Showing papers on "Lattice constant published in 1983"

Formation and coupling of magnetic moments in Heusler alloys

Abstract: The microscopic mechanisms responsible for both the formation and coupling of magnetic moments in Heusler alloys (${X}_{2}\mathrm{Mn}Y$) are identified. We find that the $X$ atoms (e.g., Cu, Pd) serve primarily to determine the lattice constant, while the $Y$ atoms (e.g., Al, In, Sb) mediate the interaction between the $\mathrm{Mn}d$ states. There is no significant direct interaction between the Mn atoms, but the occupied $d$ states of Mn are delocalized by their strong interaction with the $X$-atom $d$ states. The localized character of the magnetization results from the exclusion of minority-spin (defined locally) electrons from the $\mathrm{Mn}3d$ shell. The coupling between the localized magnetic Mn moments can be described with the Heisenberg Hamiltonian and the sign of the exchange constants results from a competition between the intra-atomic magnetic energy and interatomic $Y$-atom mediated covalent interactions between the the $\mathrm{Mn}d$ states. These effects compete because the covalent mechanism is possible only for antiferromagnetic alignments, but necessarily reduces the magnitude of the local moments. The sensitive dependence of magnetic order on the occupation of the mediating $p\ensuremath{-}d$ hybrid states accounts well for experiments by Webster in which this occupation is varied by alloying. Our analysis is based on self-consistent, spin-polarized energy-band calculations for ${\mathrm{Co}}_{2}$MnAl, ${\mathrm{Co}}_{2}$MnSn, ${\mathrm{Ni}}_{2}$MnSn, ${\mathrm{Cu}}_{2}$MnAl, ${\mathrm{Cu}}_{2}$MnSn, ${\mathrm{Pd}}_{2}$MnIn, ${\mathrm{Pd}}_{2}$MnSn, and ${\mathrm{Pd}}_{2}$MnSb, for both ferromagnetic and antiferromagnetic spin alignments.

Mechanics of Fracture

Abstract: Calculations of the theoretical strengths of crystalline solids are usually based on idealised forms of atomic force-displacement curves, in which the force is defined as the differential with respect to distance of the inter-atomic or inter-ionic energy. The energy curve is similar to that for a diatomic molecule in that it represents the resultant of inter-atomic repulsions and attractions; the nature and strength of the attractive forces depending on the bond type: ionic, covalent, metallic, or Van der Waals. Differences in character between a lattice and a molecule occur at separations of order one lattice spacing, when Friedel oscillations in the screening charge cause the long range component of the interaction potential in the lattice to undergo a damped oscillation about zero. For small displacements, the atomic force/displacement curve is linear, having a slope equivalent to Young’s modulus, E. A lattice also has shear stiffness, denoted by the shear modulus μ. Both E and μ are defined macroscopically, usually for randomly-oriented polycrystals which are assumed to be isotropic. In single crystals, both the tensile stiffness and the shear stiffness vary with the orientation of the crystal with respect to the tensile axis and these variations can be substantial: in iron, for example, the minimum value of E is in the [100]direction (132 GPa at room temperature) and the maximum value is in the [111] direction (260 GPa).

First-principles calculation of the equilibrium ground-state properties of transition metals: Applications to Nb and Mo

Abstract: We have used a self-consistent pseudopotential method to calculate the equilibrium ground-state properties of the transition metals Mo and Nb. From our calculations we obtain equilibrium lattice constants, cohesive energies, and bulk moduli which are in excellent agreement with experiment.

Dependence on volume of the phonon frequencies and the ir effective charges of several III-V semiconductors

Abstract: The mode Gr\"uneisen parameters of the LO and TO Raman phonons of AlN, BN, and BP, and the dependence of ${e}_{T}^{*}$ on lattice constant have been measured by Raman scattering in a diamond anvil cell. The results for ${e}_{T}^{*}$ are interpreted by means of pseudopotential calculations of ${e}_{T}^{*}$ versus lattice constant.

