Showing papers by "Solid State Physics Laboratory published in 1980"

A cellular atomic model for the mossbauer isomer-shift of au-197 in alloys

Abstract: It is demonstrated that the atomic model previously shown to give a semi-quantitative account of the heat of formation of transition metal intermetallic compounds, can also offer a relatively simple interpretation of the isomer shift in metallic alloys. The two main terms in the isomer shift are due to (1) electronic charge transfer between more electropositive and more electronegative metal atoms and (2) the discontinuity in the electron density that exists at the boundary between disimilar atoms if the atoms have similar charge distributions in both alloy and the pure metals. The electron density must become continuous at the cell boundary; this leads to a term in the isomer shift that usually, but not always, opposes the charge transfer term. In solid solutions there is an additional volume (size mismatch) contribution to the isomer shift which, however, is not as important as suggested by some other authors. In addition to experimental results obtained on the isomer shift of solid solutions of Au as an impurity in transition metals hosts, we discuss results on Au-base alloys and gold intermetallic compounds.

Temperature effects in silicon solar cells

Abstract: The temperature variation of the relative spectral response RSR, short circuit current Isc and open circuit voltage Voc is measured and the results are theoretically discussed. The RSR at wavelengths larger than the peak wavelengths always increases with temperature. The observed increase is found to agree with theory when temperature variation of the absorption coefficient of light and of Ln the carrier diffusion length in the base is taken into account properly. The temperature variation of short wavelength RSR depends on S, the surface recombination velocity and L p d , L p is the carrier diffusion length in the diffused layer and d is the junction depth. If S is small and L p d is large, the RSR is practically independent of temperature. On the other hand if S is large and/or L p d is small, the RSR decreases as the temperature increases. A new relation between Ln, d and the peak position of RSR is derived. The observed temperature shift in the peak position agrees well with that predicted by this relation. The temperature increase of Isc and decrease of Voc are different for AM0 and AM1 illuminations. The observed differences can be explained at least qualitatively on the basis of the results obtained in this paper.

EPR study of Ni + and Ni 3 + in x-irradiated Ca F 2

Abstract: EPR measurements of Ca${\mathrm{F}}_{2}$:Ni crystals before and after room-temperature (RT) x irradiation are reported. Depending on the rare-earth impurity content a small concentration of ${\mathrm{Ni}}^{+}$ centers is observed in "as-grown" crystals. During RT x irradiation the ${\mathrm{Ni}}^{+}$ concentration increases. At least two different kinds of ${\mathrm{Ni}}^{+}$ centers are observed in the EPR spectra. One of them has tetragonal symmetry with ${g}_{\ensuremath{\parallel}}=2.569$ and ${g}_{\ensuremath{\perp}}=2.089$ and gives a superhyperfine (SHF) structure, due to the interaction with four equivalent flouride ions, which can be resolved at liquid-nitrogen temperature (LNT). The other one has an almost tetragonal symmetry with $g$ factors and SHF structure very close to the former one but the (SHF) structure is resolved at RT. The first ${\mathrm{Ni}}^{+}$ center is associated with a ${(\mathrm{N}\mathrm{i}{\mathrm{F}}_{4})}^{3\ensuremath{-}}$ molecular ion with the ${\mathrm{Ni}}^{+}$ at a distance of about 0.37 \AA{} from the plane of the fluorine atoms. The other EPR signal is assigned to a similar center with some kind of perturbation. An isotropic EPR signal ($g=2.003$) with a well-resolved superhyperfine structure due to the interaction with eight equivalent fluorines has been detected in some of the samples after RT x irradiation. The tentative model proposed for the center responsible for this signal is a ${\mathrm{Ni}}^{3+}$ ion at a ${\mathrm{Ca}}^{2+}$ substitutional position.

Correlation between amorphous and metastable crystalline alloys

Abstract: The local chemical environments and geometrical arrangement of atoms in metallic and covalent glasses [(Fe, Ni)3B, Zr3Fe, As2Te3] were studied by Mossbauer spectroscopy. It was found that in amorphous transition metal-metalloid and metal-metal alloys the chemical short-range order is similar to that of the (meta)-stable crystalline phases (Fe3B, Ni3B, Zr3Fe). The geometrical arrangement of near neighbours in amorphous As2Te3 is different from those present in the metastable (fcc) and stable (monoclinic) crystalline counterparts.

