Lattice Constant Ratio Research Articles

Effect of BaWO4 on the Microwave Dielectric Properties of Ba(Mg1/3Ta2/3)O3 Ceramics

Physical and microwave dielectric properties of complex perovskite Ba(Mg1/3Ta2/3)O3 ceramics have been investigated as a function of the amount of BaWO4 in the temperature range from 20° to 80°C at 10.5 GHz. Up to 0.05 mol BaWO4 addition, the lattice constant ratio (c/a), ordering parameter, apparent density, and unloaded Q all increase, due to the increase in the substitution of Ta5+ ions of Ba(Mg1/3Ta2/3)O3 by W6+ ions from the melted BaWO4 at above 1430°C. With further addition of BaWO4, the unloaded Q decreases, due to an increase of the BaWO4 phase. The temperature coefficient of resonant frequency (TCF) can be controlled by the volume mixture rule of Ba(Mg1/3Ta2/3)O3 and BaWO4. When 0.09 mol BaWO4 is added, TCF becomes 0 ppm/°C.

Film Thickness Dependence of Dielectric Property and Crystal Structure of PbTiO3 Film Prepared on Pt/SiO2/Si Substrate by Metal Organic Chemical Vapor Deposition

The crystal structure of PbTiO3 film, which was prepared on a Pt/SiO2/Si substrate by metal organic chemical vapor deposition, was investigated along with its dielectric properties. The relative dielectric constant increased with increasing film thickness. Crystal structure difference was observed below 0.3 µm of film thickness, as compared to that above 0.3 µm of thickness; i.e., the lattice constant ratio of the c-axis to a-axis, the intensity ratio of (00l) to (h00) reflections and the average crystallite size decreased, and the nonuniform stress increased with decrease in the film thickness. These crystal imperfections for below 0.3 µm thickness might be related to the existence of a low relative dielectric constant layer near the substrate.

High-pressure phase transition to β-polonium type structure in selenium

Abstract High-pressure x-ray powder diffraction experiments have been carried out in VIb group selenium in the pressure range between 42 GPa and 101 GPa. A new phase transition to β-Po type structure of space group R 3 m was observed at 60 GPa. Lattice constants of the high-pressure phase at 60 GPa were a= 3.992(3) A, c= 3.119(2) A, V= 43.04(8) A3 and Z= 3. The β-Po type phase remained stable to at least 101 GPa. A negative pressure dependence of the lattice constant ratio, c/a, suggests that the post-β-Po type structure is the bcc one.

T c variation in PbMo 6S 8: A critical analysis and a comparison with pure phases

Different methods were employed to synthesize a series of high quality PbMo 6S 8 Chevrel-phase compounds. All the samples were characterized by means of X-ray diffraction, Auger electron spectroscopy, specific heat, acsusceptibility, resistivity, scanning electron spectroscopy and microprobe analysis. A marked decrease in the superconducting transition temperature (T c) and the hexagonal lattice constant ratio (c/a) were noticed on the samples prepared in quartz tubes. Based on a simple correlation between T c and c/a ratio, we present some of the significant structure-property relations and phase purity of the samples. This interdependence is discussed mainly in terms of oxygen content in the phase. For instance, oxygen-free samples have T c invariably above 14.5 K. Interestingly, the optimal T c is found in samples having rather different, from each other, crystallographic parameters, but with a same c/a ratio. On the other hand, the wide variation of T c and structural parameters of PbMo 6S 8 observed in the present investigation and those of many earlier works are explained by the presence of varying amounts of oxygen in the ternary phase. These results are checked in parallel with the oxygen-doped (PbMo 6S 8−xO x; x ≥ 0.2) samples.

Interatomic potentials and the structural properties of silicon dioxide under pressure.

