Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs

We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds in relation to crystal structure. The two compounds crystallize in two structural polymorphs, both tetragonal. The high temperature polymorph (HTP) adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying polycrystals prepared by arc melting we have observed that the rapidly cooled samples retain the HTP even at room temperature (RT) and below. Annealing such samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP and LTP were subsequently studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility and specific heat measurements. The HTP and LTP of both compounds respectively, behave as Pauli paramagnets. Superconductivity has been found exclusively in the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in LaIr2Si2). The relations of magnetism and superconductivity with the electronic and crystal structure are discussed with comparing experimental data with the results of first principles electronic structure calculations

Crystal structure and magnetism of YbT_xGa_4−x, T = Zn, Cd with the BaAl₄-type

Intermetallic compounds of the composition YbTxGa4-x (T = Zn, Cd) were synthesized and their crystal structure and magnetic properties were studied. These compounds crystallize in the tetragonal BaAI(4) type structure with a random occupation of the 4e site by Ga, Zn(Cd). The magnetic ground state of Yb in these compounds is nonmagnetic

Magnetism and structural chemistry of ternary gallides RENi_xGa_4−x(RE=; La, Ce, Pr, Nd, Sm, Gd, Tb) and LaCo_0.5Ga_3.5

Ternary gallides with the chemical formula (La, Ce, Pr, Nd, Sm, Gd, Tb) NixGa4 − x, (Ce, Pr, Nd, Sm)NiGa3 and LaCo0.5Ga3.5 have been synthesized from the elements by arc melting. From X-ray powder diffraction analysis the RENixGa4−x compounds and LaCo0.5Ga3.5 were found to be isostructural and to adopt the BaAl4-type structure. The homogeneous range at 600 °C for the CeNixGa4 − x phase has been established revealing a remarkable positive deviation from Vegard's rule. The existence of new ternary compounds RENiGa3 with the CePtGa3 type was observed for RE ; Ce, Pr, Nd, and Sm at the stoichiometric composition which, however, is found within the region of the BaAl4-type structure. Magnetic susceptibilities were determined over a temperature range from 2 to 900 K. Above LNT, magnetic behaviour of the rare earth compounds (except lanthanum and samarium) of both structural series is characterized by the paramagnetic contribution of the tripositive rare earth free-ion moments. LaNi0.75Ga3.25 and LaCo0.5Ga3.5 are weakly paramagnetic. Samarium-containing alloys are characterized by a typical Van Vleck-type paramagnetism of closely spaced multiplets. At very low temperatures (T<20 K) for the ceric-nickel-gallides ferromagnetism is encountered, whereas for (Sm, Gd, Tb)NixGa4 − x the coupling of the 4f moments is antiparallel

Structural chemistry and magnetic behavior of ternary gallides REAu_xGa_4−x, RE = La, Ce, Pr, Nd, and Sm

Two new series of ternary gallides with the chemical formulas (La, Ce, Pr, Nd, Sm)AuxGa4−x and (La, Ce, Pr, Nd, Sm)Au1.5Ga2.5 were synthesized from the elements by arc melting. From X-ray powder diffraction analysis the REAuxGa4−x series of compounds was found to be isotypic and crystallize with BaAl4 type of structure; the homogeneity range at 600°C of each of the REAuxGa4−x phases was established, revealing remarkable deviations from Vegard's rule. At 600°C the REAuxGa4−x phases of the BaAl4 type were observed to be in thermodynamic equilibrium with a structure variant crystallizing at the composition REAu1.5Ga2.5 and with a narrow homogeneous range. In case of PrAu1.5Ga2.5 the structure type was refined from X-ray single-crystal counter data (CaBe2Ge2 type, space group View the MathML source). Gold and gallium atoms were generally found on separate crystallographic sites; however, a statistical distribution of 51% Au + 49% Ga was derived for the 2b sites. 197Au Mössbauer spectroscopy confirmed the occupational mode of the gold atoms in CeAu1.5Ga2.5. For the BaAl4-type phases, X-ray powder and Mössbauer data revealed preferential occupation of the 4e sites of View the MathML source by gold atoms; practically no Au was observed on the 4d sites. Magnetic susceptibilities were determined over a temperature range extending from 2 to 1100 K. Above liquid nitrogen temperatures the paramagnetic behavior of the (Ce, Pr, Nd)AuxGa4−x and the (Ce, Pr, Nd)Au1.5Ga2.5 compounds is characterized by magnetic moments close to the ideal trivalent rare earth values. Lanthanum compounds are diamagnetic, whereas SmAuxGa4−x alloys are characterized by a typical Van Vleck-type paramagnetism of closely spaced multiplets. At very low temperatures onset of ferromagnetic ordering is observed for the BaAl4-type series of compounds. No superconductivity was encountered down to 2 K