Marcella Pani

Structural and transport properties of some UTX compounds where T = Fe, Co, Ni and X = Si, Ge

Abstract The ternary intermetallic compounds with general formula UTX (T = Fe, Co, Ni; X = Si, Ge) were analyzed by X-ray methods and some of their physical properties examined. All the studied phases belong to the well known TiNiSi-type structure. The compound UFeGe shows a phase transition at 500 K; at room temperature it crystallizes in a new monoclinic structure with a = 6.986, b = 4.308, c = 6.992A, β = 93.71° in the space group P 2 1 / m ; above 500 K it changes into the orthorhombic TiNiSi-type. Electrical resistivity measurements were carried out in the range 1.5–300 K for all samples, and up to 600 K for UFeGe. Anomalies in the resistivity behaviour were detected for UNiSi, UFeGe, UCoGe and UNiGe at 19, 80, 46 and 41 K respectively and ascribed to magnetic transitions.

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.

Phases around the 1:1:1 composition in the Yb–Au–Ge and Ca–Au–Ge systems

Abstract The systems YbAu 2− x Ge x and CaAu 2− x Ge x with 0.7 x 2 , AlB 2 , EuAuGe (occupation derivative), SrAgGe, α -YbAuGe and β -YbAuGe (the last two being centrosymmetric variants of the CaCuGe type). The equiatomic compound CaAuGe shows a phase transition at 1238 K. YbAuGe crystallizes in three different structural modifications as a function of temperature: α -YbAuGe up to 1003 K, β -YbAuGe in the range 1003 to 1308 K and γ -YbAuGe at T >1308 K. Resistance measurements carried out in the range 14–400 K for the high-temperature forms of YbAuGe and CaAuGe show normal metallic behaviour. Magnetic susceptibility data of YbAuGe are in agreement with the occurrence of divalent ytterbium.

Crystal structure of the RCuZn, RAgZn and RAgAl intermetallic compounds (R = rare earth metals)

Abstract The RCuZn, RAgZn and RAgAl intermetallic compounds (R = rare earth metals) were synthesized and structurally studied by powder and single crystal X-ray diffraction. Among the 28 examined phases, YbAgAl crystallizes in the MgZn2 structure type, while all the others belong to the CeCu2 type. The usual decrease of the cell parameters from lanthanum to erbium is observed, and the clear positive deviation of the three ytterbium compounds can be imputed to Yb divalency. The application of the structural map method to the RMX compounds, where M and X are elements of the 10 to 15 groups, allows some predictions about unknown phases to be proposed.

High temperature structural study of Gd-doped ceria by synchrotron X-ray diffraction (673 K ≤ T ≤ 1073 K).

The crystallographic features of Gd-doped ceria were investigated at the operating temperature of solid oxides fuel cells, where these materials are used as solid electrolytes. (Ce(1-x)Gd(x))O(2-x/2) samples (x = 0.1, 0.3, 0.5, 0.7) were prepared by coprecipitation of mixed oxalates, treated at 1473 K in air, and analyzed by synchrotron X-ray diffraction in the temperature range 673 K ≤ T ≤ 1073 K at the Elettra synchrotron radiation facility located in Trieste, Italy. In the whole temperature span a boundary was found at x ∼ 0.2 between a CeO2-based solid solution (for x ≤ 0.2) and a structure where Gd2O3 microdomains grow within the CeO2 matrix, taking advantage of the similarity between Gd(3+) and Ce(4+) sizes; the existence of the boundary at x ∼ 0.2 was confirmed also by measurements of ionic conductivity performed by impedance spectroscopy. Similar to what observed at room temperature, the trend of the cell parameter shows the presence of a maximum; with increasing temperature, the composition corresponding to the maximum moves toward lower Gd content. This evidence can be explained by analyzing the behavior of the coefficient of thermal expansion as a function of composition.

