S. J. La Placa

Formation of thin films of NiSi: Metastable structure, diffusion mechanisms in intermetallic compounds

The formation of NiSi films from the reaction of Ni2Si with (100) and (111) silicon substrates was found to be controlled by a lattice diffusion process with an activation energy of 1.70 eV. In order to correlate kinetic information obtained by Rutherford backscattering with x‐ray diffraction data, ‘‘standard’’ diffraction powder patterns for both Ni2Si and NiSi have been established. The existence of a metastable hexagonal form of NiSi has been confirmed. Observations on the formation of Ni2Si confirm previous investigations. The diffusion process at work during the formation of NiSi is discussed in terms of the crystalline anisotropy of this compound and compared to what is known about diffusion in other silicides.The formation of NiSi films from the reaction of Ni2Si with (100) and (111) silicon substrates was found to be controlled by a lattice diffusion process with an activation energy of 1.70 eV. In order to correlate kinetic information obtained by Rutherford backscattering with x‐ray diffraction data, ‘‘standard’’ diffraction powder patterns for both Ni2Si and NiSi have been established. The existence of a metastable hexagonal form of NiSi has been confirmed. Observations on the formation of Ni2Si confirm previous investigations. The diffusion process at work during the formation of NiSi is discussed in terms of the crystalline anisotropy of this compound and compared to what is known about diffusion in other silicides.

Crystallography and microstructure of Tl-Ca-Ba-Cu-O superconducting oxides

We report on the crystallography and microstructure of six oxides in the Tl‐Ca‐Ba‐Cu‐O system with nominal compositions Tl1Ba2Cu1O5, Tl1Ca1Ba2Cu2O7, Tl1Ca2Ba2Cu3O9, Tl2Ba2Cu1O6, Tl2Ca1Ba2Cu2O8, and Tl2Ca2Ba2Cu3O10. The structures consist of one, two, or three Cu perovskite‐like units sandwiched between Tl‐O monolayers or bilayers. The predominant defects in the crystals with bilayer and trilayer Cu perovskite‐like units are stacking faults that produce local intergrowths of related structures. The density of stacking defects in these oxides correlates with changes in the superconducting transition temperatures.

Non-integral charge transfer in an organic metal: the structure and stability range of (TTF)Br0.71–0.76

Abstract The organic metal (TTF)Br 0.71–0.76 crystallizes in a monoclinic lattice comprised of mixed valence (0, +1) TTF molecules and Br - arranged in segregated stacks along the c -axis. Its structure can be viewed in terms of two separate, range of the salt was deduced from the composition dependence of the bromide sublattice parameters. The relationship of the unusual stoichiometry of (TTF)Br 0.71–0.76 to the degree of charge transfer in 1 : 1 Π-donor/ TCNQ salts is examined and discussed.

Silicides of ruthenium and osmium: Thin film reactions, diffusion, nucleation, and stability

The reactions of ruthenium and osmium thin films with their silicon substrates lead to the formation of the following phases: the isomorphous Ru2Si3 and Os2Si3, and OsSi2. Ru2Si3 forms by a diffusion controlled mechanism from approximately 450 to 525 °C; the activation energy is 1.8 eV. Os2Si3 grows by the same process and with the same activation energy as Ru2Si3, but at slightly higher temperatures (for equivalent rates). OsSi2 grows at about 750 °C by a nucleation controlled mechanism. With an alloy of ruthenium containing 33 at.% rhodium, one obtains RuSi, which forms by a diffusion controlled mechanism with an activation energy of 2.4 eV from 400 to 475 °C, and Ru2Si3. In the case of the ruthenium alloy, the formation of Ru2Si3 is not diffusion controlled; the controlling process is akin to a nucleation mechanism. Ru2Si3 and Os2Si3 form by silicon motion. This is also true of OsSi2, but the high formation temperature results in some metal motion also during the formation of that phase. The diffusion ...

Annealing treatment effects on structure and superconductivity in Y1Ba2Cu3O9−x

We report the effects of heat treatment and ambient on the structure and superconducting properties of Y1Ba2Cu3O9−x. The structure undergoes an orthorhombic‐to‐tetragonal transition on heating at about 700 °C, caused by oxygen loss and disordering of oxygen vacancies on the copper plane between the barium layers. Heat treatments that promote maximum ordering of the oxygen vacancies result in superior superconducting properties.