{ital First}-{ital principles} interatomic potentials based on {ital ab} {ital initio} Hartree-Fock self-consistent-field calculations have recently been proposed for silicon dioxide. These pairwise potentials yield accurate ambient-pressure internal-structure parameters and dynamically stable polymorphs for several known forms of silica. We use these potentials to analyze crystallographic data for {alpha}-quartz under pressure. We find the potentials do not describe the changes in the internal parameters of {alpha}-quartz under pressure. Specifically, the potentials do not give the correct trends for the O-Si-O angles or the {ital c}/{ital a} lattice-constant ratio. We have included bond-bending terms to modify the pairwise potentials. If the angles in question are fit to reproduce the experimental data, then the bulk modulus is in poor agreement with experiment. Our findings reinforce the need for a covalent'' term in the interatomic potential, which is not based on quadratic expansions of bond-bending and -stretching forces.

The Microstructural Dependence of Piezoelectric Properties in PbTiO<sub>3</sub> Ceramics

The grain size dependence of lattice constants (a and c), dielectric constant (e/e0), dissipation factor (tanδ), electromechanical coupling factors (κt and κp), and mechanical quality factor (QM) was investigated in ceramics with the compositions of 0.99⋅Pb1-XLa2X/3TiO3+0.01⋅MnO2 with X=0.05 (grain size=0.37-1.32μm) and X=0.10 (grain size=0.30-1.14μm). With decreasing grain size, c/a (lattice constant ratio), tanδ (of non-poled), κt/κp (anisotropy of coupling factor), and QM decreased, while e/e0 (of both non-poled and poled) increased. When poled, tanδ showed little grain size dependence. A close correlation was observed between the lattice constant ratio and the dielectric constant for all the materials, and the grain size dependence of these properties has been attributed mainly to the effect of internal stress in grains. The grain size dependence of tanδ may be interpreted on the basis of the effect of grain size on the motion of do-main boundaries. The values of QM-1 and tanδ did not agree with the relationship, QM-1∝tanδ, usually observed in ferroelectric materials. This is possibly explained in terms of the ceramic structure-dependent component of inner friction (QM-1), which becomes significant with decreasing grain size.

Computer simulation of relaxation process of deposited films

The relaxation process for deposited films has been simulated by the molecular dynamics (MD) method. The system calculated was that involving a monatomic layer of fcc(111) on a bcc(110) substrate. For simplicity, the inter-atomic potential was assumed to be a morse function and only the lattice constants were fitted to the experimental values. The systems with lattice constant ratios of 2 √3 , √ 3 2 , 4 3 and √2 were constructed and relaxed by the MD method. The kinetic and potential energies of the system were monitored during the calculation. The relaxed structures strongly depended upon the depths of the interaction potentials. Some systems showed coherent interfaces under the strong effects of the substrate potential. These systems showed some definite epitaxial relationship when the strength of interaction potentials of the deposit-substrate and that of the deposit-deposit were comparable.

Correlations among c/b, Tc, and Madelung potentials in the system of RBa2Cu3Ox superconductors.

We have observed a correlation between the lattice-constant ratio {ital c}/{ital b} and the superconducting transition temperature {ital T}{sub {ital c}} in {ital R}-Be-Cu-O superconductors (where {ital R} is a rare-earth element). Madelung potentials and the bond length of Cu(1)-O(3) are calculated and correlated with the distribution of {ital T}{sub {ital c}} over the various {ital R}Ba{sub 2}Cu{sub 3}O{sub 7} superconductors. Further consideration of YBa{sub 2}Cu{sub 3}O{sub {ital x}} with variable {ital x} of suitable oxygen content leads to an explanation of the correlation of decreasing {ital c}/{ital b} ratio with increasing {ital T}{sub {ital c}}. A Coulomb-type ionic mechanism is therefore suggested so that shortening of the {ital c} axis and increased chain length provides (1) a channel of charge-transfer excitation from chain to Cu-O layers and (2) a long-range Coulomb interaction between the two Cu-O layers, both of which result in stronger coupling between the two Cu-O layers and higher {ital T}{sub {ital c}}.

A molecular statics and dynamics study of epitaxial growth fronts

We study the MBE growth process employing both an energetics calculation and a molecular dynamics simulation. The importance of various growth parameters: lattice-constant ratio, relative orientation, and relative displacement is examined by calculating the total energy of a model fcc(in)/bcc(110) system. Using a molecular dynamics method, the actual motion of a large number of atoms at the growth front is monitored and the temperature dependence of the diffusion constant is obtained for various substrate potentials and surface coverages.

