Pietro Manfrinetti

The phase diagram of the Ca–Si system

Abstract The phase diagram of the Ca-Si system has been investigated across almost the whole composition range (0 to 70 at% Si) using differential thermal analysis, metallographic analysis, X-ray diffraction and electron microscopy. Six intermediate phases have been found: CaSi2 (CaSi2-type), Ca14Si19 (Ca14Si19-type), Ca3Si4 (Ca3Si4-type), CaSi (CrB-type), Ca5Si3 (Cr5B3-type) and Ca2Si (anti-PbCl2-type). Ca3Si4, not previously known, is a Zintl phase representative of a new structural type: hexagonal hP42, a=854.1, c=1490.6 pm, space group P6 3 /m. Three eutectics occur in this system: at about 3.5 at% Si and 795°C, 42.5 at% Si and 1230°C, 72.0 at% Si and 1020°C.

Significant enhancement of irreversibility field in clean-limit bulk MgB2

Low resistivity MgB2 bulk samples annealed in Mg vapor show an increase in irreversibility field μ0H*(T) by a factor of ∼2 in both transport and magnetic measurements. The best sample displayed a magnetic irreversibility field μ0HM*>14 T at 4.2 K and ∼6 T at 20 K. These changes were accompanied by an increase of the 40 K resistivity from 1 to 18 μΩ cm and a lowering of the resistivity ratio from 15 to 3, while the critical temperature Tc decreased by only 1–2 K. These results show that systematic processing changes can make MgB2 attractive for magnet applications.

Neutron irradiation of MgB211: From the enhancement to the suppression of superconducting properties

In this letter, we present the effect of neutron irradiation up to fluences of 1.4×1020cm−2 on the superconducting properties of MgB2. In order to obtain a homogeneously distributed disorder, the experiment was carried out on bulk samples prepared with the B11 isotope. Up to fluences of 1018cm−2, the critical temperature (Tc) is slightly diminished (36K) and the superconducting properties are significantly improved; the upper critical field is increased from 13.5T to 20.3T at 12K and the irreversibility field is doubled at 5K. For the largest neutron fluence, Tc is suppressed down to 9.2K and the superconducting properties come out strongly degraded.

Vaporization thermodynamics of MgB2 and MgB4

The vaporization behavior of MgB2 and MgB4 under thermodynamic conditions has been studied by the Knudsen effusion-mass spectrometry technique. In the temperature range explored (883–1154 K), magnesium borides are observed to decompose by loss of gaseous Mg only. The equilibrium pressures of Mg(g) have been measured during high-temperature decompositions involving MgB2/MgB4 and MgB4/MgB7 two-phase mixtures and the corresponding standard reaction enthalpies were determined. The decomposition temperatures for MgB2 and MgB4 were also inferred by the relevant Van’t Hoff plots.

Thermodynamic stabilities of intermediate phases in the Ca–Si system

Abstract Vaporization thermodynamics in the binary system calcium–silicon has been studied by Knudsen effusion-mass spectrometry and vacuum microbalance techniques. The equilibrium partial pressure of Ca(g) over the two-phase regions in the composition range 20–75at.% Si has been measured and the standard enthalpy changes for the appropriate vaporization reactions were determined from the temperature dependence of the measured vapor pressures. The standard reaction enthalpy changes were also evaluated by the third-law method using the pressure data in conjunction with estimated Gibbs energy functions. Standard enthalpies of formation of the calcium silicides were derived from the standard reaction enthalpy values at room temperature. The results obtained for ΔfH°298 were the following: Ca2Si=−56.1±3.1, Ca5Si3=−55.3±3.5, CaSi=−49.6±2.2, Ca3Si4=−40.6±1.5, Ca14Si19=−44.4±2.3, CaSi2=−37.8±1.6 all in kJ/mol atoms. The results for Ca2Si, CaSi and CaSi2 may be compared with previous measurements, all other results are first determinations.

Crystal structure of R3Co8Sn4 compounds (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y)

Abstract The R 3 Co 8 Sn 4 compounds (R=rare earth) have been synthesized and studied by single crystal and powder X-ray diffractometry. The phases with R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y crystallize in the Lu 3 Co 7.77 Sn 4 structure type, but with full occupancy of all atomic positions, giving 3:8:4 stoichiometry in all cases except the lutetium phase. The studied phase was not found in the systems with La, Ce and Sc. These compounds should replace the previously reported RCo 3 Sn phases. The slightly anomalous dependence of the lattice constants on the rare earth size can be ascribed to strengthening of the Co–Sn bonds.

Synchrotron powder diffraction Rietveld refinement of MgB20 crystal structure

Abstract The crystal structure of MgB 20 has been investigated by X-ray and synchrotron powder diffraction at room temperature. As it happens in other metal-boron systems, the Rietveld refinement of this magnesium boron-rich phase shows a structure derived directly from β-boron. The substantially unaltered B–B framework hosts magnesium atoms in three different atomic positions (D, E and F holes); the occupancy of these sites is partial. The resulting unit cell is trigonal with space group R 3 m , the refined hexagonal axes a =1.09830(4) and c =2.41561(15) nm.

Magnetic properties of R3Cu4Sn4 (R=Ce, Gd and Y)

Abstract R 3 Cu 4 Sn 4 (R=rare earth) compounds crystallise in the orthorhombic, Gd 3 Cu 4 Ge 4 -type structure with space group Immm . There are two inequivalent crystallographic sites for the rare earth ions in this structure. We have studied the magnetic properties of compounds with R=Ce, Gd and Y, respectively. Both the symmetry inequivalent Ce-ions are found to be in the trivalent state. Multiple magnetic transitions are observed in Ce 3 Cu 4 Sn 4 and Gd 3 Cu 4 Sn 4 with comparable highest transition temperature (∼10.4 and ∼13 K, respectively), which is incompatible with the de Gennes scaling. There is no signature of Kondo effect in Ce 3 Cu 4 Sn 4 . Y 3 Cu 4 Sn 4 is a Pauli paramagnet.

Self-magnetic compensation and shifted hysteresis loops in ferromagnetic samarium systems

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Magnetic properties of R3Cu4Sn4 (R= La, Pr, Nd and Sm)

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