Formation of intermediate phases, Ni3Si2 and Pt6Si5: Nucleation, identification, and resistivity

The formation of Ni3Si2 from the reaction of Ni2Si with NiSi, and that of Pt6Si5 from the reaction of Pt2Si with PtSi have been investigated by Rutherford backscattering, x‐ray diffraction, resistance measurements, and optical and electronic microscopy. Standard x‐ray diffraction patterns were calculated for Pt6Si5 and for the high‐temperature form (hexagonal) of Pt2Si. These are shown to match experimental diffraction patterns. Both Ni3Si2 and Pt6Si5 form quite suddenly (at 470 and 535 °C, respectively) according to the pattern of nucleation‐controlled reactions which are anticipated when the free energies of formation of the new phases are sufficiently small. These observations are discussed with respect to the absence of both Ni3Si2 and Pt6Si5 from the sequence of phases which form when Ni and Pt thin films react with Si. Resistivity measurements are reported for Ni3Si2, Pt6Si5, and for the two forms (low and high temperature) of Pt2Si.

Structure of La2Cu2O5 by high-resolution synchrotron X-ray powder diffraction

Dicopper(II) dilanthanum pentaoxide, La 2 Cu 2 O 5 , M r =484.90, orthorhombic, Pbam. At T=300 K: a=5.5490 (1), b=10.4774 (2), c=3.8796 (1) A, V=225.557 (8) A 3 , Z=2, D x =7.139 g cm -3 , λ=1.200 A. Final R I =6.20, R p = 14.6 and R wp =20.61%, 124 independent reflections observed. The structure has been refined from high resolution synchrotron X-ray powder diffraction data using the Rietveld method. It is of the oxygen defect perovskite type and is composed entirely of corner-shared CuO 5 square pyramids, which share oxygen vacancies forming vacancy tunnels along the c axis. The La atoms reside at a perovskite-like A-site and are tenfold coordinated by oxygen

Defect structure in the alkali gallate fast ion conductors

Abstract K-β-Ga 2 O 3 , Na-β-Ga 2 O 3 , and Na-β″-Ga 2 O 3 are alkali fast ion conductors which crystallize with the β and β″ Al 2 O 3 structures. These compounds exhibit an excess of alkali ions which must be compensated by negatively charged defects to maintain charge neutrality. Single crystal X-ray diffraction has been used to elucidate the defect structures. Compensation in K-β-Ga 2 O 3 occurs through the presence of interstitial oxygen, as observed in Na-β-Al 2 O 3 . Compensation in Na-β-Ga 2 O 3 occurs through two processes. One third of the excess sodium is compensated by interstitial oxygen. The remainder is compensated by immobile sodium, which has replaced gallium on tetrahedral sites. Compensation in Na-β″-Ga 2 O 3 is achieved exclusively by immobile sodium. The observed site distributions in K-β-Ga 2 O 3 and Na-β-Ga 2 O 3 do not fit the WGC or Wolf models, but can be accounted for in terms of a new type of cluster.

Superconductivity in Bi‐Sr‐Ca‐Cu‐O (invited)

We report on the structure and superconducting properties of a single phase polycrystalline composition given by Bi2.2Sr1.7Ca1.2Cu2O8+δ as well as measurements on single crystals. X‐ray and electron diffraction analyses of both single‐crystal and single phase materials show an incommensurately modulated orthorhombic superlattice, derivable from a pseudotetragonal substructure. The various compositions have the common feature that the major portion of the superconductivity disappears above 85 K but there is a small tail in the diamagnetic susceptibility extending to 110 K, above which the sample becomes paramagnetic. Single crystals, grown by the flux method, exhibit the same incommensurately modulated pseudotetragonal structure as the ceramic, but show a higher diamagnetic shielding and in some cases also have tails. The critical currents in the (ab) plane are only about one‐fifth of the value found for YBa2Cu3O7 crystals and the anisotropy is smaller. These reduced values may be an intrinsic property of ...

Magnetic-field-dependent susceptibility of single-crystal Ho-Mo-S Chevrel phase at very low fields

We report measurements of the temperature dependence of the ac and dc susceptibility of single‐crystal HoMo6S8 oriented both parallel and perpendicular to the magnetic axis in static magnetic fields varying from 0.005 to 100 Oe. We find Tc1 ≂1.70 K and Tc2 ≂0.65 K with significant supercooling in low fields at Tc1 and very long time effects associated with cooling through Tc2 but not on warming. We find a significant difference between the ac and dc susceptibility changes at both transitions. The lower critical field Hc1 is determined to be ∼20 Oe.