exchange-correlation potential. Moreover, the modified Becke–Johnson approximation was also used for the band structure calculations. We examined the composition effect on the lattice constants, bulk modulus and band gap. The microscopic origins of the band gap bowing were characterized in detail using the approach of Zunger and colleagues. Pressure and temperature effects on the lattice parameter, heat capacity, Debye temperature, Grüneisen parameter, and thermal expansion coefficient were predicted using the quasi-harmonic Debye model. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing, ∆Hm, and the phase diagram. It was shown that these alloys are stable at high temperature.

Temperature evolution of structural and electrical property for Sr7Mn4O15 at high temperature

Temperature dependence of structural evolution for single phase Sr7Mn4O15 is determined using the in-situ X-ray diffraction with Rietveld refinements in the temperature range of 300–1200 K. It's found that thermal expansion of Sr7Mn4O15 lattice parameters increase linearly with temperature. The structure of face-sharing clusters prefers expansion in the transverse, and corner-sharing clusters prefers the stretch in the longitudinal. Resistance decreases sharply with increasing temperature at high temperature, which means that the Sr7Mn4O15 is a well candidate in applications as negative temperature coefficient thermistor. Based on the fitted atomic parameters given from Rietveld refinements, temperature dependent band gap of Sr7Mn4O15 is estimated with first principle calculation, which exhibits a dramatic reduction from 1.141 eV to 0.349 eV with increasing the temperature.

Surface microstructure and properties of Cu-C powder metallurgical alloy induced by high-current pulsed electron beam

This paper investigates the microstructure and properties of Cu-6C powder metallurgy (PM) alloys irradiated by high-current pulse electron beam (HCPEB). After 20-pulsed HCPEB irradiation, the surface was melted completely and the remelted layer was formed in 2–3 μm thickness, which consisting of ultrafine Cu grains. Within the near irradiated surface, high-density dislocations and vacancy clusters were observed which provide a large amount of paths for carbon atoms to diffuse into Cu matrix, and it leads to the formation of the supersaturated solid solution, causing Cu lattice parameter expansion. In addition, the extreme rapid cycle generated the ultrafine carbon particles at the surface of Cu-6C metallic matrix. The results of microhardness measurements and sliding wear resistance testing indicated that the irradiated sample exhibited better properties than the initial one. The ultrafine carbon particles played an important role in the antifriction properties, which was attributed to the formation of carbonic tribolayer on the hard surface.

Ionic ASi2N3 (A=Li, Na, K and Rb) stabilized by the covalent Si–N bonding: First-principles calculations

The structural, elastic and electronic properties of LiSi2N3 and its substitutions by Na, K and Rb were investigated through first-principles computations. The expansion of lattice parameters of ASi2N3 from Li, Na, K to Rb is found to be determined by the bond angle of Si–N1–Si, which suggests a possible way to improve the lithium ionic conductivity by substitutions. ASi2N3 (A=Li, Na, K and Rb) shows the similar elastic behaviors, while the electronic band gap gradually decreases from 5.1 to 3.4eV from LiSi2N3 to RbSi2N3. Interestingly, the analysis of electronic structure, crystal orbital Hamiltonian populations and Bader charges shows that the covalence of Si–N bonding is critical for the stability of ASi2N3 phase. Among ASi2N3 phases, there is a relatively high ionicity in NaSi2N3; the Si–N bond strength in [Si2N3]− net for KSi2N3 and RbSi2N3 is comparable to LiSi2N3, but stronger than NaSi2N3.

Exchange interactions and Curie temperatures in Fe2 NiZ compounds

Using the spin-polarized relativistic Korringa-Kohn-Rostoker method, we study the exchange interactions and Curie temperatures of Fe2 NiZ (Z= Al, Ga, Si, Ge) with the Hg2 CuTi structure. The Heisenberg exchange coupling parameters and Curie temperatures are obtained by calculating the exchange interactions between the constituents. It is found the nearest neighbor exchanges play a leading role in interactions, the calculated Curie temperatures, which are higher than the room temperature, decrease when changing Z from III to IV group. In addition, we study the influence of lattice constant on magnetic moments and Curie temperatures. It is found the Fe magnetic moments become more larger due to the reduction of atomic orbital overlap as the expansion of lattice parameters. Only small changes are observed for the moment of Ni atom, while for the Z atom, the magnetic moment is nearly constant. Finally, the Curie temperatures of Fe2 NiZ compounds under different lattice constants are acquired by using the mean-field approximation.