Structure and transport properties of the R2Co2Al compounds (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y)

Abstract The new R2Co2Al compounds (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y), studied by single crystal and powder X-ray diffraction, crystallize with the W2CoB2 structure type. Pr2Co2Al is dimorphic, showing also its own monoclinic structure (a=9.595 Å, b=5.602 Å, c=7.753 Å, β=103.89°), which is closely related to the W2CoB2 type. Both structures are based on different packing of chains of cobalt-centred double trigonal prisms formed by rare earth and aluminum atoms. The electrical resistivity, measured in the 13-300K temperature range for all phases (save Y2Co2Al), always shows a transition point at temperatures going from 14K for Tm2Co2Al to 94K for Tb2Co2Al. As already found for Gd2Co2Al, these points should correspond to an order-disorder magnetic transition, and the deviations from de Gennes scaling indicate the influence of both the crystalline electric field and the strong coupling between 4f and conduction electrons.

Pyrrolidines, Pyrrolines and Pyrroles from 1,4-Diaryl-2,3-dinitro-1,3-butadienes via a 5-endo-trig Cyclization

The reactions between 1,4-diaryl-2,3-dinitro-1,3-butadienes 1a-d and primary alkylamines gave high yields of N-alkyl-2,5-diaryl-3-alkylamino-4-nitropyrrolidines 2 as pure all-trans diastereomers via an unusually favoured 5-endo-trig ring closure. The stereochemistry of compounds 2 has been attributed through an X-ray crystal structure analysis of the acetyl derivative 5 of 2ai. Amine elimination from 2 gave the N-alkyl-2,5-diaryl-3-nitro-3-pyrrolines 3 which could be easily aromatized to the corresponding pyrroles 4.

Crystallographic, magnetic and magnetocaloric properties of GdMgX intermetallic phases (X=Al, Ga, In)

Abstract Experimental studies of the physical properties of the three equiatomic compounds GdMgX (X=Al, Ga, In) are reported. GdMgAl (MgCu 2 -type structure) orders ferromagnetically below 59 K. Between 70 and 300 K it follows the Curie–Weiss law with μ eff =8.59 μ B and θ P =+70 K. The magnetisation, in the ordered state, saturates almost completely at 5 K in a magnetic field of 9 Tesla ( μ sat =6.71 μ B ). From several magnetisation isotherms between 10 and 120 K, the total entropy change and the refrigerant capacity of the material at 1.5, 3 and 5 T magnetic fields is evaluated. The crystal structure of the two new phases GdMgGa and GdMgIn (hexagonal ZrNiAl-type structure) is determined by single crystal X-ray analysis and the positional and displacement parameters refinements of both compounds are reported. GdMgGa orders antiferromagnetically at 15.3 K; above 50 K the compound follows the Curie–Weiss law with θ P =−124.5 K and μ eff =7.52 μ B . On the contrary, no evidence of a magnetic order is observed for the GdMgIn phase. From the Curie–Weiss law, applied in the 70–290 K range, θ P =−130.5 K and μ eff =7.75 μ B are obtained. The magnetisation of the two compounds does not saturate at 5 K in an applied magnetic field of 9 T ( μ sat (GdMgGa)=0.88 μ B , μ sat (GdMgIn)=1.05 μ B ).

Electrical resistivity of the RMBi phases with R = Ca, Yb and M = Cu, Ag, Au

Abstract The electrical resistivity of the phases CaAgBi, CaAuBi, YbAgBi in the range 10–300 K and CaCuBi, YbCuBi, YbAuBi in the range 10–600 K was measured. All compounds show metallic behaviour. The phonon contribution with an additional cubic term are used to represent the temperature dependence of the resistivity for the gold and silver phases. Anomalous features in the thermal cycling resistivity, imputable to the occurrence of microcracks, are found for the CaCuBi and YbCuBi samples. Moreover, YbCuBi shows a phase transition at 375 K, probably due to a crystallographic transformation from the LiGaGe to the ZrBeSi structure type.

The CaAg2-CaZn2-CaAl2 pseudo ternary system : crystal structures and structural stability

The CaAg2—CaZn2—CaAl2 pseudo ternary system has been studied by micrographic and X-ray diffraction analyses of 37 alloys, melted and annealed at 973 K or 873 K in tantalum crucibles. Single crystal and powder X-ray data show a distribution of the phases among four structure types: MgCu2, MgZn2, MgNi2 (the Laves phases) and CeCu2 (for the first time reported for KHg2). A good separation of the structural domains was obtained by means of a two-dimensional map based on the size and electronic factors. The different volume effects shown by the CeCu2-type phases as regards those of the Laves phases were accounted for on the basis of the hard-sphere space filling model.