Large single-crystal growth of RBa2Cu3O7−x by the flux method (R=Y and rare earths)

Il Nuovo Cimento D - In this work the thermal conditions required for single-crystal growth in RBa2Cu3O7−x systems (R=Y or rare earths apart from Ce and Tb) have been systematically revised...     

Thermodynamic and magnetic properties of LaSn3

Thermal expansion coefficient between 77 and 900K, isothermal compressibility in the 0–80 Kbar pressure range, magnetic susceptibility between 77 and 1300 K and heat capacity at constant pressure in the 20–300 K temperature range were determined for the LaSn₃ compound. From the experimental data, the specific heat at constant volume was calculated and the thermal dependence of the Debye's parameter θ_D was obtained. The electron contribution to the heat capacity was also determined from the high temperature data. The magnetic properties confirm that there is no evidence of the existence of a magnetic moment localized on La atoms, in contrast with a previous report and in agreement with the general assumptions. A little anomaly found in the expansion coefficient, in the isothermal compressibility and in the specific heat is discussed in terms of a lattice order-disorder phenomenon.     

Electronic structure of CaSi and CaSi2

The electronic properties of calcium silicides (CaSi and CaSi₂) are investigated through a joint experimental and theoretical study using Bremsstrahlung Isochromat spectroscopy (BIS) and self-consistent calculation of the electronic states by the Linear Muffin-Tin Orbitals (LMTO) method in the Atomic Sphere Approximation (ASA). The peculiar crystal structure of CaSi₂ with two inequivalent Si atoms is responsible for a well defined BIS feature. We found that the calculated high energy DOS features are consistently lower in energy compared to the BIS spectra. Inclusion of the cross-sections of different states into the calculations improves the agreement between the experimental spectra and the calculated curve. We also show that considerable covalent character is present in the bond of calcium silicides.     

Abnormal magnetic properties of the Ce24Co11 hexagonal phase

The lighter rare earth-cobalt and thorium-cobalt binary systems were revised on the rare earth or thorium side, by metallographic, electron microprobe and X-ray methods. In agreement with previous works, it is confirmed that, in these systems, the first intermetallic compound corresponds to the following stoichiometry: 3:1 for trivalent La, Pr and Nd; 7:3 for tetravalent Th; 24:11 for Ce. The electronic structure of Ce in the hexagonal (P6₃mc) Ce₂₄Co₁₁ phase was investigated via magnetic susceptibility measurements in the 4.2–1300 K temperature range. The results show that the Curie-Weiss law is not followed, no magnetic order occurs down to 4.2 K and a very small change in the thermal behaviour of the magnetic susceptibility appears above the melting point of the phase (750 K). The abnormally low values of the magnetic susceptibility of Ce₂₄Co₁₁ could be understood by assuming Co is a non-magnetic state and Ce in a temperature dependent mixed valence state.     

Magnetic spectrometry of powders of highT c superconductors using a magnetic levitometer

Gli autori riferiscono intorno a misure delle proprietà diamagnetiche di superconduttori ad alta temperatura, siano monocristalli o polveri, mediante un nuovo levitometro magnetico. Il nuovo strumento ha consentito di: 1) determinare la diminuzione della magnetizzazione e della suscettività al crescere del campo magnetico nei cuprati di ittrio e bario; 2) confrontare il comportamento delle polveri con quello del monocristallo; 3) analizzare lo spettro di suscettività e la struttura dei flussoidi dei grani levitanti. Lo strumento si è rivelato essere un ottimo spettrometro magnetico adatto alla separazione di qualità delle diverse fasi superconduttrici presenti nei grani. (A cura dell'editore).

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(Conferenza tenuta dal Prof. G. Morpurgo il 27 marzo 1990)     

Thermodynamic properties of the CeSn3 mixed valence compound

We have measured on the CeSn₃ compound, the expansion coefficient between 80 and 800 K at normal pressure, the isothermal compressibility in the 0–8 GPa pressure range at room temperature and the heat capacity at constant pressure in the 60–300 K temperature range. The experimental data were compared with those previously found for the isomorphous LaSn₃ phase, assumed as a proper reference material for the study of the intermediate valency states in CeSn₃. Both the thermal expansion (3α) and the isothermal compressibility (k) of CeSn₃ show behaviours quite different from those of LaSn₃: for instance, in the standard conditions, 3α is 55 × 10^?6K^?1for CeSn₃ and 38 × 10^?6K^?1for LaSn₃; k is 15 × 10^?12 Pa^?1 and 12 × 10^?12 Pa^?1 respectively for CeSn₃ and LaSn₃. The thermal behaviour of the molar specific heat at constant pressure of CeSn₃ is similar to that of LaSn₃ for temperatures lower than 50 K. In the 70–300 K temperature range, the heat capacity of CeSn₃ is clearly higher than that of LaSn₃, ΔC_p being maximum near 150 K. The analysis of the calorimetric data show that the electronic coefficient γ of CeSn₃ is temperature dependent: its value varies from 53 mJ K^?2 mole^?1 at low temperature 24 mJ K^?2 mole^?1 at 300 K.