Transitions in poly(vinylidene fluoride) by thermally stimulated techniques

Abstract: Multiple transitions in poly(vinylidene fluoride)-PVF2 - have been studied by thermally stimulated techniques: Thermostimulated creep -TSCr - and thermostimulated current -TSCu -. The resolving power of these methods have allowed us to investigate the fine structure of transitions in PVF2. A relaxation mode is observed around the glass transition temperature -Tg -; it can be described by a distribution of relaxation times following an Arrhenius equation and it has been attributed to microbrownien motions of the amorphous chains. Another relaxation mode is revealed ∼50° higher than Tg. It corresponds to a distribution of relaxation times obeying a Vogel equation and has been assigned to the fluidification of the amorphous chains. In both cases, two sub-modes can be distinguished:The sub-mode observed at lower temperature has been attributed to the amorphous regions free from constraint, while the other one at higher temperature, has been assigned to amorphous chains under constraints from crystall...

Structure of Liquid Na-K Alloys

Abstract: X-ray and neutron diffraction data are presented for a number of liquid Na-K alloys covering the whole composition range. Additionally, for the composition 50 at.% Na - 50 at%K, a computer experiment in molecular dynamics has been carried out. An attempts is made to analyze the results in terms of three partial structure factors. To this aim several models for the liquid state, subject to the condition of compatibility with the sets of diffraction data, are considered. The number density structure factor SNN(q) has been determined rather accurately. For the 50% alloy the results are compared with those of the computer experiment.

Optical-properties of ni+ centers in caf2-ni and srf2-ni

Abstract: The optical properties of X-irradiated CaF2:Ni and SrF2:Ni are studied. An absorption band at 255 nm in CaF2 and 270 nm in SrF2 is created by X-irradiation. It is concluded from optical bleaching experiments in low-temperature X-irradiated samples that those absorption bands are due to Ni+ ions. Optical dichroism experiments indicate that these Ni+ centres are oriented along the directions. A comparison with previous EPR experiments of X-irradiated CaF2:Ni indicates that the centres responsible for the 255 nm band in CaF2:Ni and the 270 nm band in SrF2:Ni are Ni+ ions displaced from the centre of the cube of fluorines toward one of the faces of that cube. On a etudie les propietes optiques du CaF2:Ni et Srf2:Ni apres les avoir irradie avec rayons X. Une bande d'absorption placee a 255 nm dans le CaF2 et a 270 nm dans le SrF2 est cree par le rayonnement X. Experiences de blanchiment optique sur les cristaux irradies a basse temperature indiquent que ces bandes sont associees au Ni+. Experiences de dichroisme montrent que ces centres sont orientes au long des directions . Comparaisons avec des precedentes experiences de EPR sur les cristaux du CaF2:Ni indiquent que le centre responsable des bandes d'absorption a 255 nm dans le CaF2:Ni et a 270 nm dans le SrF2:Ni est un ion Ni+ deplace de centre du cube des fluors vers une des faces de ce cube.

PHASE DIAGRAM OF THE ORDER-DISORDER TRANSITION IN Ni3Fe

Abstract: The phase diagram of the binary system Fe-Ni has been determined in the composition range 69–77 at% Ni and in a temperature region around 780 K, where an order-disorder transition is known to occur between the ordered L12 and the disordered f.c.c. structure. By employing Mossbauer Effect Spectroscopy it was possible to determine in considerable detail the location of the ordus and the disordus and the extent of the equilibrium two-phase region between the phases. A discussion in the literature about the possible violation of Gibbs' phase rule in the top of the phase diagram was resolved experimentally in favour of Gibbs. The existence of the 15 K wide hysteresis zone between ordus and disordus is explained as a magnetic effect. ab|fr|Nous avons determine le diagramme de phases du systeme binaire Fe-Ni dans le domaine compris entre 69 et 77 at% Ni, pour des temperatures autour de 780 K, ou l'on sait qu'il se produit une transition ordre-desordre entre la phase ordonnee L12 et la structure desordonnee c.f.c. Grace a la spectroscopie Mossbauer, il a ete possible de determiner en detail la position de Tordus et du disordus, ainsi que l'extension de la region diphasee a l'equilibre. Une discussion de la litterature concernant une violation possible de la regle des phases de Gibbs vers le haut du diagramme de phases a ete close experimentalement en faveur de Gibbs. Nous expliquons par un effet magnetique l'existence de la zone d'hysteresis large de 15 K entre l'ordus et le disordus.

