H. Saitovitch

Perturbed angular correlation measurements at 181Ta in Hf2Pd and Zr2Pd tetragonal MoSi2-type phases

Abstract The electric quadrupole interaction at 181 Ta in Hf 2 Pd and Zr 2 Pd compounds was studied by the perturbed angular correlation method in a broad temperature range. The powder X-ray analysis verified a single-phase C11 b structure of the samples. The least-squares fits of the perturbation factor to the experimental spectra measured for Hf 2 Pd and Zr 2 Pd compounds yielded an electric interaction with a well-defined single quadrupole frequency ( v Q = 305(2) MHz for Hf 2 Pd and 291(4) MHz for Zr 2 Pd) and asymmetry parameter η = 0, reflecting the existence of one axially symmetric probe site. A decrease in the quadrupole interaction frequency with an increase of temperature is very rapid for both Hf 2 Pd and Zr 2 Pd compounds.

PAC study of nuclear relaxation in solids:181Ta in hafnium hydrides

The electric quadrupole interaction (QI) at the site of181Ta in different hafnium hydrides HfHx has been investigated as a function of temperature by TDPAC measurements. Below 300 K the QI is static within a 300 nsec time window. Above 300 K a dynamic QI related to the diffusion of hydrogen atoms leads to relaxation effects in the TDPAC spectra. The average residence time between hydrogen jumps, evaluated on the basis of the theory of stochastic perturbations, ranges from 100 nsec at 400 K in the ɛ-phase to 1 nsec at 700 K in the α-phase of the hafnium-hydrogen system. The activation energy for hydrogen jumps was found to be independent of the hydrogen concentration: Ea=0.53(12) eV.

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.

TDPAC characterization of Ni2HfF8·12H2O and its decomposition products

The hyperfine interaction in Ni2HfF8·12H2O has been determined between 77 K and 1100 K by means of the time-differential perturbed angular correlation technique. From 200 K on, the one-site phase existing at lower temperatures undergoes a gradual phase transition until, at room temperature, the populations of both phases attain a 2:1 ratio. While the quadrupole frequencies characterizing them exhibit aT3/2 thermal dependence, their population ratio seems to obey a Boltzmann distribution. At 350 K, when the η-value of the high temperature phase electric field gradient approaches its maximum value, the starting compound decomposes to NiHfF6·6H2O. A kinetics study of the Ni2HfF8·12H2O recovery at room temperature seems to indicate that a tri-dimensional diffusion mechanism is responsible for the corresponding reaction process. The first decomposition product of NiHfF6·6H2O left to atmospheric pressure is found to be NiHfF6·4H2O at 368 K and, between 414 K and 590 K, the high temperature cubic phase of NiHfF6 and Hf2OF6 can be simultaneously observed. Finally, monoclinic HfO2 appears from 1020 K on, having been preceded by an interaction which can be though of as depicting a preliminary stage in hafnia formation.

Electric field gradient and its temperature variation at a 181Ta probe in Zr2Fe and Zr2Co intermetallic compounds

Abstract In this paper we report a time differential perturbed angular correlation study of electric field gradients and their temperature variation in Zr 2 T (T = Fe,Co) intermetallic compounds (C16 crystallographic structure). Measurements were performed at a 181 Ta radioactive probe in the temperature range 20 K–1173 K. In both samples only one strong asymmetric quadrupole interaction was observed, consistently with one possible crystallographic position of the probe atom in the studied structure. The quadrupole coupling constant is equal to 1039(8) MHz with asymmetry parameter η = 0.82 and 889(7) MHz with η = 1 at room temperature for Zr 2 Fe and Zr 2 Co respectively. In the investigated temperature range a linear dependence of electric field gradient was observed in contrast to the T 3 2 dependence found in most intermetallic compounds and non-cubic metals.

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

Angular correlation study of hyperfine interactions in vanadium sesquioxide V2O3

The hyperfine interaction at dilute 111Cd impurities in vanadium sesquioxide V2O3 has been investigated as a function of temperature by time differential perturbed angular correlation measurements. The authors have observed a pure electric quadrupole interaction in the metallic phase (M) and a combined magnetic dipole and electric quadrupole interaction in the antiferromagnetic, insulating phase (I). The electric field gradient (EFG) undergoes a first-order change at the M-I transition at Tt=160K from 8.2*1017 V cm-2 in the I phase to 6.3*1017 V cm-2 in the M phase, but varies smoothly with temperature in the region of the high-temperature resistivity anomaly. In the metallic phase the EFG increases with increasing temperature. A point-charge lattice-sum calculation suggests that this behaviour results from the thermal variation of symmetry and dimension of the oxygen sublattice. The magnetic hyperfine field at 111Cd in the antiferromagnetic phase of V2O3 has a saturation value of 15(1) kOe and forms an angle beta =68(2) degrees with the direction of the principal EFG component. The field vanishes at the M-I transition temperature Tt. Its temperature variation follows a Brillouin function with an extrapolated Neel temperature TN(TN>Tt), which depends sensitively on the impurity concentration of the sample.