Showing papers on "Lattice constant published in 1972"

Structure and Phase Transitions of Solid Heavy Methane (CD4)

Abstract: The orientational structures and the phase transitions (27.0 and 22.1°K) of solid CD4 are investigated by coherent neutron scattering from powder samples as well as from single crystals. In its high temperature phase, CD4 is found to crystallize within the space group Fm3m with four molecules per unit cell and a lattice constant a0=5.96 A at 77°K. The molecular orientations are completely disordered at 35°K, while an indication of partial order was detected at 77°K. The transition to phase II is accompanied by critical fluctuations with a correlation length of about 24 A (at 27.7°K) and appears to be of second order (critical exponent β =0.4± 0.1. In phase II six of eight molecules order with a local symmetry 42m, while the remaining two are orientationally disordered. The space group is cubic Fm3c with 32 molecules per unit cell and a0=11.64 A at 24.5°K. The transition to phase III is predominantly of first order. With the present data the structure of phase III cannot be determined unambiguously. Superlattice reflections can be indexed cubic primitive. The number of molecules per unit cell remains 32, with a0=11.61 A at 17.5°K. The data suggest, that the low temperature structure is to be understood by an ordering within the sublattices of molecules disordered in phase II together with slight distortions of the other sublattices. The structures of phases I and II agree with the ones predicted by James and Keenan under the assumption of octupole—octupole interaction between nearest neighbour molecules. For phase III the model of James and Keenan, as well as other predictions, hitherto published, must be ruled out.

Further studies on the nickel–aluminium system. I. β‐NiAl and δ‐Ni2Al3 phase fields

Abstract: New lattice parameter and density results have been obtained for alloys in the β-NiAl and 6-Ni2Al3 phase fields of the nickel–aluminium system. The lattice parameter of the β-NiAl phase (CsCl-type) falls linearly from 2.8870 A at 50 at.% Ni to 2.8618 A at 66. at.% Ni, with 2.00 atoms per unit cell. On the other hand, the lattice parameter on the Al-rich side of NiAl falls linearly from 2.8870 A to 2.8652 A, while the number of atoms per unit cell falls from 2.00 to 1.817 by the creation of vacancies in normally nickel sites. The trigonal 6-Ni2Al3 phase-structure, which is essentially an extension of cubic β-NiAl, but with every third plane of nickel atoms perpendicular to the trigonal axis missing, shows a minimum in the a and c spacings at stoichiometric Ni2Al3. Density measurements indicate that the vacancies formed by the missing planes are progressively filled as nickel is added to Ni2Al3, but that a substitutional solid solution is formed on the aluminium-rich side of stoichiometric Ni2Al3 with aluminium replacing nickel atom by atom. In the NiAl phase, the number of valence electrons increases from 2.28 per unit cell at 61.9 at.% Ni to 3.00 at stoichiometric NiAl, and remains constant at 3.00 as the vacancies form until Ni2Al3 is reached, at which stage the number of vacancies will have reached a maximum when there are only 1.67 atoms per pseudo-cubic cell. The number of electrons per pseudo-cubic unit cell then begins to rise and reaches the phase boundary value of 3.12 at 37.6 at.% Ni.

Optical waveguides of single‐crystal garnet films

Abstract: We report light‐wave propagation experiments in single‐crystal epitaxially grown garnet films. The discussion includes refractive index and lattice constant considerations for various garnets, and also the use of iron garnet films on gallium garnet substrates as magneto‐optical waveguides useful in integrated optics.

Measurement of the Lattice Parameter of Wustite at High Temperatures

Abstract: The lattice parameter of wustite (Fe2O) single crystals was measured at high temperatures. Plots of both lattice parameter vs composition and lattice parameter vs temperature were linear, and no appreciable changes in slope were detected at the boundaries of the subphases suggested by Vallet and Raccah.

Magnetic susceptibility and expansion coefficient of the intermetallic compounds YbAl2 and YbAl3

Abstract: Magnetic susceptibilities and lattice constants of the intermetallic compounds YbAl2 and YbAl3 have been measured in the temperature range −180 °C to 550 °C. YbAl3 shows a behavior closely resembling that of an isomorphous trivalent REAl3 compound but YbAl2 appears to contain a percentage of YbIII which increases with temperature.

On the formation of solid solution in Co3-xMnxO4 system

Abstract: The formation of solid solution in the Co3−xMnxO4 system in atmospheres of oxygen, air and argon was examined at a constant temperature of 1000 °C. In oxygen, a small amount of the NaCl-type compound was found to co-exist with the cubic spinel in the composition rangex≤0.1. A single phase of the cubic spinel was found in the range 0.1 to 1.3 and the tetragonal spinel above 1.9. In the rangex=1.3 to 1.9 where the cubic and tetragonal spinels co-exist they both have very broadened diffraction line profiles. In air, the identified phases and the changes in their lattice constants with composition were very similar to those in oxygen, except that the NaCl-type compound and the cubic spinel co-existed over a larger range. In argon, the cubic spinel was not observed over any of the composition range and the NaCI-type compound and the tetragonal spinel co-existed in the wide range of 1.1 to 2.3. The experimental results are discussed with regard to the cation distribution in the spinel and also to the relative stability of Co3+- and Mn3+-ions under the low partial pressure of oxygen.

