Paulo R. J. Silva

Strongly disordered Heisenberg spin-1 chains : vanadium warwickites

Abstract The structure of the compound MgVOBO3 is characterized by the presence of ribbons where the spin-1 V3+ ions are randomly located. We present X-ray, susceptibility χ(T) as a function of temperature and high-field magnetization measurements m(H) (up to 45 T) on powdered samples. These give evidence that in this system, with strong disorder on the magnetic chains, there are arbitrarily low-energy excitations which are responsible for the power-law dependence of χ(T) and m(H) that we observe. We find that the magnetic behaviour is reminiscent of the disordered spin-½ antiferromagnetic chain. Our theoretical results for the strongly disordered spin-1 antiferromagnetic Heisenberg chain substantiate the experimental observations.

Interações hiperfinas em catalisadores metálicos

Heterogeneous catalysts are of fundamental importance in several modern chemical processes. The characterization of catalysts is an issue of very present interest as it can provide a better understanding of the fundamental aspects of the catalytic phenomena, thus helping in the development of more efficient catalysts. In order to extend and improve the characterization of catalysts, new and less conventional methods are being applied, such as nuclear spectroscopies. In this paper we focus on the application of angular correlation, with can be used to resolve different local environments of probe atoms in solids and can be applied, as shown here, in the characterization of heterogeneous catalysts. A brief theoretical introduction is given and experimental results related to catalytic systems of alumina and niobia-supported Pt-In and Pd-In catalysts are presented.

Hyperfine interactions studies on Pt–In/Nb2O5 catalysts

Abstract Pt–In/Nb 2 O 5 catalysts were investigated using temperature-programmed reduction (TPR) and time differential perturbed angular correlation (TDPAC) experiments. The results indicated the presence of In–O surface complexes for low loading In/Nb 2 O 5 and Pt–In/Nb 2 O 5 catalysts after calcination. These complexes did not form a In 2 O 3 crystalline phase. After reduction of the Pt–In/Al 2 O 3 catalyst, In is present in different states. A fraction of In atoms is bonded to niobia surface, as a surface complex that does not show crystalline structure similar to bulk In 2 O 3 . Other fraction of In atoms interacts with platinum, in the form of an alloy, in locations that present trigonal symmetry.

Angular correlation measurements in (Ag, In)/CdCr2Se4

We report time differential perturbed angular correlation (TDPAC) measurements above and below the Curie temperature of the ferromagnetic semiconductor (Ag, In)/CdCr2Se4. The TDPAC spectra for the Ag and In dopants have profound differences which we attribute to distinct site occupations of these impurities in the spinel structure.

Electric quadrupole interaction measurements in YBa2Cu3O x and related compounds

In this work we present Electric Quadrupole Interaction (EQI) measurements, made by Time Differential Perturbed Angular Correlation (TDPAC), on111Cd in YBa2Cu3Ox and some related compounds. These studies were intended to determine the relationship between the EQI and the actual probe site. The probes were introduced into the materials as a diluted111In-complex or via In(111In)2O3. Our observations indicated that there is no need to suppose the presence of many probe sites in YBa2Cu3Ox to explain the experimental results.

Mo¨ssbauer study of the superconductor57Fe: YBa2 Cu3OY

Abstract YBa 2 (Cu 1−x Fe x ) 3 O Y superconductive compounds were prepared in the pure orthorhombic phase with x=0.001, 0.005, 0.01, 0.03 and 0.10. The DC magnetization measurement revealed that the iron atoms induce only a broadening in the superconducting transition for x⩽0.01 but T c clearly decreases for higher x values. Even with x=0.1 the samples exhibit a superconducting transition despite the fact that the 4.2 K Mo˝ssbauer spectra (MS) show that iron has a magnetic moment. The MS indicated that for x

Hyperfine interactions on Pt-In catalysts

The effect of indium loading in Pt-In/Al 2 O 3 catalysts was investigated by time differential angular correlation. For high In loading In/Al 2 O 3 samples, spectra similar to bulk In 2 O 3 were obtained after calcination, and metallic bulk In spectra were obtained for the reduced samples. For low loading In samples, frequency distribution spectra were obtained at low temperatures and relaxations at higher temperatures. In the case of high loading Pt-In/Al 2 O 3 samples, there was not a formation of bulk-like alloy structures. For lower contents of Pt and In, frequency distribution spectra were observed at low temperatures and relaxations at higher temperatures. The interaction between Pt and In was confirmed, and a mechanism of promotion was proposed.

A study of platinum-supported catalysts through hyperfine interactions

The effect of indium addition on alumina-supported platinum catalysts was investigated by measurements of hyperfine interactions. Via lime differential perturbed angular correlation spectroscopy (TDPAC) on111Cd, Pt/Al2O3 catalysts were studied in the flow of a heptane/H2 gas stream. The results indicate that some amount of indium sticks to platinum which is then dispersed on the support surface. The amount of In that is free from platinum is mobile under reaction conditions, being capable of diluting platinum particles and draining off coke precursors from the platinum surface.