Superior Oxide Ion Conductivity of Novel Acceptor Doped Cerium Oxide Electrolytes for Intermediate-Temperature Solid Oxide Fuel Cell Applications

Novel compositions of Nd3+ and Dy3+ codoped cerium oxide according to the system of Ce0.8Ndx–yDyyO2–{x–y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) have been synthesized by a simple sol–gel method and studied as electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Thermal, microstructural, optical, and electrical properties have been enhanced to different extents by the addition of Nd3+ and Dy3+ ions in cerium lattice, in particularly Dy3+ ions because of their low ionic radius mismatch. Thermogravimetric/differential scanning calorimetric (TG/DSC) analysis exhibited a small weight loss and high thermal stability in the intermediate temperature range (400–800 °C). The addition of Dy3+ ions stabilized the cubic fluorite structure, which is confirmed from X-ray diffraction (XRD) studies. Lattice parameter expansion and contraction have been observed on account of their ionic radii trend. The formation of cubic fluorite structure has been confirmed by high-resolution transmission electron mi...

Magnetic properties of Sm-doped rutile TiO2 nanorods

Sm-doped TiO2 nanorods are synthesized by a molten salt method. The expansion of lattice parameters of Sm-doped TiO2 samples verified by XRD and TEM characterizations suggests the successful incorporation of Sm ions into TiO2 matrix. M-H loops show weak room temperature ferromagnetism in Sm-doped samples. Sm and N co-doped TiO2 nanorods are also prepared to study the magnetic behaviour. The results indicate that the co-doping does not enhance the room temperature ferromagnetism but leading to a strong paramagnetic signal at 5 K.

Comparison of stress, strain, and elastic properties for porous silicon layers supported by substrate and corresponding membranes

This paper describes characterization of mechanical properties of porous silicon (PS) layers with different porosities using high resolution XRD. The XRD measurement determined various mechanical properties of PS such as; Young modulus, Poisson's ratio, and lattice parameter expansion. Our results indicated that mechanical properties reduce with increasing porosity. Also, the mechanical properties of two different porous layers, either supported by or detached from the substrate were examined. Comparison of the two porous layers showed that the constraint in the interatomic spacing is the origin of the lattice constant expansion in the planes perpendicular to the surface. This phenomenon can be useful for gas sensor applications.

Room-Temperature Ferromagnetism in TiO2 Nanocrystals Synthesized by the Controlled Hydrolysis Procedure.

TiO2 nanocrystals were prepared by a controlled hydrolysis procedure at room temperature. The effect of V-doping, N-doping and V/N codoping on the lattice parameters and magnetic properties of TiO2 nanocrystals was investigated by means of X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and vibration sample magnetometry. Doping performed at room temperature causes the expansion of lattice parameters. Undoped and doped TiO2 nanocrystals show room-temperature ferromagnetism. A monotonic correlation between saturation magnetization and the ratio of the lattice parameter c to a (c/a) was observed. Saturation magnetization of TiO2 nanocrystals increases with the value of c/a.

Studies on the structural, morphological and optoelectrical properties of spray deposited CdS:Pb thin films

Undoped and Pb-doped CdS (CdS:Pb) thin films were prepared on glass substrates by a spray technique using a perfume atomizer. The effects of Pb doping on the structural, morphological and optoelectrical properties of CdS thin films were studied. All of the films are polycrystalline in nature exhibiting hexagonal crystal structure. Crystallite size estimated from the Scherrer formula increased from 18.42 nm to 34.76 nm with increases in Pb doping concentration. The expansion of lattice parameters experienced with Pb doping might be due to the larger ionic radius of Pb2+ ions compared to that of the host Cd2+ ions. Increased optical transparency is observed for the doped films and the optical band gap experienced band bowing with increase in Pb doping concentration. Electrical resistivity values of the films were found to be on the order of 101 Ω cm. From the obtained results, it is observed that the CdS thin film coated with 6 wt.% Pb doping concentration exhibit better structural, morphological, optical and electrical properties.

Low temperature structural studies of SrSnO3

High resolution powder neutron diffraction measurements on polycrystalline SrSnO3 between 8 and 350 K are described. SrSnO3 retains the orthorhombic Pbnm structure over this temperature range. Examination of the thermal expansion of the individual lattice parameters reveals an anomaly near 230 K that may reflect the presence of polar nanodomains associated with local disorder of the octahedral tilts.

Thermal expansion of polycrystalline gallium nitride: an X‐ray diffraction study

Gallium nitride is studied at ambient and nonambient temperatures by powder X‐ray diffraction followed by Rietveld refinement of the structure. The structure is reported for the ambient temperature on the basis of laboratory data. The diffraction data collected using a synchrotron beam serve for derivation of the lattice parameter and thermal expansion dependencies on temperature. The variation of unit‐cell size on temperature was studied in detail in a broad range 11 K < T < 1073 K. The obtained dependencies of a, c and V and thermal expansion coefficients are smooth, and the results are consistent with previously collected data. The c/a ratio shows an almost linear decreasing trend with rising temperature. The magnitude of thermal expansion obtained in the described experiments is similar to a recently reported theoretical one. Copyright © 2015 John Wiley & Sons, Ltd.

