Alejandro Ayala

Temperature dependence of the nuclear quadrupole interaction at Zr-Ti sites in in the Zr-rich rhombohedral and cubic phases

In this paper, the hyperfine quadrupole interaction at the Zr-Ti site of (PZT) polycrystalline samples is studied for the first time. Powders for x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9 and 1 were prepared and characterized by XRD. The lattice constants and space groups were obtained as functions of Ti concentration. PAC analyses were done for ferroelectric PZT for x = 0.9 and 0.8 as a function of temperature, and for x = 0.6 at RT. Also antiferroelectric was analysed. Low-concentration nuclei were used as probes. In the ferroelectric and paraelectric phases of PZT compounds two sites were occupied by probes. For each site the quadrupole frequency, asymmetry and relative distribution width parameters were obtained as functions of temperature above and below the Curie temperature. For only one site was detected with a static quadrupole interaction below and a nuclear spin-relaxation process above it. Furthermore, the temperature dependence of the hyperfine parameters, the possible origin of both quadrupole interactions in PZT and the perturbation that characterizes the cubic phases of and compounds are discussed.

Stabilization of β-Bi2O3 by hafnia

Abstract The addition of HfO 2 into Bi 2 O 3 causes stabilization at RT of the β-phase. This material is characterized by XRD, TEM, EDAX and time-differential perturbed-angular-correlation spectroscopy (PAC). The β and δ phases were detected and characterized. The oxygen vacancy occupation is compared with β-Bi 2 O 3 stabilized by other oxides.

Hyperfine Interactions in the Cubic Phase of BaTi x Hf 1− x O 3

The hyperfine quadrupole interaction at probes in sites with cubic point group symmetry was measured in many perovskite-type compounds. This interaction is commonly associated to the presence of oxygen vacancies close to probes. The effect of this point defect on 181 Ta probe in BaTi x Hf 1 m x O 3 for x = 0.70, 0.50, 0.30, 0.10, 0.05, and 0.01, is studied. The lattice constant of these oxides at room temperature were measured using XRD technique. The quadrupole parameters corresponding to probe-defect interaction were obtained by means of Perturbed Angular Correlation (PAC) spectroscopy as functions of both, composition and temperature. A static, asymmetric and distributed quadrupole interaction was fitted to all PAC data. This interaction was observed to be strongly composition dependent. These results and those corresponding to compositions 0.75 h x < 1 were compared to the ones obtained using the point charge method for the calculation of the electric field gradient. The proposed model took into account polarized oxygen vacancies, different covalence of Ti--O and Hf--O bonds and using computational simulation for cations and oxygen vacancies lattice positions.

Phase transitions in the SrHfO3 perovskite measured with 111In/111Cd PAC

The temperature dependence of the hyperfine parameters in SrHfO3 powder samples has been investigated by means of Perturbed Angular Correlation spectroscopy using implanted 111In probes. Three quadrupole interactions have been established, with the largest fraction showing a pronounced dynamic interaction. We assign this fraction to 111In / 111Cd probe atoms on substitutional Hf sites. The temperature dependence of the dynamic interaction has been associated to the Pnma↔Imma phase transition at ∼700 K. We discuss the results in relation to those obtained for 181Hf / 111Ta-probes in AHfO3 (A=Ba, Sr, Ca) and for 111In / 111Cd-probes in PbZrO3 and BaTiO3.

Hyperfine interactions in cubic perovskites

ABO3 perovskites display several physical properties determined by the characteristics of A and B cations. These compounds have cubic structure at high temperature. Lower symmetry cells that are distorted cubes are found at low temperature. Defects modify the properties of these compounds. Under standard conditions oxygen vacancies are produced. Cation substitution also alters the characteristics of perovskites. These materials have been studied by Perturbed Angular Correlation (PAC) spectroscopy and other hyperfine techniques. In this way abundant information is available to determine charge distributions close to probes. In the cubic phase perturbations were detected that are produced by the interaction of probes with defects. To show up these effects we analyze the quadrupole interaction at 181Ta in several compounds: ABO3 with A=Ca, Sr and Ba, BaTi1−xHfxO3 and PbZr1−xTixO3 for 0⩽x⩽1. Three different quadrupole interactions were found and are interpreted in terms of distinct probe-oxygen vacancy configurations.

About the perturbation factor for static and distributed quadrupole interaction in PAC spectroscopy

The traditional analysis of electric quadrupole interaction in disordered materials can give hyperfine parameters that would mask the actual values. As the relative distribution width δ increases, the correlation of this parameter with the rest produces strong shifts in all of them. Motivated by the values δ ⩾ 0.4 fitted in BaTi1-xHfxO3 for 0 ⩽ x ⩽ 0.2 and other perovskites we extend previous calculations for Gaussian distribution function to the interval 0.3 ⩽ δ ⩽ 1. We also include the study of correlation between quadrupole parameters for Lorentzian distribution function in the 0 ⩽ δ ⩽ 1 interval.

The stability of PbZr0.52Ti0.48O3 prepared by the sol-gel method

Samples of PbZr 0.52 Ti 0.48 O 3 (PZT) were fabricated by the Sol-Gel method and characterized by X-ray diffraction analysis. The Rietveld method was used to obtain lattice parameters and grain sizes as a function of the annealing temperature. A powder sample previously annealed at 873 K was irradiated to activate the 181 Ta probe. Then, Perturbed Angular Correlation (PAC) spectra were measured at RT and temperatures above 873 K to study the thermal behaviour on samples capsulated in SiO 2 tubes. The hyperfine parameters observed as a function of temperature show changes in this PZT compound that would correspond to thermal evolution of defects. Furthermore, the reaction of PZT with the substrate was observed at 1173 K, obtaining at this temperature the hyperfine parameters corresponding to SiZrO 4 . At this temperature no trace of the PZT was left.