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

A neutron diffraction study of Fe7Se8

An investigation of Fe7Se8 was undertaken to study the magnetic ordering and phase transitions in this compound. On a sample slowly cooled from 600 °C a superstructure was identified corresponding to the 3c structure found by Okazaki [4]. This is derived from the small Ni-As-type cell by doubling of the a-axis and tripling of the c-axis. Neutron diffraction diagrams were made at several temperatures between 4.2 °K and 493 °K. The unit cell dimensions found were : The space groups C 46 — P62 and C 56 — P64 proposed by Okazaki and Hirakawa [3] are not applicable. A possible space group is P31, with vacancies at (1/2, 1/2, 0) (1/2, 0, 1/3) (0, 1/2, 2/3). The magnetic moments are ferromagnetically aligned within each (0001) sheet and antiferromagnetically aligned in neighbouring sheets. At low temperatures the moment direction is perpendicular to the c-axis, but between 120 °K and 130 °K the moments turn to a direction closer to the c-axis. The intensity of the (0003) reflection indicates a tilt of the spin with respect to this axis. No indication has been found of the gradual change in the axis of easy magnetization which was measured by Hirakawa [5] on a torque magnetometer.Une étude de Fe7Se8 a été entreprise afin de déterminer l'ordre magnétique et les transitions de phase dans ce composé. Sur un échantillon refroidi lentement depuis 600 °C, une surstructure correspondant à la structure 3c trouvée par Okazaki [4] a été mise en évidence. Celleci est obtenue à partir de la maille de NiAs en doublant l'axe a et en triplant l'axe c. Le diagramme neutronique a été fait à différentes températures entre 4,2 °K et 493 °K. Les dimensions de la maille sont : Les groupes d'espace C46 —P62 et C5 6— P64 proposés par Okazaki et Hirakawa [3] ne sont pas applicables. Un groupe possible serait P31, avec des lacunes en (1/2, 0), (1/2, 0, 1/3), (0, 1/2, 2/3). Les moments magnétiques sont alignés ferromagnétiquement dans chaque plan (0001) et antiferromagnétiquement entre deux plans voisins. A basse température, la direction des moments est perpendiculaire à l'axe c, mais entre 120 °K et 130 °K, les moments tournent en se rapprochant de l'axe c. L'intensité de la réflexion (0003) indique une inclinaison des spins par rapport à cet axe. Aucune indication n'a pu être trouvée sur le changement progressif de l'axe de facile aimantation qui avait été mesuré par Hirakawa [5]

MAGNETIC STRUCTURES OF ORTHORHOMBIC RARE EARTH COMPOUNDS

Magnetic structures of HoCoSi2 and TbRhGe were determined by neutron diffraction method. At 4.2 K HoCoSi2 orders antifferromagnetically with magnetic moments of Ho3+ parallel to the c-axis. The magnetic structure of TbRhGe is incommensurate of modulated transverse spin-wave type