Hall effect studies in ytterbium through the f.c.c.-h.c.p. transformation

The Hall coefficient (R_H) of polycrystalline Yb showing the f.c.c.-h.c.p. transformation has been determined at 20 koe in the range 1.8–420°K. Both the structural and the associated magnetic transformation would separately be expected to modify R_H, but the evidence suggests that the observed behaviour through the transition arises primarily from the effects of the change in structure. A more detailed interpretation is presently not possible, however, because of the lack of knowledge of the band structure of h.c.p. Yb.     

Pressure dependence of the Néel temperature of f.c.c. stainless steel

The Néel temperature T_N of f.c.c. stainless steel has been measured as a function of pressure using the Mössbauer effect. A sample with approximately 74 wt.% Fe, doped with ^57Co, and having T_N = 58.4 ± 0.1 K at zero pressure, shows T_N rising with pressure with increasing slope. Our results indicate that T_N is raised to room temperature by a pressure of 175 ± 5 kbar.     

The effect of hydrogen on the superconducting and structural properties of b.c.c. Nb-Ru alloys

The superconducting transition temperature (T_c) has been measured before and after the introduction of hydrogen into Nb_(1?x)Ru_x$\text{(0.20?x?0.33)}$. In all cases, the presence of appreciable amounts of this interstitial component led to a sharp increase in the T_c. All the evidence suggests that conversion of the host metal lattice to f.c.c. is necessary for the appearance of the elevated T_c.     

Concentration and temperature dependence of asphericity of 3d-electrons in f.c.c. Co-Ni and Co-Fe alloys

The asphericity of 3d-electrons was measured as a function of temperature in some f.c.c. ferromagnetic Co alloys by means of the polarized neutron technique. The asphericity, as measured by the population of the e_g and t_2g sub-levels, was found to be almost independent of temperature. Simple models failed to explain the data.     

A re-examination of the induction (crystal field) energy contribution to the relative stabilities of the f.c.c. and h.c.p. rare-gas crystal structures

A new calculation of the induction (crystal field) energy of the f.c.c. and h.c.p. structures of the rare-gas crystals of neon and argon gives an energy in favour of f.c.c. which is more than sufficient to counter the summed pair-potential which favours h.c.p. Predicted values of (E _f.c.c. - E _h.c.p.)/E _f.c.c. for neon and argon are 0·9 × 10^-3 and 1·3 × 10^-3 respectively. These values are similar to those that have been obtained for a modified dispersion energy and it is concluded that they should be added to these dispersion energies. Calculations on solid helium show a negligible difference in the induction energy for the two close-packed structures.     

B.c.c.-f.c.c. allotropy, F-bands,and metallization in xenon and krypton

APW-Xα calculations of a f.c.c.-b.c.c. phase transition in Xe prior to metallization give P ? 950 kbar, contrary to the 820 kbar result of Hama and Matsui; their use of non-touching APW spheres (f.c.c.) may be the crucial difference. Hartree-Fock plus correlation calculations in Kr indicate that f-bands play no role in rare gas crystal metallization.     

Mean-square atomic displacements in f.c.c. crystals

The mean-square displacements of volume atoms, surface atoms and atoms near a vacancy are calculated for f.c.c. metals and rare gas solids, using a simple series expansion of the Wallis formula. We employ a model with rotationally invariant bond bending forces, unlike the de Launay model used by Masri and Dobrzynski. For atoms near a vacancy and for rare gas solids this method has been applied for the first time. The calculations are done to the R⁴ term in the expansion. The inclusion of the R⁴ term together with temperature-dependent elastic constants give results which are in better agreement with experiment. For rare gas solids Lindemann ratios calculated by this method agree closely with the values obtained from elaborate computations as well as those deduced from entropy data. The present calculation also supports the view that the melting of solids is initiated at the crystal surfaces and in the neighbourhood of vacancies.     

Observation of occupied 5d states in f.c.c. Yb metal

We propose experimental evidences for the sd character of the occupied band states of f.c.c. Yb metal. Photoionization cross-section σ¹ in the range 24–65 eV, covering the 5p core electron energies have been measured for both Yb (4ƒ¹⁴6s²5d⁰) and Lu (4ƒ¹⁴6s²5d¹). Both the hv dependence of σ¹ for the Yb valence band and the presence of photoemission resonances above the 5p edges of Lu and Yb indicate the presence of occupied 5d states at the top of the valence band of solid Yb. The Yb 4ƒ σ¹ is presented in order to distinguish the 5d band resonances from the 5p-5d giant dipole autoionization decays. L_{2, 3} X-ray absorption white lines for Yb provide a measure of the 5d bandwidth.     