Liquid phase epitaxial growth of Ga x In 1-x P

Abstract: Single crystal epitaxial layers of Gaxln1−xP alloys have been grown by the steady-state liquid phase epitaxial growth technique on (111)B GaAs substrates. The crystal growth process has been studied in detail and the resultant epitaxial layers have been characterized with respect to their structural, electrical and optical properties. Epitaxial layers of good structural quality could be grown only in the composition range x = 0.48 to 0.53, where the lattice parameter is close to that of the GaAs substrate. The band gap of these crystals was in the range 1.86 – 1.92 eV as determined by optical absorption and photoluminescence measurements.

Self-Consistent Energy Bands in Vanadium at Normal and Reduced Lattice Spacings

Abstract: Self-consistent energy-band calculations for vanadium were performed by the augmented-plane-wave (APW) method, for different values of the statistical exchange parameter $\ensuremath{\alpha}$, at normal and reduced lattice constants. Comparisons have been made, with Fermi-surface, soft-x-ray, photoemission, and electronic specific-heat experiments, and reasonable agreement was found.

Superlattice of voids in neutron‐irradiated tungsten

Abstract: Transmission electron microscopy of tungsten irradiated at about 550°C to a fast neutron fluence of approximately 1×1022 neutrons cm−2 reveal the formation of a superlattice of voids. The void lattice parameter is 195 A and the lattice parameter to average void size is about six. Both the formation of the superlattice and the temperature dependence of the lattice parameter in tungsten, a material considered to have isotropic elastic properties, is in contradiction to present theories based on the elastic anisotropy of the host metal.

Molecular packing analysis

Abstract: A general method of molecular packing analysis based on the minimization of the crystal lattice energy is described. The lattice energy was approximated by a pairwise sum over nonbonded atoms in different molecules, using (exp–6–1) potential functions. Increased speed of calculation of the lattice sums was achieved by a convergence acceleration technique. The variables considered were six rigid body rotations and translations for each molecule in the asymmetric unit, and the six lattice constants. Molecular flexibility was allowed in the form of internal rotations about bonds (subrotations). In this event additional subrotation potentials of the cos2 ψ type could be used to allow for conjugation energy, and the subrotation angles were additional variables. An optional thermal correction, based on the mean square thermal amplitudes and the potential anharmonicities, was applied to calculate the anisotropic thermal expansion. Nonbonded potential parameters, including conjugation energies, were found by fitting them to observed crystal structures.

Disorder-Activated Acoustic Mode in Raman Spectrum of Ga x Al 1-x As

Abstract: We report the observation of an acoustic phonon mode, usually not Raman active, which is interpreted here as a disorder-activated longitudinal acoustic mode. Because of good matching of lattice constants and nearest-neighbor force constants in the alloy system ${\mathrm{Ga}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$, a simple theory has been successfully applied to obtain information on the electronic states.

Long-range order and ordering kinetics in CoPt 3

Abstract: The Co-Pt system is well suited for studies of order-disorder by field-ion microscopy. Because of this, and the fact that detailed information on ordering is available for only one Ll2 alloy (Cu3Au), an extensive X-ray study was made of CoPt3. Long-range order falls to lower values (S ≈ 0.64) in this alloy than in Cu3Au. There is a two-phase field nearTc (685°C) extending for about 20°C. The ordered phase has a smaller lattice parameter than the disordered phase close toTc, but at room temperature the reverse is true. Ordering (when the domain size is large) follows Rothstein’s kinetic theory, with an activation energy of 74.0(9) kcal per g-atom. Domain growth is similar to grain growth, with a time exponent of about 0.45 and an average activation energy of about 62 kcal per g-atom. The antiphase domain structure is isotropic, in contrast to Cu3Au. There is little difference in atomic volume of cobalt and platinum in the ordered state as compared to the pure elements.

Solubility of Oxygen in ZrC

Abstract: Zirconium carbide specimens containing significant quantities of dissolved oxygen were synthesized using ZrO2 and graphite as reactants in an atmosphere of CO. These specimens were studied by X-ray diffraction and analyzed chemically for C, O, and Zr. The data indicate that oxygen substitutes in the ZrC lattice, replacing more than 1 C atom/O atom, thus decreasing the lattice parameter. Under the conditions of the study, the reaction product is a solution of oxygen in the ZrC phase rather than a distinct oxycarbide phase.

The crystal structure and magnetic and electrical properties of β‐US2

Abstract: The crystal structure of β-US2 was refined using single crystal X-ray diffraction data. The lattice constants at room temperature are: a = 8.4803 A, b = 7.1139 A, and c = 4.1205 A. The uranium atom is coordinated by nine sulphur atoms: two at a distance of 2.713 A, two at 2.896 A, two at 3.141 A, one at 2.747 A, one at 2.826 A, and one at 2.888 A. The magnetic measurements were carried out in the temperature range 4.2 to 1000 °K. The increase of the effective magnetic moment from 3.08 to 3.28 BM with temperature is explained on the assumption that the ground state is composed of four closely spaced singlets. The semiconducting character in the temperature range 4.2 to 650 °K was established with a forbidden gap ΔE≈1.2 eV. [Russian text Ignored].