Characterization of thin layers on perfect crystals with a multipurpose high resolution x‐ray diffractometer

Abstract: A high resolution x‐ray diffractometer is described in which a compact four‐crystal monochromator produces a highly parallel and monochromatic incident beam of tuneable wavelength (divergence Δθ=5″, wavelength band Δλ/λ=2.3×10−5, intensity 3×104 photons/s mm2). The instrument is very suitable for the nondestructive measurement of concentration depth profiles produced in semiconductor single crystals by epitaxy, diffusion, or implantation. The equipment is useful for measuring x‐ray rocking curves of any lattice plane of any material and also lattice constants can be determined on absolute scale using the Bond procedure. A special solution has been derived from the dynamical theory of x‐ray diffraction, which gives the reflectivity of an absorbing layer as a function of layer thickness. This new expression takes the effects of absorption and extinction into account and has been used to determine the layer thickness of epitaxial layers grown on [001] oriented InP.

In x Ga 1-x As-In y Ga 1-y As strained-layer superlattices: A proposal for useful, new electronic materials

Abstract: Strained-layer superlattices grown from lattice-mismatched layers of InGaAs are proposed as useful electronic materials with tailorable electronic properties. The first study of the electronic properties of these structures has been carried out. Band-gap energies are calculated as a function of layer thicknesses and compositions. The results demonstrate for the first time that the lattice constant, band gap, and transport properties of ternary strained-layer superlattices can be independently varied. This new capability could permit InGaAs strained-layer superlattices to significantly broaden the range of applications associated with the InGaAs ternary compound.

Lattice-parameter dependence of ferromagnetism in bcc and fcc iron

Abstract: A series of self-consistent-band calculations have been performed for a ferromagnetic state of iron in both the bcc and fcc structures. The dependence of the magnetic moment per atom on the lattice parameter was obtained. The linear combination of Gaussian orbitals method was employed in conjunction with a local-density exchange-correlation potential. An explanation is given for the abrupt increase in the magnetic moment in the fcc case near ${r}_{s}=2.7$ in terms of behavior of the band structure.

Intermediate valence state of cerium in CeNi

Abstract: Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3 , an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.

Properties of NbN thin films deposited on ambient temperature substrates

Abstract: The preparation of cubic NbN films by reactive dc magnetron sputtering is described. These superconductive films are deposited at a sufficiently low temperature (<90 °C) that photoresist liftoff techniques and can be used in fabricating Josephson junctions. The superconducting transition temperature has been measured as a function of gas composition and pressure. It reaches a maximum of 14.2 K at 15% N2–85% Ar and 1.06 Pa total pressure. The resistivity ratio of these films is close to unity. Structural studies by transmission electron microscopy and electron diffraction show that films 100‐nm thick or less are randomly oriented, and noncolumnar with a crystallite diameter of 5 nm and a lattice parameter of 4.46 A, which is significantly higher than the bulk value for cubic NbN. The films are dense with void diameters no larger than 0.7 nm. Films 300‐400 nm thick show a small degree of texturing in x‐ray studies with a Read camera. Auger analysis shows a monotonic increase in the N/Nb ratio with increase in the N2/Ar ratio in the sputtering ambient up to 30% N2. From 30% to 50% N2 in the sputtering mixture the N/Nb ratio is constant. Small amounts of carbon impurity are found in all films.

Curie temperature and lattice constant of oxidized titanomagnetite

Abstract: Summary. Low-temperature oxidation of titanomagnetites (x = 0.3, 0.5 and 0.7) was investigated in this study. Starting materials were carefully prepared so that their Curie points and lattice constants were in agreement with reliable values. Changes in Curie points and lattice constants during the process of low-temperature oxidation were then studied in detail. Independent heatings were performed under various conditions and oxidation parameters (z) were determined by chemjcal analyses. Though this method could not provide continuous z values like the weight increase method, reproducibility was very high when samples were heated in the same condition and the various states were consistent with each other. Oxidation states from z = 0.0 to 1.0 were realized in three compositions. When titanomagnetite in basalt was heated under various conditions a change occurred, either oxidation, decomposition or reduction. Natural titanomagnetite minerals closely followed the oxidation-reduction trends established for synthetic samples. The present results are consistent with those of Readman & O’Reilly with some exceptions. The differences may have been caused by the differences in starting materials. The results of Keefer & Shive are similar to those of Ozima & Sakamoto and are completely different from ours. They detected a rhombohedral phase and indicated that the decomposition process proceeds in the same manner reported by Ozima & Sakamoto. We are unable to account for their results. It may be useful to consider the results of Ozima & Sakamoto as a decomposition process. This idea has not been presented previously in the literature.