Study of ball milled nanocrystalline Fe-50%Al alloy: structural and magnetic properties

Structural, morphological and magnetic changes induced by ball milling in Fe–50%Al alloy have been investigated using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques. The MA process induces a progressive dissolution of Al into Fe, resulted in the formation of an extended nanostructured Fe(Al) solid solution with BCC structure only after 5 h of milling. The grain size of constituents was decreased to the nanometre range (between 6 and 8 nm) and the constituents dissolved at the nanograin boundaries. Expansion in lattice parameter accompanied by reduction in crystallite size occurs during transition to nanostructured alloy. The corresponding magnetic measurements show continuous decrease in saturation magnetisation and increase in coercivity, retentivity and squareness ratio with increase in milling time. The systematic variation of the magnetic properties has been correlated with the evolution of microstructure, reduction in grain size and enhanced intergranular exchange coupling.

High thermal expansion in the solid solution series BaM2−x Ni x Si2O7 (M = Zn, Mg, Co)-the effect of Ni-concentration on phase transition and expansion

In crystalline phases with the compositions BaMg2Si2O7, BaCo2Si2O7, and BaZn2Si2O7, the components MgO, CoO, and ZnO can be partially substituted by NiO. The formed solid solutions exhibit a phase transition from a low to a high temperature phase which runs parallel to an increase in volume, which is most pronounced in the case of BaZn2Si2O7. The phase transition temperatures and the thermal expansion behavior of these solid solutions are investigated as a function of the NiO concentration. The phase transition temperature of pure BaZn2Si2O7 increases from 280 up to 915 °C, if ZnO is partially replaced by NiO. Furthermore, the phase transition temperatures of the MgO- and the CoO-containing compounds also increase with the increasing NiO concentration. The thermal expansion behavior was investigated by dilatometry and for selected samples by high-temperature X-ray diffraction in a temperature range between room temperature and a temperature below the respective phase transition. The measured coefficients of thermal expansion lie in the range from 6.2 to 21.4×10−6 K−1. The thermal expansion of all studied compounds depends on the crystallographic direction. The largest variation is observed in the compound BaZn2Si2O7. The smallest differences in the expansion in different directions are measured for the sample BaCo1.5Ni0.5Si2O7, where the maximum difference between the coefficients of thermal expansion of the different lattice parameters was just 3.9×10−6 K−1.

Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe2As2 family of materials.

We report a combination of Fe Kβ x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe2(As1-xPx)2. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe2As2 [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in CaFe2(As1-xPx)2. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe2(As1-xPx)2 (x=0.055) and Ca0.78La0.22Fe2As2 at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the c-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe2As2 family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.

Crystal structure and size effects on the performance of Li[Ni1/3Co1/3Mn1/3]O2 cathodes

Abstract

Copper intercalation at the interface of graphene and Ir(111) studied by scanning tunneling microscopy

We report on the intercalation of a submonolayer of copper at 775 K underneath graphene epitaxially grown on Ir(111) studied by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) at 77 K. Nucleation and growth dynamics of Cu below graphene have been investigated, and, most importantly, the intercalation mechanism has been identified. First, LEED patterns reveal the pseudomorphic growth of Cu on Ir under the topmost graphene layer resulting in a large Cu in-plane lattice parameter expansion of about 6% compared to Cu(111). Second, large-scale STM topographs as a function of Cu coverage show that Cu diffusion on Ir below graphene exhibits a low energy barrier resulting in Cu accumulation at Ir step edges. As a result, the graphene sheet undergoes a strong edges reshaping. Finally, atomically-resolved STM images reveal a damaged graphene sheet at the atomic scale after metal intercalation. Point defects in graphene were shown to be carbon vacancies. According to these results, a Cu penetration path beneath graphene is proposed to occur via metal aided defect formation with no or poor self healing of the graphene sheet. This work illustrates the fact that Cu intercalation is harmful for graphene grown on Ir(111) at the atomic scale.

Equation of state and thermal expansion of metals with FCC structure: Application to Cu, Al and Ni

The equation of state, the expressions of lattice parameter and thermal expansion coefficient in general form are obtained by the statistical moment method. Applying to Cu , Al and Ni metals, we determine these properties in simple analytic form for each metal. Numerical results for the thermal expansion coefficient of these metals in different temperatures and pressures are in good agreement with experiments.