Showing papers on "Lattice constant published in 1970"

Martensitic transformations in gold-titanium, palladium-titanium and platinum-titanium alloys near the equiatomic composition

Abstract: Alloys of compositions near AuTi, PdTi, and PtTi are B19-type orthorhombic at room temperature and, at higher temperatures, have an ordered CsCl-type structure (B2-type). The martensitic transformations involved were studied by X-ray diffraction. The composition dependence of the martensitic-transformation characteristics, such as Ms, Mf, As and Af-temperatures, were determined. Lattice constants of the high-temperature phases are given.

Lattice Dynamics of Alkali Metals in the Self-Consistent Screening Theory

Abstract: The self-consistent treatment of correlations in the electron liquid recently given by Singwi et al. is applied to calculations of the lattice dynamics of alkali metals. With the Ashcroft form for the pseudopotential, in which the only parameter is the core radius, good agreement with the measured dispersion curves is obtained for the four metals for which such data exist. For Na and K the fitted values of this parameter are close to those derived from Fermi-surface and liquid-resistivity data; for Li and Rb the fitted values lie between those obtained from the other physical properties. Phonon lifetimes due to the electron-phonon interaction are calculated for K. Finally, the cohesive energy, lattice parameter, and compressibility are derived; agreement with the measured values of all three quantities is obtained with a not unreasonable adjustment of the Hartree energy. The relation between the compressibility sum rule and the long-wavelength limit of terms containing third and fourth powers of the electron-phonon matrix element is evaluated for the case of Na within the same framework.

Lattice parameters of ZnO from 4.2° to 296°K

Abstract: The temperature dependence of the lattice parameters of ZnO was measured by x‐ray powder‐diffraction methods. Retrograde behavior was found to occur perpendicular to the c axis at 93°K. Lattice parameters at 4.2°K were a0=3.24826±0.00015A, c0=5.2033±0.0018 A. These results are compared with those for some other II‐VI compounds.

Magnetic Properties of some Cubic Rare‐Earth‐Iron Compounds of the Type RFe2 and RxY1−xFe2

Abstract: Magnetic measurements in fields up to 30 kOe and for temperatures between 4.2° and 1000°K have been performed for cubic Laves phase compounds RFe2 (R=Gd, Tb, Dy, Ho, Er, and Y) and RxY1−xFe2 (R=Gd, Tb, and Er). The Curie temperatures and saturation moments at 4.2°K are given. A compensation in the temperature dependence of the magnetization is only observed for the various compounds in a certain range of x values. Conditions for the occurrence of a compensation point are derived. For the compounds RxY1−xFe2 the iron moment remains constant for 0≤x≤0.8. Above x=0.8 the iron moment increases strongly. The variation of the Fe moments with x is discussed in terms of a nonlocalized moment associated with the iron ions. The moments shown by Er and Tb in ErFe2 and TbFe2 are both 8.3 μB. For most of the compounds RFe2 the lattice constants are given.

Thermal Expansion of AlAs

Abstract: The lattice parameter of AlAs has been very accurately measured as a function of temperature between 15° and 840°C, and the coefficient of thermal expansion has been determined. The lattice parameter at 0°C is 5.6605±0.0005 A and the coefficient of thermal expansion is (5.20±0.05) ×10−6/°C. A comparison is made to GaAs which indicates that there is a perfect lattice match at about 900°C and 0.14% lattice mismatch at room temperature.

Lithium tantalate single crystal stoichiometry

Abstract: Accurate lattice parameter, hydrostatic density and Curie temperature measurements have been made on lithium tantalate single crystals grown from different melt compositions. Crystal compositions were determined by comparing crystal Curie temperatures with those determined from well reacted ceramic specimens of known composition. These measurements showed that a stoichiometric melt produces nonstoichiometric crystals and that the congruently melting composition is 49.0 mole % Li2O. The resulting defect model for nonstoichiometric lithium tantalate is Li[1−(5−n)x]Ta(1+x)nxe−O3 where n≃2 and the existence region is 46.0 to 50.4 mole % Li2O at room temperature.

Thermal expansion of NaCl, KCl and CsBr by X‐ray diffraction and the law of corresponding states

Abstract: The coefficients of thermal expansion of NaCl, KCl and CsBr are determined accurately at different temperatures using a diffractometer, Geiger counter, chart recorder and a specially designed furnace. Equations are given for the variation of the lattice constants with temperature. The temperature dependence of the thermal expansion at high temperatures is shown to be related to the concentration of thermally generated Schottky defects. The energies of formation of Schottky pairs in the three halides are estimated and are found to be consistent with those deduced from ionic conductivity studies. The reduced parameters α/(α)M/2 and T/TM give a common curve for all the halides, TM being the melting point and (α)M/2 the value of α at T = ½ TM. The curve is a straight line in the limits ~ 0.30 < T/TM < ~ 0.65 and is found to deviate considerably at higher temperatures. Assuming that the deviation is due to defects, the energies of formation of Schottky defects for the varies halides of Li, Na, K, Rb and Cs are estimated and found to agree excellently with the experimental and theoretical values.