Thermoelectric Determination of Cation Distributions in

Abstract: Thermoelectric coefficient measurements in the system Fe3O4-Fe2TiO4 were used to determine the cation distributions from 600° to 1300°C. The cation distributions obeyed the relation -RT In K=-9.61+2(-0.0154T)b+0.0256T (kJ/mol) where KFecd is the iron distribution constant and b is the tetrahedral Fe3+ concentration. Low-temperature extrapolations agreed with experimental magnetometry and lattice parameter results. With the incorporation of a subregular size-mismatch term, agreement with high-temperature thermodynamic data was achieved. A solvus at intermediate temperatures was correctly predicted by the model.

On precipitation in rapidly solidified aluminium-silicon alloys

Abstract: The precipitation of silicon in rapidly solidified AlSi alloys was studied. For alloys with 2.4 and 11.0 wt % Si (2.3 and 10.3 at % Si, respectively) the lattice parameters of the Alrich and of the Si-rich phases were measured after ageing at 397,425 and 448 K. For alloys with 2.6 and 13.0 wt % Si crystallite sizes and lattice strains were determined by analysis of the X-ray diffraction line broadening. After ageing the lattice parameters of the Al-rich and the Si-rich phases were influenced by the difference in thermal expansion between both phases. After correction for this effect the amount of silicon dissolved in the Al-rich phase was estimated as a function of ageing time. Quenched-in (excess) vacancies influenced the precipitation kinetics. Activation energies for precipitation appeared to depend on the extent of transformation. Further, quenched-in vacancies caused anomalous maxima in the lattice parameter curves. The behaviour of the lattice microstrains on ageing was explained as a result of the disappearance of stresses due to quenching and the introduction and subsequent dissipation of stresses due to precipitation. After completed precipitation stresses due to the difference in thermal expansion between both phases still exist at room temperature.

Ternary alkaline earth and rare earth metal palladium phosphides with ThCr2Si2- and La6Ni6P17-type structures

Abstract: The new compounds APd2P2 (A  Ca, Sr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb), La6Pd6P17 and Ce6Pd6P17 were prepared by reacting the constituent elements in silica tubes. Their Guinier powder patterns show that they crystallize with ThCr2Si2- and La6Ni6P17-type structures. Their lattice constants are reported. A comparison of the cell volumes suggests that europium and ytterbium have intermediate valence in their respective compounds.

Investigations of the magnetic properties of CeRh2Si2 using lattice parameter, LIII edge and extended X-ray absorption fine structure measurements

Abstract: The lattice parameters of tetragonal CeRh2Si2 are anomalous compared with those of the other members of the series CeM2Si2 (M ≡ Ru, Rh, Pd and Ag). Measurements of the susceptibility of CeRh2Si2 as a function of temperature in the range 15–300 K reveal interesting features. At high temperatures the susceptibility obeys a Curie-Weiss law with an effective moment per cerium ion that is 20% higher than that of the free Ce3+ ion. There is a cusp-like peak at about 37 K and a relatively smaller peak at 5.2 K. The lattice parameters a and c show a slight decrease between 300 and 40 K and a rather small increase between 40 and 12 K. The LIII absorption edge and the extended X-ray absorption fine structure spectra are unchanged between 20 and 300 K. This clearly indicates that the valence of the cerium ion does not change and that the first-nearest-neighbour environment (eight rhodium atoms and eight silicon atoms) of the cerium ion is not affected by the transition at 37 K.

Cohesive Properties of Alkali Halides and Simple Oxides in the Local-Density Formalism

Abstract: The standard self-consistent KKR band calculations are carried out to five alkali halides LiF, NaF, LiCl, NaCl and KCl, three oxides MgO, CaO and BeO and a sulfide CaS within the limit of the local density approximation. The equilibrium lattice constant, the total energy, the bulk modulus and the pressure-volume relation are evaluated by the knowledge of the one-electron energy and the wave functions at several lattice parameters.