J. B. Torrance

Synthesis, crystal structure, and properties of metallic PrNiO3: Comparison with metallic NdNiO3 and semiconducting SmNiO3

Abstract The compound PrNiO3 has been prepared for the first time. Using only moderate oxygen pressure, PrNiO3, NdNiO3, and SmNiO3 were made and found to crystallize in the GdFeO3—type orthorhombically distored perovskite structure. Structural refinements for all three compounds reveal NiO6 octahedra with an average NiO distance of 1.94–1.95 A, about the same as in the more distorted HoNiO3. The electrical conductivity of PrNiO3 is metallic at room temperature, but undergoes a transition at 130 K to an insulating state. Examination of the conductivities of the corresponding Sm, Nd, and La compounds reveals a monotonic pattern of behavior: as the rare earth radius is increased, the compounds become more conducting because the metal-insulator transition temperature decreases from 400 K(Sm) to 200 K(Nd) to 130 K(Pr) to none for La. From DSC and lattice constant measurements, this transition is shown to be first order, with a ∼0.2% contraction upon heating into the metallic state. Low temperature neutron measurements in the insulating phase reveal new diffraction peaks, probably related to magnetic ordering of Ni and/or RE moments.

A new, simple model for organic ferromagnetism and the first organic ferromagnet

Abstract A new and simple model is presented for an organic ferromagnet containing segregated stacks of radical ions. This new model gives a conceptual framework for selecting candidate molecules, and hence appears to be a promising, general guide for designing new organic ferromagnetic materials. The discovery of the first organic ferromagnet is reported. It is based on a polymer obtained from reacting s-triaminobenzene with iodine. The reaction is complex and the resulting polymer is not very reproducible. Nevertheless, on a number of occasions a ferromagnet material has been obtained. This material remains ferromagnetic to high temperatures, until it decomposes near 400°C.

New class of high Tc structures: Intergrowth of multiple copper oxide perovskite-like layers with double sheets of BiO

Abstract The structures of two new BiCuO superconductors are determined from their X-ray powder diffraction patterns. Both materials are members of a new structural type: intergrowths of two BiO sheets with n ( n =1,2,3…) copper oxide perovskite-like layers. The n =1 compound has a single La 2 CuO 4 -like layer, a composition (BiO) 2 Sr 2 CuO 4 and T c =6 K . The n =2 compounds has layers containing two CuO 2 sheets, similar to La 2 CaCu 2 O 6 . It has a range of compositions near ( BiO ) 2 Sr 2− x Ca 1+ x Cu 2 O 6 , with T c near 75IK.It is speculated that the phase with T c near 120K corresponds to another compound in this new series with n > 2.

Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene–chloranil (TTF–CA) charge transfer complex

The infrared and Raman spectra for the room temperature, quasineutral, and the low temperature, quasi‐ionic, phases of the mixed stack charge transfer complex tetrathiafulvalene–chloranil (TTF–CA) are reported. The analysis of the analogous data for a newly synthesized room temperature phase point to a dimerized segregated stack structure. All the vibrational data are interpreted and exploited through a clear identification of the differences, for the two types of stacks, in the spectroscopic effects due to the vibronic interaction, i.e., the coupling between electron and molecular vibration (e‐mv). It is shown that for distorted mixed stack complexes both Raman and infrared spectra can be substantially influenced by the vibronic interaction, whereas the dimerized segregated stack complexes, as already known, display striking vibronic effects only in infrared. The theoretical model which explains the origin of these effects is briefly summarized and its extension to mixed stack structures successfully use...

New procedure for determination of [Cu-O]+p charge and oxygen content in high Tc copper oxides

Abstract The properties of High Tc Superconductors are critically dependent on the degree of oxidation, i.e., on the average [Cu-O]+p charge (or formal oxidation state Cu+2+p of copper). This fact makes it important to accurately characterize High Tc copper oxide samples in term of this key parameter. We describe here a simple and reliable procedure for determining the effective copper oxidation level and, as a result, the oxygen content. Using a modification of standard iodometric titration techniques, our procedure involves the ratio of the results of two titrations: one to determine the copper concentration in the sample and the second to measure the degree of oxidation beyond Cu+. Using this ratio, numerous errors and difficulties are minimized. In addition, the exact stoichiometry of the sample need not be known. Experimental results on YBa2Cu3Oy are discussed which demonstrate the accuracy, reproducibility and reliability of our procedure.