Magnetism from mössbauer spectroscopy

Abstract: The effect of neighbours on the hyperfine field will be discussed in magnetically ordered, mostly iron based insulators, semiconductors, alloys and metallic glasses. It will be show that the value and the temperature dependence of the hyperfine field offer complementary information on the local and nonlocal magnetic interactions.

Isothermal Anderson-Grüneisen Parameter δ T of Alkali Halide Crystals

Abstract: The isothermal Anderson-Gruneisen parameter δ T has been studied with the help of interaction approach. The necessary equation for δ T has been derived interms of various lattice properties and tested for some cubic crystals, on the basis of wellknown Born, Lennard-Jones (12: 6) and modified Varshni-Shukla potential energy functions. The potential parameters of these models have been calculated employing the lattice equilibrium conditions at hydrostatic pressure instead of the generally utilised equilibrium lattice conditions at atmospheric pressure ( p =0). The results have been discussed in detail and it is inferred that the computed values of δ T , except for the Lennard-Jones function, are quite satisfactory with the experimental values keeping in view the limitations involved in the present treatment, which is obviously based on the least possible approximations.

Electrical conduction in polystyrene-chloranil system.

Abstract: The charge-transport behaviour in polystyrene–chloranil system has been studied by measuring the dependence of current on voltage, temperature, electrode material, and chloranil concentration The experimental results are discussed in terms of the Jonscher–Ansari modified Poole–Frenkel mechanism which takes into account the effect of traps The conductivity was found to increase with an increase in the chloranil concentration An explanation for this has been attempted on the basis of charge-transfer-complex formation between chloranil molecules and the phenyl group of polystyrene molecules

Structural relaxation in ferromagnetic metallic glasses

Abstract: The dependence of the Curie temperatures on heat treatment measured by DSC and ~assbauer effect in different metallic glasses [(Fe, N i ) B , Fe40Ni40P14Bb, F ~ N ~ C ~ P B ] is correlated to the change in the iron hyperfine field distribution. The results suggest the occurence of compositional short-range ordering in the structural relaxation. One of the most significant features of the glassy state is its deviation from thermodynamic equilibrium. It is clearly manifested in its relaxation behaviour, i.e. the time dependence of different physical properties below the glass temperature. In the rapid cooling process involved in the fabrication of the metallic glasses the solidification indicates the loss of thermal equilibrium of the undercooled melt. The quenched-in excess energy and volume can be reduced by annealing. Besides by direct experiments [1,2] the details of the process can be studied by monitoring the time dependence of some structure sensitive property at different temperatures. The investigation of ferromagnetic Curie point /T / ageing is used quite frequently for this C purpose [3-81. In this paper the TC changes determined by Differential Scanning Calorimetry are corbe observed because it is within the experimental accuracy. Only indirect evidence is available for the existence of CSRO [7] which is a change In short range correlation among the different atoms due to the annealing. Depending on the number and kind of atoms involved in the rearrangement the CSRO would result in a distortion of the shape of p(H) because of the change in the number of iron atoms with different local neighbourhoods. In the present paper the flrst observation of this effect will be reported. related with the results of Mossbauer spectroscopy l 3 t / A ' in order to characterize the changes in compositional and/or topological short range order on annea590 K ling. 11 In principle, the iron hyperfine field distribution, 0 0 610 K p(H) can provide information on the two dominant mechanisms of the structural relaxation, namely on the decrease in excess free volume and on compositional short-range ordering (CSRO). The decrease in free volume is manifested in a sharpening of the peaks of the pair distribution functions [1,2], in an increase of the density [9] by about 0.5% which results in better defined atomic positions. The smaller scatter of the atomic positions would cause a narrowing (and perhaps some shift) of p(H) but its shape would remain unchanged. Unfortunately, the expected change is smaller than 1% which cannot *on leave from the Central Research Institute for Physics, Budapest. 1 I I I 3