Crystal Structure of Luminescent ZnSiP2

Abstract: ZnSiP2 crystallizes with the chalcopyrite structure: there is no detectable difference in x‐ray scattering between luminescent and nonluminescent crystals. The level of impurity, either in the form of chemical dopant, physical defect, or a departure of less than about 1% from complete order at the Zn and Si sites hence determines if the crystal luminesces or not on excitation by an electron beam. The lattice constants of this tetragonal crystal at 298°K are a = 5.399 ± 0.001 and c = 10.435 ± 0.002 A. The space group is I42d, and there are four formulas in the unit cell. A total of 2023 reflections were measured with the luminescent crystal, and 1600 with the nonluminescent crystal, using pexrad. The final agreement factor, based on a combination of 197 symmetry‐independent reflections from the first crystal and 176 from the second, is 0.049. Each Zn is tetrahedrally surrounded by 4 P atoms at 2.375 ± 0.001 A distance, and each Si by 4 P atoms at 2.254 ± 0.001 A. All phosphorus tetrahedra have four P–P ed...

Elasticity of a magnetic lattice near the magnetic critical point

Abstract: The isothermal elastic constants and the coefficient of anomalous thermal expansion of a magnetic lattice are discussed. The spin system is described by the Ising model with an exchange coupling depending on lattice spacing. A behavior of the elastic constants and the coefficient of thermal expansion is found which is in qualitative agreement with experiments. The isothermal compressibility remains positive nearTc and no thermo-mechanical instability occurs (which would lead to a first-order phase transitions), in contrast to earlier theories.

The Preparation and Crystallography of Cadium Zinc Sulfide Solid Solutions

Abstract: Single crystals and powders of were grown using chemical transport with iodine. These components form a continuous series of solutions over the entire range of x, 0 to 1. The wurtzite structure was produced over the entire range. The lattice parameters and the composition of the solid solutions were determined to approximately 1% using x‐ray diffractometry techniques. The results provide a definitive calibration of lattice constants for the wurtzite structure of solid solutions of and and allow the analysis of samples. The variation of lattice parameters with composition was found to obey Vegard's law. The variation of the c/a ratio with composition was also linear. The ideal ratio, assuming close packing of hard spheres, occurred at 70% . The volume relationship with composition showed a slightly negative deviation from linearity. A least squares method for refining lattice parameters is described.

Macro lattice from segregated amorphous phases of a three block copolymer

Abstract: Extruded plugs of the styrene-butadiene-styrene amorphous copolymer Kraton 102 were found to exhibit discrete X-ray reflections in the low angle region such as are characteristic of an individual single crystal. This crystal lattice is hexagonal, with a lateral lattice spacing of 300 A and an ‘infinite’ period along the hexagonal axis which coincides with the extrusion direction. Annealing improved the ‘crystal’ perfection, in fact a large portion of the macroscopic sample could become a ‘single crystal’. The orientation of the lattice could be influenced by deformation procedures in a consistent fashion. The diffraction effects are consistent with the styrene phase being in the form of cylinders. The cylinder diameter was estimated both from the lattice spacing coupled with the known molecular composition, and from missing reflections: both methods gave a diameter of close to 150 A. Some suggestions are made on the molecular origin of this remarkable ordering phenomenon, together with pointing out some of the possible further consequences of such an order.

Preparation of Thin BaTiO3 Films by dc Diode Sputtering

Abstract: Direct current diode sputtering of reduced BaTiO3 ceramic has been applied to the preparation of thin BaTiO3 films. Sputtering was carried out over a range of apparent sputtering rates of ∼500–1200 mg/Ah in atmospheres of argon and air. As the substrate, Pt sheet and fused quartz were principally used. Grain size of the deposited film was ∼1000 A for the Pt substrate at 400°C. The lattice constants were in good agreement with those of ordinary BaTiO3 when the Pt substrate was kept at a temperature above 1000°C, although they were slightly larger for substrates below 900°C. Measurements of optical density and observations of electron micrographs indicated that the sputter‐deposited films have more favorable properties of transparency, uniformity, and crystallization than vacuum‐evaporated films. The film prepared on a Pt substrate at 1000°C in an air atmosphere had a dielectric constant of 1700 and dissipation factor of 1.8% without post‐deposition treatment. However, the remanent polarization was very sma...