Investigation of Ferrimagnetic Cobalt‐Doped Gamma‐Ferric Oxide Micropowders

Abstract: The magnetic properties of cobalt‐doped γ‐Fe2O3 were investigated, and the crystallographic properties were examined using the Mossbauer technique and x‐ray diffraction. An increase in lattice parameter with cobalt concentration was observed. A large increase in coercive force, especially pronounced at low temperatures, was proportional to cobalt concentration, and may be accounted for by single‐ion anisotropy. The magnetic moment changed very little. Measurements on partially aligned particles indicated that the particles were single domain. The Mossbauer spectra provided no evidence for the presence of Fe2+ or Fe4+ ions. Also, at helium temperature the anisotropy was such that the samples were not saturated at 50 kOe. The experimental data are consistent with the hypothesis that the cobalt ions replace iron on B sites and also fill B‐site vacancies. Conversion to α‐Fe2O3 on heating was never attained completely, and it is proposed that cobalt ferrite is present in the heated samples.

Superconductivity in the Nb-rich Nb--Al alloys

Abstract: Nb‐rich Nb–Al alloys were prepared by both sputtering and quenching from the liquid state. Lattice constants and superconducting transition temperatures were measured and compared for as‐quenched and as‐sputtered samples and also for the same samples after annealing. For the samples with between 75 and 80 at.% Nb, annealing produced nearly single‐phased β‐W alloys with the highest transition temperatures of 18.6 °K for quenched samples and 16.6 °K for sputtered samples.

Theory of the void lattice in molybdenum

Abstract: A model is proposed for the study of various physical properties associated with the void lattices as recently observed in several metals. The model gives the correct value for the equilibrium void lattice spacing in molybdenum and also explains the stability of the void lattice.

Fourier Representation Methods for Electronic Structures of Linear Polymers

Abstract: The Fourier representation method described in the previous paper of this series is used to make electronic structure calculations for a linear chain of equally spaced hydrogen atoms. The electronic wavefunction is assumed to be a determinant of doubly-occupied crystal orbitals of modulated-plane-wave type, built from one 1s Slater-type orbital of screening parameter ζ centered on each atom. The energy is calculated from the electrostatic zero-order Hamiltonian with exact evaluation of all Coulomb and exchange contributions, and is optimized with respect to the lattice spacing and ζ value. Good agreement with work by others is noted, indicating a near-equivalence of modulated-plane-wave and tight-binding wavefunctions for this half-filled-valence-band system. The linear chain is calculated to be far more stable than cubic three-dimensional hydrogen crystals. This fact sheds light on the unusually large calculated nearest-neighbor distances in the cubic crystals, and is related to a suggestion that under certain conditions the most stable structure for solid atomic hydrogen may be of lower symmetry than cubic.

The iron-palladium-hydrogen alloy system. (Mossbauer studies)

Abstract: Using the results of previous workers a phase diagram has been constructed for the AgPdH alloy system showing the hydrogen to metal atomic ratio as a function of Ag concentration. From the results of X-ray and Mossbauer studies reported earlier by the authors, an analogous phase diagram has been obtained for the FePdH system. In each case, the limits of stability of the alpha (low hydrogen content) and beta (high hydrogen content) phases are shown. Further analysis of Mossbauer spectra has determined the variation with temperature of hyperfine field at Fe57 nuclei for various concentrations of iron in alpha and beta phase hydrides. The saturation fields for alpha max and beta min iron-palladium hydrides are identical to within +or-5 kG at a given iron concentration. The observed Fe57 isomer shift difference between alpha and beta phase hydrides is shown to result entirely from changes in lattice parameter. An extension of the model for Fe impurities in a Pd lattice is proposed to interpret these effects. On introducing an iron atom into palladium, rearrangement of band structure occurs which localizes 4d(eg) type magnetized d states on nearest and next nearest Pd atoms. Polarization of the band t2g d states of Pd occurs to large distances via a RKKY mechanism. In the hydrides, the t2g moment is quenched but the localized eg moment is unchanged except at hydrogen concentrations beyond the normal solubility limit.

Crystal Structure of the Solid Electrolyte(C5H5NH) Ag5I6 at -30°C

Abstract: The crystal structure of pyridinium hexaiodopentaargentate, (C5H5NH) Ag5l6, is unique among those of the halide and chalcogenide solid electrolytes in that face-sharing iodide octahedra as well as face-sharing tetrahedra and face-sharing between octahedra and tetrahedra provide the paths for silver ion transport. There are two formula units in a hexagonal cell, space group P6/mcc (D6h2). At -30°C, the lattice constants are a = 11.97 ± 0.02, c = 7.41 ± 0.01 A. The structure has three sets of sites for the silver ions. At -30°C two of these sets are apparently filled with the ten silver ions per unit cell, while the third set of tetrahedrally coordinated general positions is empty. Therefore, the conductivity at this temperature is limited by the thermal excitation of the silver ions into the empty tetrahedra.