A.c. conductivity in AsF5 doped polyphenylacetylene (PPA)

The a.c. conductivity of semiconducting cis-cisoid polyphenylacetylene (PPA) pellets has been studied at temperatures between 230 and 290 K and frequencies, ?, from 37 to 10⁵ Hz. The a.c. conductivity (σ_a.c.) is found to be strongly temperature dependent. σ_a.c. is proportional to ?^s with s varying from 0.45 to 0.75 as temperature is raised from 230 to 290 K. Both frequency dispersion and strong temperature dependence of σ_a.c. are best explained by the mechanism of hopping conduction in the band-tails.     

The density of states of b.c.c. Nb3Au: A comparison with A-15 Nb3Au

When Nb₃Au is converted from the A-15 structure to the b.c.c. allotrope, a sharp decrease in the superconducting transition temperature, T_c, occurs (11.1 to 1.5 K). To study possible causes for this sharp decrease, specific heat measurements have been made on the b.c.c. form of Nb₃Au yielding values of γ=19.2 mJ/mole-K², β=0.274 mJ/mole-K⁴ and θ_D=305 K. These parameters compared with published values for the A-15 Nb₃Au indicate that a sharp drop in the density of states at the Fermi level, N(0), occurs when the A-15 structure is converted to the b.c.c. form.     

Single crystal neutron inelastic scattering measurement of stiffness constant in disordered f.c.c. Ni-Mn alloys

Inelastic neutron scattering measurement of stiffness constant D on simple crystals of disordered f.c.c. Ni-Mn alloys has been performed at differen Mn concentrations (1.5%, 6%, 12%, 18% at.Mn). Stiffness constant D show similar behaviour vs concentration as observed in Ni-Fe alloys by Hennion et al. [Solid State Commun.17, 899 (1975)]. This behaviour does not agree with that observed by inelastic small-angle magnetic scattering measurements by Menshikov et al. [Int. J. Magn.1, 91 (1975)]. A noticeable enhancement of the spin-wave width is also observed at higher Mn content and higher spin wave energies. The values of pair exchange integrals are also given.     

Index of refraction and d. c. electrical conductivity in Ge40−xSbxS60 glasses

The spectral dependence of the index of refraction (n) in Ge_40?xSb_xS₆₀ glasses has been described by a simple one-oscillator approximation using the Wemple-DiDomenico treatment (W-D). The possibility of application of the Moss formula or W-D in some amorphous materials for estimation of the optical gap (E _g) fromn or vice versa has been examined. From the results of optical and d.c. electrical conductivity measurements and bond statistics suggestions it can be concluded that many types of defect states are created in Ge_40?xSb_xS₆₀ glasses, which pin the Fermi level in the vicinity ofE _g/2.     

Field dependence of a.c. Electroluminescence in ZnS: Mn,Cu, Cl film panels

Electroluminescent (EL) film panels of ZnS: Mn, Cu, Cl operated by a.c. electric field are studied at room temperature. The emission spectrum consists of a single peak at 590 nm. The EL emittance B varies with frequency f of the applied a.c. electric field as B = B_s ? B_m exp ? f/f_c where B_s, B_m and f_c are constants. This equation indicates that B approaches a saturation value B_s when f?f_c and a linear relation between B and f when f?f_c. At a fixed frequency f, B is found to depend on the applied voltage V as B = A exp [-G/F + V^{$\text{1}\text{2}$})] where A, G and F are constants. This formula is valid at all stages of the operating life of the film panel.     

Ordering behaviors of the F.C.C. and B.C.C. phase alloys in the InMg,LiMg and AlZn systems

Expressions of the band-structure energy and the electrostatic energy characterized by the usual long-range and short-range order parameters are given to a binary alloy of simple metals with the f.c.c.-type or b.c.c.-type structure.The ordering energy and the local ordering energy are calculated for the InMg, LiMg and AlZn systems. The numerical results explain successfully the facts that the InMg system has the Ll₀-type and Ll₂-type ordered phases, each of which exists over a wide range of concentration and that the LiMg system has a local order with a negative short-range order parameter, while the AlZn system has that with the positive one. A lattice distortion in the Ll₀-type ordered structure of InMg is also discussed.     