The eta carbides in the Fe−W−C and Co−W−C systems

Abstract: The η carbides in Fe−W−C are Fe6W6C with an edge length of the cubic unit cella=10.956 to 10.958A and Fe3W3C witha=11.102 to 11.146A. In Co−W−C the η carbides solidity from the melt as Co6W6C and ∼Co2W4C. Co6W6C retains a narrow homogeneity range down to 1000°C with an observed range of lattice constants between 10.894 and 10.902A. The compositional range of Co2W4C spreads out between Co3W3C and Co2W4C at 1400°C and narrows back down to a small range around Co2W4C at 1000°C. Its lattice constants vary between 11.066 and 11.251A. In Fe-W-C a new hexagonal phase is found at FeW3C. It has the structure reported by Schonberg for Co3W9C4 and its lattice parameters area=c=7.806 to 7.810A.

Partial Molar Volumes from High-Temperature Lattice Parameters of Iron–Carbon Austenites

Abstract: The lattice parameters of a number of iron–carbon alloys have been measured in the austenite field for temperatures up to 1200° C (1473 K). By extrapolation and interpolation of these data the variation with carbon concentration of the lattice parameter of austenite has been determined for a wide range of temperatures, and used to calculate partial molar volumes of iron and carbon in austenite. Both partial molar quantities show little variation with composition but V c, γ decreases with increasing temperature, while V Fe, γ increases. The values obtained for the partial molar volume of carbon in austenite are appreciably lower than those cited earlier. It is probable that the differences previously reported between the measured and calculated effect of pressure on the composition of austenite in equilibrium with cementite at 727° C (1000 K), are due primarily to the use in the calculations of too high a value for the partial molar volume of carbon in austenite.

Ferroelectric Paraelastic Paramagnetic Barium Cobalt Fluoride, BaCoF4, Crystal Structure

Abstract: Barium cobalt fluoride, BaCoF4, ferroelectric and paramagnetic at room temperature, crystallizes in the orthorhombic system with lattice constants a = 5.8519 ± 0.0003, b = 14.628 ± 0.002, and c = 4.2102 ± 0.0003 A at 298°K and space group A21am. The integrated intensities of 1526 reflections within a hemisphere of reciprocal space of radius (sinθ) / λ = 1.02 A−1 were measured with one crystal, and 2986 reflections with a second crystal of BaCoF4, using PEXRAD. The crystal structure was refined by the method of least squares, using 386 symmetry‐independent Fmeas from the first crystal and 875 from the second. The combined set of 1261 Fmeas, together with anisotropic thermal coefficients for all atoms, resulted in a final agreement factor R = 0.0679. The structure is isomorphous with BaMnF4 and contains CoF6 octahedra sharing corners to form puckered sheets parallel to (010). These sheets are linked by Ba2+ ions only. The octahedra are distorted, with Co–F distances ranging from 1.972 to 2.109 A, with an av...

Energy Bands of Hexagonal NiS

Abstract: Energy bands are calculated for hexagonal NiS using lattice constants and crystal potential appropriate to the metallic phase. Group theoretic results for $s$, $p$, and $d$ bands in the NiAs structure are obtained, and the calculation is performed using a first-principles tight-binding method. The energy band structure is characterized by a $3d$ band about 3 eV wide, hybridized with a broad $s\ensuremath{-}p$ band, indicating $s\ensuremath{-}d$-type conductivity. The Fermi energy lies just below the top of the $d$ bands and the density of states exhibits a strong sharp peak about 1 eV below the Fermi energy. Since the energy bands suggest considerable structure in the low-energy region of the reflectivity, selection rules, polarizations, and energies for some of the spectra are presented. The band structure is discussed in terms of the metal-to-semiconductor transition which has been observed for hexagonal NiS.

Lattice Dynamics of Wurtzite: CdS. II

Abstract: A new model for the lattice dynamics of semi-ionic compounds is presented and applied to the computation of phonon dispersion in CdS. The calculated two-phonon density of states as a function of frequency is in agreement with measured infrared absorption in the two-phonon region. Improved values of the microscopic dielectric, elastic, and piezoelectric coefficients result from a self-consistent least-squares fit of model parameters. The model includes valence-band forces, rigid-ion Coulomb forces, and electronic and ionic polarization; it may be generally applicable to other semi-ionic II-VI compounds.

Formation and magnetic properties of ultrafine spinel ferrites

Abstract: Investigations of the formation and magnetic properties of Mn δ Co 1-δ Fe 2 O 4 formed in alkaline aqueous solution by the coprecipitation method are covered. The lattice constant of these ferrites apparently reduced with decreasing average particle size below about 500 A. The coercive force ratio of MnFe 2 O 4 and Co-Fe 2 O 4 is 1 to 50 and equal to that of the crystal magnetic anisotropy coefficient K 1 of sintered ferrites. As the average particle size became larger, the coercive force and the apparent remanence increased linearly, and superparamagnetic critical size was determined. The sizes range, for example, from 50 to 100 A for CoFe 2 O 4 and from 150 to 200 A. for MnFe 2 O 4 . From the correlation between the average particle size and their magnetization, the limit size of ferromagnetic critical particle was also determined. Results obtained in this experiment are approximately in accord with Neel's Version of the theory on thermal fluctuation aftereffect of magnetic fine particles.