Systematik der Seltenerdmetall-Blei-Legierungen

Zusammenfassung Das systematische Verhalten (sowie dessen Fehlen) der Seltenerdmetall-Blei-Legierungen wird untersucht. Es wurden 12 verschiedene Verbindungsstöchiometrien beobachtet. Fast alle dreiwertigen Lanthanidenelemente bilden die PhasenR ₅Pb₃,R ₅Pb₄ undRPb₃. Von den anderen möglichen Stöchiometrien kommen, mit Ausnahme vonRPb₂, nur von einigen Lanthaniden Verbindungen vor. Die bekannten kristallographischen Daten werden durchbesprochen und die Existenz und Nichtexistenz einiger Phasen auf Grund kristallchemischer Theorien diskutiert. Das Schmelzverhalten derR-reichen Eutektika und der PhasenR ₅Pb₃,RPb₂ undRPb₃ wird dargestellt. Die bekannten Informationen über diese Legierungen werden auf der Basis der relativen Volumskontraktionen und des 4f-Beitrags zur Bindung überprüft und die Ergebnisse zu Voraussagen über die relativen Stabilitäten der VerbindungenR ₅Pb₃,RPb₂ undRPb₃ verwertet.

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Systematics of the rare earth-lead alloys

The systematic behavior (and also the lack of it) in the rare earth-lead alloys is examined. Twelve different compound stoichiometries have been observed. Nearly all of the trivalent lanthanide elements form theR ₅Pb₃,R ₅Pb₄, andRPb₃ phases. For the other stoichiometries, except for possiblyRPb₂, compounds exist for only a limited number of the lanthanide elements. The known crystallographic data are reviewed, and the existence and nonexistence of some of the phases are discussed on the basis of crystal chemistry theories. The melting behaviors of theR-rich eutectic and theR ₅Pb₃,RPb₂ andRPb₃ phases are presented. The known information on these alloys is examined on the basis of the relative volume contractions and the 4f contribution to the bonding, and the results are used to predict the relative stabilities of theR ₅Pb₃,RPb₂, andRPb₃ compounds.

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Mit 8 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet.

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Die Arbeiten wurden im Ames-Laboratorium der US-Atomenergie-kommission durchgeführt (Beitrag Nr. 2972).     

DETECTION OF DNA-PSORALEN PHOTOADDUCTS in situ

Abstract— An immunological method, with the use of specific immune serum, has been developed for detection of 8-methoxypsoralen (8-MOP) photoadducts to DNA, formed in situ in cell nuclei, after combined treatment with 8MOP and UV-A irradiation (Zarçbska et al. , 1978). Lymphocytes fixed on slides or in suspension, and cryostat sections of different mammalian tissues, served as antigenic substrate, after treatment with 8-MOP and UV-A in vitro. Specific fluorescence in these substrates was detected in the nuclei after treatment with 30 ˜ 140 kJ/m² UV-A in the presence of 0.1-0.3 μg/cm² 8-MOP. PHA-stimulated-lymphocytes appeared to be the most sensitive substrate.
However, hairless mice treated with high doses of UV-A in vivo , 70 ˜ 360 kJ/m² did not reveal a specific fluorescence of epidermal nuclei, unless a high local concentration of 8-MOP was attained.
The apparent discrepancy in the level of photoadduct detection between the in vitro and in vivo treated specimens was explained by the low number of DNA-8-MOP-photoadducts formed in vivo under these experimental conditions. The relevance of these findings to the role of DNA-8-MOP-photoadducts formed during PUVA photochemotherapy is discussed.     

Low temperature heat capacity of electrotransported scandium

The heat capacity of two scandium samples containing 1.6 and <5 ppm atomic Fe was measured from 1 to 20 K. The weighted values for γ and θ_D are: 10.340 ± 0.018 mJ/g-at. K² and 346.1 ± 0.9 K, respectively. This is the first time anyone has obtained the same heat capacity results on two different scandium samples. The iron impurity concentration appears to be critical.     

Transition into a magnetic state without magnetic scattering in a rare earth system: CeAl3

Susceptibility measurements on CeAl₃ down to 0.5 K show both the absence of magnetic ordering and a reduced effective moment such as found in dilute Kondo alloys. The resistance decreases a factor of 30 below the maximum at T = 37 K and the magneto resistance which is negative above 0.5 K apparently becomes positive for T < 0.5 K.     

ON QUOTIENTS OF BRAID GROUPS

ABSTRACT.

Let G be the group ?[t, t ^?1] x ?. By studying the action of the braid group B_n on the set Gⁿ , we obtain representations of B_n into a wreath product of the symmetric group and the general linear group over ?[t, t ^?1]. This in particular recovers the Burau representation of the braid group. Furthermore, some quotients of the braid group are obtained by using the representations found.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Microstructure and magnetocaloric effect in cast LaFe11.5Si1.5Bx (x=0.5, 1.0)

Phase formation, structure, and the magnetocaloric effect (MCE) in as-cast LaFe_11.5Si_1.5B_x (x=0.5, 1.0) compounds have been studied. The Curie temperatures, T_C, are ∼211 and 230 K for x=0.5 and 1.0, respectively, which are higher than that of annealed LaFe_11.5Si_1.5 (T_C=183 K), while the maximum magnetic entropy changes at the respective T_C under a magnetic field change of 0-5 T are 7.8 and 5.8 J/(kg K). Wavelength dispersive spectrometry (WDS) analysis shows that only a small fraction of boron atoms is dissolved in the NaZn₁₃-type structure phase, and that the compositions of the as-cast LaFe_11.5Si_1.5B_x (x=0.5, 1.0) alloys are much different from the intended nominal compositions. These as-cast alloys exhibit second-order magnetic phase transitions and low MCEs. However, based on the relative cooling power, the as-cast LaFe_11.5Si_1.5B_x alloys are promising candidates for magnetic refrigerants over a wide temperature range.     

Magnetoresistance of Sc3In

The magnetoresistance of the weak itinerant-electron ferromagnet Sc₃In was measured from 1.5 to 40 K in magnetic fields up to 14 T. The measurements were made on three samples containing 24.1, 24.3 and 24.4 at% In with Curie temperatures at H = 0 of T_c(0) = 5.5, 6.0 and 6.3 K, respectively. By assuming that the characteristic spin-fluctuation temperature is equal to T_c(0), the negative magnetoresistance in Sc₃In was systematically and reasonably analyzed in terms of the quenching of spin fluctuations by magnetic fields.