Intensity of the atomic oxygen lines at 8446 and 7771 A in an Ar-O2 d.c. glow discharge

An equation for the intensity ofO(3³P?3³S) at 8446 A and ofO(3⁵P?3⁵S) at 7771 A has been derived as a function of electron-, Ar- and O₂-densities assuming applicability of the excitation processes proposed by Bennett et al. and by Tunitskii and Cherkasov. The electron temperature has been assumed to be constant. The results are compared with experimental data obtained in the d.c. glow discharge of Ar-O₂ mixtures.     

Density of states and asphericity of electron distribution in ferromagnetic Co-Ni alloys

The coherent potential approximation has been extended in order to treat the case of multiple bands in ferromagnetic substitutional binary alloys. The concentration dependence of total magnetic moments and their aspherical contribution, e_g and t_2g, were derived and compared with experimental data in Co-Ni f.c.c. alloys.     

The enhancement of dark d.c. conductivity by gas adsorption on thin films of macrocyclic copper complexes

The dark d.c. conductance of thin films of copper complexes of phthalocyanine (α-form), hemiporphyrazine and dihydrodibenzo [b, i] [1, 4, 8, 11] tetra-azacyclotetradecine has been studied in vacuo and in the presence of an extensive range of gases. Only those gases which can be classed as electron acceptors increased the conductance, with NO₂/N₂O₄ (“NOX”) being particularly effective. Spectroscopic evidence is provided to show that the NOX-copper phthalocyanine interaction generates the organic radical cation and NO^-₂, and that substantial quantities of gas are accumulated within the thin film. A new class of extrinsic organic semiconductor is described.     

A.c. conductivity and relaxation in LiCoVO4 ceramics

The LiCoVO₄ compound is synthesized by solution-based chemical method. X-ray diffraction analysis exhibits a single phase nature of the compound with cubic structure. The dielectric constant (ε_r), tangent loss (tanδ) and a.c. conductivity (σ_ac) have been studied as a function of frequency and temperature using complex impedance spectroscopy (CIS) technique. The variation of (ε_r and tanδ) with frequency at studied temperatures shows a dispersive behavior at low frequencies. Frequency dependence of σ_ac at different temperatures obeys Jonscher’s universal power law governed by the relation: σ_ac = σ_dc + Aωⁿ, where n is the frequency exponent in the range of 0 ⩽ n ⩽ 1 and A is a constant that depends upon temperature.     

Electronic structure,pairwise interactions and ordering energies in binary f.c.c. transition metal alloys

Effective pair interactions for binary f.c.c. transition metal alloys have been computed using a generalized perturbation method starting from the completely disordered state. Their properties, as a function of interatomic distance, band filling, disorder and concentration are discussed. The ordering energy computed from the pair interactions is succesfully compared, for the Cu₃Autype structure, to the energy derived from the band structure determined by the recursion method.     

Ne(I) spectral lines from a Ne-O2 d.c. glow discharge

Measurements have been made of intensities of the spectral lines emitted from an Ne-O₂ d.c. discharge with small discharge current (1–4 mA) under the following conditions: gas pressures of 2 and 3 torr and oxygen partial pressures (P₀₂) up to 0.1 torr. All of the Ne(I) line intensities observed decrease when O₂ is added. The Ne(I) λ5852 line (1s₂-2p₁) has been studied in detail as a representative example. The population density of the 2p₁ level of neon has been obtained from the intensity measurements as a function of P₀₂. The energy-distribution function of electrons has been determined using Druyvesteyn's method in order to calculate the population density for a corona model. The high-energy tail of the measured distribution function is markedly reduced when O₂ is added. It is shown that inelastic collisions of electrons with O₂ produce large energy losses for the electrons. These cause a decrease in population density of the 2p₁ level when O₂ is added. The population density of the 2p₁ level at a gas pressure of 2 torr is 1.2×10⁴ cm^-3 in pure neon and 5.2×10² cm^-3 in an Ne-O₂ mixture (P₀₂ = 0.01 torr). The electron densities and average electron energies are 3.5×10⁸ cm^-3 and 8.7 eV and 1.7×10⁸ cm^-3 and 5.3 eV, respectively, for the specified two cases.