R.M. Waterstrat

Atomic ordering and superconductivity in A-15 compounds

Twenty-six different binary A-15 compounds were investigated to determine if superconductivity could be related to the degree of long-range atomic ordering on the crystallographic lattice sites. Significant changes in the superconducting transition temperatures and critical fields were produced by quenching from high temperatures followed by low-temperature annealing. These changes inTcwere accompanied by changes in the degree of long-range ordering as determined using x-ray diffraction methods. The results can be interpreted within the framework of Wegers “linear-chain” model only when the B-element is a nontransition element. When both the A-and B-components are transition elements, however, the superconducting behavior will apparently depend on the nature of “d-electron” interactions between the component atoms. Complete ordering is not always an essential requirement for optimizing the superconducting properties.

Crystallographic structures and phase transformations in ZrPd

Abstract Phase transformations and crystal structures in the equiatomic compound ZrPd were examined using in-situ heating and cooling in a transmission electron microscope (selected-area diffraction and high resolution imaging) and Rietveld refinement of high resolution neutron powder diffraction data. Three phases were characterized: a high temperature cubic B2-type phase (Pm3m; Z = 1; a = 0.335 97(3) nm at 800°C); an orthorhombic CrB-type phase formed by martensitic transformation from the B2 phase (Cmcm; Z = 4; a = 0.333 19(3) nm, b = 1.030 1(1) nm and c = 0.441 11(4) nm at 400°C) and a monoclinic variant of the CrB-type phase at room temperature (assumed space group Cm; Z = 8; a = 0.666 11(6) nm, b = 0.874 99(7) nm, c = 0.542 35(6) nm; β = 108.21(1)°). The monoclinic ⇌ orthorhombic phase transformation was found to occur reversibly around 200°C.

Nb‐Si A15 compounds produced by liquid quenching

A metastable A15‐type structure has been obtained in binary Nb‐Si alloys containing 18.8±0.5 at.% Si by very rapid quenching (splat cooling) from the liquid state. Superconducting onset temperatures (Tc) of about 4 K were measured on these samples. The relatively low Tc values can be a result of the nonstoichiometric (Nb‐rich) composition, residual antisite disorder, or other types of defect structures. Ternary additions such as Sn, Pt, or Ir seem to improve the stoichiometry of the A15 phase and a Tc of 13 K was obtained for the analyzed composition Nb75Si16Sn9.

Analysis of selected alloys in the systems Cr-Pd, Cr-Ru, V-Pd and Ta-Pt

Abstract Cr-Pd, Cr-Ru, V-Pd and Ta-Pt alloys were selected, appropriately annealed and subjected to electron microprobe analyses. The data enable a basis to be established for a more accurate location of phase boundaries which have not been accurately defined in previous studies.

The palladium–zirconium phase diagram

Abstract The palladium–zirconium phase diagram has been re-investigated over its entire composition range. The revised diagram includes two intermediate phases, Pd 4 Zr 3 and Pd 11 Zr 9 , which did not appear in previous studies. The crystal structures of the new phases resemble the B2-type structure although the atoms are displaced significantly from the cubic lattice sites. The present observations are discussed and compared with similar results from the related systems Ni–Ti and Rh–Zr.

Electronic properties, superconductivity and stability of the ZrRh alloys

Abstract Experimental information on the electronic properties of ordered ZrRh alloys is presented through low temperature specific heat and magnetic susceptibility measurements. In general, a low density of states at the Fermi level is deduced and this is explained by a split band regime consistent with the known DOS calculatios for the type of ordered structures considered. The existence of a split band is considered as typical of stable ordered structures. Zr 2 Rh, which is a superconductor with T c = 11.2 K possesses a very high electronic specific heat coefficient. It is suggested that this high λ value is associated with a decrease of stability of the ordered structure and an increased ability to form amorphous alloys when the concentration of Zr increases.

New high‐pressure phases of Nb3Si produced by recrystallization of metastable sputter deposits

Threee new phases of the alloy Nb3Si have been produced by a relatively low‐temperature (∼800 °C) recrystallization anneal under pressures of up to 100 kbars. The new phases are apparently formed only when the Nb3Si starting material has a metastable body‐centered‐cubic structure which is prepared by sputtering. They are not formed when the Nb3Si starting material has the Ti3P‐type structure. One of the new high‐pressure phases has a superconducting transition temperature Tc of 5.45 K, but its crystal structure has not yet been identified. The phase formed at 60 and 100 kbars has a tetragonal Ni3P‐type structure, but the structure of the 80‐kbar phase is not known.

Superconducting behavior of A15 compounds

Abstract Superconductivity has been discovered in the A15 compounds V 1− x Ni x , V 3 Pd and Ta 85 Pt 15 and a new feature has been observed in the T c vs. e/a correlation for A15 compounds. The anomalously low T c of V 3 Pd is attributed to magnetic interconnections. Critical magnetic field data is presented for Nb 3 Os, Cr 3 Rh and Cr 3 Ir.

Crystal structure of the Zr3Pd4 phase

Abstract A new binary ZrPd phase with Zr3Pd4 stoichiometry was found. The structure of the phase was investigated by transmission electron microscopy and powder neutron diffraction. The structure belongs to the Pu3Pd4 structure type, which has a rhombohedral R 3 space group with 42 atoms in a hexagonal cell. Modifications of the ZrPd phase diagram which accommodate the new phase are suggested.

Electronic properties, superconductivity and stability of the ordered alloys of the Ti-Rh, Zr-Rh and Hf-Rh isoelectronic systems

Abstract Experimental information on the electronic properties of ordered Ti-Rh, Zr-Rh and Hf-Rh alloys has been obtained through low-temperature specific heat and magnetic susceptibity measurements. The relatively low density-of-states at the Fermi level for TiRh, ZrRh, HfRh, TiRh 3 , ZrRh 3 and TiRh 3 is explained by a split-band regime which is typical of relatively stable ordered structures. The minimum value of the density-of-states at the Fermi level n ( E F ) is not reached by ZrRh and HfRh and even less by TiRh; this is related to CsCl → ZrIr-type and CsCl → AuCuI-type martensitic transformations respectively. Hf 2 Rh (Ti 2 Ni-type structure) is a superconductor with T c = 1.60 K and has a moderate γ value. Ti 2 Rh (MoSi 2 -type structure) has a low γ value and no detectable T c whereas Zr 2 Rh (Al 2 Cu-type structure) has T c - 11.2 K and a high γ value. The type of structure is apparently the basis for these property variations.