Temperature behaviour of the tetragonal distortion and thermal expansion of Cu-III-VI2 chalcopyrite semiconductors

The tetragonal unit cell parameters a and c of Cu-III-VI2 chalcopyrite semiconductors have been accurately determined at various temperatures in the range from 300 K to about 950 K by X-ray diffractometry. These data were used to evaluate the tetragonal distortion Δ (= 2−ca) and the linear thermal expansion coefficients αa and αc of the lattice parameters a and c respectively at various temperatures. The lattice constant ratio ca decreases with increasing temperature, resulting in an increase in the tetragonal distortion Δ with temperature. Because of the axial symmetry of these compounds the linear thermal expansivities αa and αc are thermally anisotropic (αa >αc), and this anisotropy increases with decreasing lattice constant ratio ca.

Structure of the Oxygen Point Defect in SnMo6S8and PbMo6S8

Rietveld structural refinement of neutron powder diffraction data for Chevrel-phase samples in both the Sn-Mo-S-O and Pb-Mo-S-O systems has led to a model for an oxygen-containing point defect. The oxygen substitutes for the sulfur located in the special position along the $\overline{3}$ axis and the metal ion is displaced \ensuremath{\simeq} 0.8 \AA{} toward the oxygen impurity, presumably forming a covalent bond. This defect is responsible for the wide variation reported for ${T}_{c}$ and $\frac{c}{a}$ (hexagonal, lattice-constant ratio).

ROLE OF PHONONS IN ORIENTATIONAL INTRA-PLANE ORDERING IN GRAPHITE INTERCALATION COMPOUNDS

Theoretical study of orientational intra-plane ordering in the second stage graphite intercalation compounds is presented with particular emphasis on the role of phonons. It is shown that the rotation angle of intercalant layers relative to the graphite layers is predominantly determined by intra = plane transverse modes and also depends on the ratio of lattice constants of both intercalant and graphite layens.

Relation between lattice and continuum theories of two-dimensional solids

A general model is proposed to describe the melting of adsorbed monolayers with a lattice constant which is commensurate to the underlying substrate. The model introduced here is a vector generalization of the discrete planar (clock) model. Dislocations are incorporated in the theory, and the translational invariance of the substrate is preserved. Exact duality relations can be used to obtain properties of the model. The phases which occur and which are considered in most detail can be described by all combinations of long-range, quasi-long-range (algebraic), and short-range order in the two perpendicular displacement fields. Depending on the symmetry of the ordered phase relative to the substrate, an intermediate "floating" phase, with algebraic decay of positional order can exist on a triangular lattice if the ratio of lattice constants of the overlayer to the substrate exceeds $2\sqrt{3}$.

Trends in the thermal expansion coefficients of the AIBIIIC2VI and AIIBIVC2V chalcopyrite compounds

AbstractEmpirical relations are derived for the average linear thermal expansion coefficient αL and the linear thermal expansion coefficients αa and αc of the lattice parameters a and c, respectively, of the AIBIIIC2IV and AIIBIVC2V compounds. It is shown that the thermal expansion coefficients of all tetrahedrally coordinated compounds can be described within the same model. The anisotropy of the thermal expansion coefficients depends essentially on the lattice constant ratio c/a. There exists a critical c/a value below of which αc becomes negative.

SmCo<SUB>5</SUB>-SmCu<SUB>5</SUB>系合金における磁気異方性と組織について

Directional magnetic properties are observed in SmCo5-SmCu5 system alloy, which was subjected to freezing in a direction normal to the cold surface of the copper hearth. The maximum energy product, (BH)max, of 65 wt%SmCo5-35 wt%SmCu5 alloy is 5.78 MGOe in the as-cast state when the measurement is carried out in the direction perpendicular to the chill plane. By annealing at 300°C for 1 hr, (BH)max value increases to 7.73 MGOe with Br=6450 G and BHc=3400 Oe. The magnetic properties in the direction perpendicular to the chill plane are superior to the other directions and the chill plane shows weak texture, (0001). These alloys have the easy direction of magnetization, which is the c-axis of the hexagonal crystal as SmCo5.The lattice constants of both a- and c-axes of the hexagonal crystal enlarge linearly with increasing copper content. The value of lattice constant ratio c⁄a is approximately 0.802 in all the alloys. The results of X-ray micro analysis suggest that the alloys of this system alloy have congruent points and miscribility gaps in solid solution on a quasi-binary diagram.