Why are some oxides metallic, while most are insulating?

Abstract A large variety of undoped binary and ternary transition metal oxides, including formally divalent, trivalent and tetravalent metal cations, have been examined. These 76 compounds are classified as either “metals”, “insulators”, or having a “metal-to-insulator” transition. In an attempt to understand these variations, the Zaanen-Sawatzky-Allen framework was used in which each compound can be characterized by three parameters: the Coulomb correlation or disproportionation energy (U′), the charge-transfer energy (Δ) and the bandwidth (W). Assuming W is constant, we have calculated U′ and Δ using a simple ionic model, which includes only the gas phase ionization potentials and the bare electrostatic Coulomb interactions between the ions. With this model, the occurence of metallic conductivity is remarkably well accounted for in these oxides.

Role of the Madelung energy in hole conductivity in copper oxides: Difference between semiconductors and high-Tc superconductors.

The difference in Madelung site potential, {Delta}{ital V}{sub {ital M}}, between a hole on a copper site and one on an oxygen site can control the holes site preference and ability to delocalize. Among a variety of copper oxide structures, {ital e}{Delta}{ital V}{sub {ital M}} is shown to vary by {similar to}3 eV and to clearly separate two classes of compounds: Those with {ital e}{Delta}{ital V}{sub {ital M}}{gt}47 eV are metallic with high {ital T}{sub {ital c}} and delocalized holes, whereas those with lower {Delta}{ital V}{sub {ital M}} are semiconducting because their holes are self-trapped on oxygen.

Behavior of charge‐transfer absorption upon passing through the neutral‐ionic phase transition

The charge‐transfer band is determined from reflectance measurements on single crystals of TTF‐chloranil from 300 to 45 K, passing through the neutral‐ionic phase transition at 84 K. As the temperature is decreased from 300 K toward the transition, hνCT decreases slowly from 0.66 to 0.55 eV, perhaps the lowest energy observed for a charge‐transfer band. Below this transition, hνCT increases slightly and a second band appears at higher energy. From the measured oscillator strength, the temperature dependence of the degree of charge transfer has been determined. It is found to increase from ∼0.20 to ∼0.30 as the temperature is lowered from 300 to 100 K. Below TC, the degree of charge transfer rises to a value near 0.70.

Simple and perovskite oxides of transition-metals: Why some are metallic, while most are insulating

Abstract Some 76 simple and perovskite transition-metal oxides are classified as “metals,” “insulators,” and those exhibiting metal-insulator transitions. Using the framework of Zaanen, Sawatzky, and Allen and a simple ionic model to estimate the two relevant energies ( Δ 0 and U ′ 0 ), we can find boundaries which separate the insulating oxides from two types of metals: low Δ 0 metals and low U ′ 0 metals. In addition, compounds with metal-insulator transitions are found to be on (or near) these boundaries. It is concluded that the large differences in conductivity behavior of oxides are largely due to differences in the ionization potentials of the transition metal cations.

Extra oxygen in electron superconductors: Ce and Th doped Nd2CuO4+δ and Gd2CuO4+δ

Abstract Using iodometric titration techniques, we have systematically studied the oxygen content of the title compounds. Our oxidized and reduced samples both show an excess of oxygen above the nominal value of 4. This extra oxygen tends to partially compensate the effect of the tetravalent cation doping and must be considered for a complete understanding of the doping of electron superconductors. The partial removal of some of this extra oxygen is presumably the reason for the necessity of a reduction step in the preparation of electron superconductors. In the case of our Ce doped Gd 2 CuO 4 samples, there is an especially high concentration of extra oxygen and we are unable to make them superconducting.