M. L. Fornasini

Phase diagram of the Ca–Sn system

Abstract The phase diagram of the Ca–Sn system has been investigated in the whole composition range using differential thermal analysis, metallographic analysis, single-crystal and powder X-ray diffraction. Seven intermediate phases have been found. Four of them have been confirmed: CaSn 3 (AuCu 3 -type), CaSn (CrB-type), Ca 31 Sn 20 (Pu 31 Rh 20 -type) and Ca 2 Sn (Co 2 Si-type); while Ca 7 Sn 6 (Ca 7 Sn 6 -type), Ca 36 Sn 23 (Yb 36 Sn 23 -type) and Ca 5 Sn 3 (Cr 5 B 3 -type) have been identified and characterized for the first time. Four eutectic points occur in this system: at about 5 at.% Sn and 780°C, 48 at.% Sn and 995°C, 68 at.% Sn and 610°C, and at less than 1 at.% Ca and 225°C.

Structure and magnetism of Gd2Co2Ga, Gd2Co2Al and Gd14Co3In2.7

The paper reports on the crystal structure determination and magnetic measurements of the Gd2Co2Al, Gd2Co2Ga and Gd14Co3In2.7 phases. Gd2Co2Ga and the new intermetallic compound Gd2Co2Al crystallise in the Pr2Ni2Al structure, while single crystal data show that Gd14Co3In2.7 is closely related to the Lu14Co2In3 phase, but with a different occupation of the Co and In sites. Gd2Co2Al and Gd2Co2Ga order ferromagnetically below 78.2 and 76 K, respectively, and the isothermal magnetisation in the ordered state (5 K) saturates completely to the same value for both compounds (μsat.=6.6 μB/Gd atom). The intermetallic compound Gd14Co3In2.7, on the contrary, is antiferromagnetic below TN=37 K and reveals a metamagnetic transition at μ0H=1.4 Tesla, detected also by AC susceptibility measurements.

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.

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.

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 ).

Physical properties of GdNiIn

Abstract Single-crystal X-ray analysis, electrical resistivity and magnetisation measurements were performed on GdNiIn. The compound is ferromagnetic with T c =93.5 K. From the hysteresis loop at 77 K, a remanence of 5 mT and a coercive field of 15 mT were obtained. The magnetic moment at 4.2 K shows a saturation value of 7.2 μ B .

Ba5Ga6: a phase with octahedral clusters of gallium

Abstract The crystal structure of Ba 5 Ga 6 was determined by single-crystal diffractometry. It is hexagonal, hP22, space group P 6 c2 (n. 188), a = 7.771(2) A , c = 14.376(4) A , Z = 2 , refined with anisotropic displacement factors, R = 0.076 with 358 reflections, and contains a disordered Ba position with partial occupation. The structure is characterized by octahedral Ga 6 clusters, isolated and surrounded by a cuboctahedron of twelve Ba atoms. The barium cuboctahedra, by sharing of edges and faces, are condensed in a tridimensional framework. Application of Wades rules to the Ga 6 naked cluster gives an excess of two electrons per formula unit. The compound Ba 5 Ga 6 replaces the phase “BaGa” given in the original Ba-Ga phase diagram.

New phases in the thorium-iron-tin system: ThFe0.22Sn2 and Th4Fe13Sn5☆

Abstract The structures of two new phases were determined by single crystal diffractometric data. The phse ThFe0.22(2)Sn2, orthorhombic, space group Cmcm (No. 63). Pearson symbol S16-3.12, a = 4.479(1), b = 17.045(3), c = 4.430(1) A , V = 338.2(1) A , Z = 4, R = 0.038 for 260 reflection with F > 4σ(F) has a defective structure derived from the CeNiSi2 type. The phase Th4Fe11Sn5, tetragonal, space group P4/mbm (No. 127). Pearson symbol IP44, a = 8.290(1), c = 11.946(1) A , V = 821.0(2) A , Z = 2, R = 0.061 for 760 reflections with F11 > 4σ(F11) shows a structure of a new type, formed by two segments which alternate along the c-axis. One segment shows an atomic arrangement very similar to that of the Ce(Mn,Ni)11 structure, while the other corresponds to a half-cell of the Cr5B3 type. Slabs of centered icosahedra containing only iron atoms are a characteristic feature of this structure, common also, besides the cited Ce(Mn,Ni)11, to the Ce2Ni12Si4 and Nd6Fe11Si for La6Co11Ga) structures. For the phase Th4Fe11Sn5 a transition to the ferromagnetic state at 375 K was observed by differential scanning calorimetry, electrical and magnetic measurements.

Valency changes of ytterbium in YbMnGe and in the YbMnSi1-xGex pseudo-ternary system

Abstract X-Ray diffraction on powders and single crystals has been used to study some phases in the YbMnSi1−xGex system. YbMnGe is dimorphic: the high-temperature form, stable above 1423 K, crystallizes in the hexagonal ZrNiAl-type structure, while the low-temperature form, stable below 1073 K, is orthorhombic with TiNiSi-type. The structural transition is accompanied, and probably caused, by a valence change of the ytterbium atoms, from trivalency (in the HT form) to divalency (in the LT form). The corresponding large volume increase is the cause of the observed explosive nature of the transition. The same valence transition, even if at lower temperatures, occurs also in some phases on the germanium side of the YbMnSi1−xGex system (for x=0.8, 0.9, 0.95, 0.975), but both HT and LT forms are TiNiSi-type, showing sharply different cell volumes. No transition was observed in YbMnSi and YbMnSi0.5Ge0.5, which crystallize in the TiNiSi structure and probably contain trivalent ytterbium atoms.

Ba15Al13.4Ga14.5, a disordered structure derived from Sr5Al9

La structure du compose du titre peut etre obtenue a partir de la structure ordonnee Sr 5 Al 9 par redistribution des atomes Ga et Al. Elle a des caracteristiques qui sont apparentees aux phases de Laves et peut etre decrite comme une intercroissance de feuillets MgZn 2 et Ba 3 Al 5 (69%) comme dans Sr 5 Al 9 et de feuillets Rb 2 Au 3 Sn 2 et Ba 3 Al 5 (31%).