Directional Magnetic Properties and Structures in the SmCo<SUB>5</SUB>-SmCu<SUB>5</SUB> System Alloy

The coercive force BHC of the intermetallic compound SmCo5 is not sufficient for its application as a permanent magnet in the cast state. This study deals with increasing of the coercive force of the SmCo5 alloy system. Specimens are melted in an arc melt furnace which has the tungusten electrode and inactive gas Ar.The directional magnetic properties are observed in the SmCo5-SmCu5 system alloy, as a result of freezing in a direction normal to the cold surface of the copper hearth. The maximum energy product, (BH)max, of the 65 wt% SmCo5-35 wt% SmCu5 alloy has 5.78 MGOe in the as-cast state, when the measurement is carried out in a direction perpendicular to the chill plane. By annealing at 300°C for 1 hour, the (BH)max value increases to 7.73 MGOe with Br 6450 G and BHC 3400 Oe. The magnetic properties in the direction perpendicular to the chill plane are superior to those in other directions and the chill plane shows a weak texture, (0001). These alloys have the easy direction of magnetization, which is the c-axis of the hexagonal crystal as in the case of SmCo5.The lattice constants of both a- and c-axes of the hexagonal crystal enlarge linearly with increasing copper content in alloy. The value of lattice constant ratio c⁄a keeps approximately 0.802 in all the alloys investigated.

Mechanical Dispersion and Molecular Motion in Crystals of Polyethylene and Other Polymers

Dynamic mechanical loss of solution-grown polyethylene crystals is measured by the suspension method as a function of temperature and the crystalline dispersion (α dispersion) is observed above 40°C in just the same manner as in melt-grown polyethylene. In the temperature range of this dispersion, the ratio of crystal lattice constants b/a progressively changes toward that of a higher symmetry hexagonal form with increasing temperature. NMR line width of the broad component becomes gradually narrow with rising temperature in this temperature range. It is concluded from these reasons that the α dispersion of polyethylene should be ascribed to the rotational vibration of chain molecules in crystalline phase about c-axis. Similar behavior of low molecular weight paraffin crystals below the rotational transition point affords an additional confirmation of the above conclusion. Crystalline dispersions of other polymers are briefly discussed.

Refractivity and Lattice Constant Ratios of Lithium-6- and Lithium-7-rich Fluorides

THE Industrial Group of the United Kingdom Atomic Energy Authority has provided small quantities of lithium compounds, containing different proportions of the isotopes lithium-6 and lithium-7, for use by its Research and Development Laboratories at Capenhurst.

Studies on manufacture of electric carbons V.Oxydation of carbons with sulfhric acid-Potassium dichromate mixture.

Conditions of wet oxydation degree determination of carbons with sulfuric acid-potassium dichromate mixture were discussed.Suitable condition was as follows: -0.0500g puiverised carbon samples will be taken in a flask having a reflex condencer.Digested with mixture of sulfuric acid (d20 1.290) 50.0c.c.and K2Cr2O7 5.000g for 60min.at100°C, filtered with a weighed glass filter, washed throghly with distilled water, dried at 105°C to constant weight and then weighed.The increase of the filter considered as residual part of carbon of the sample.As the growing of graphite crystals of carbon articles, decrement of wet oxydation degfee was observed.The variation of wet oxydation degree have mutual resemblance to the variation of the carbon crystal lattice constant ratio c/a.The oxydation of carbon with sulfuric acid-potassium dichromate mixture occurs only from surface of carbon particles.Oxydation by penetration of the agents between (ool) layer of carbon crystals could not considered.