“TDPAC study of the phase transitions in PbTi1-xHfxO3”.

R. E. ALONSO; A. LÓPEZ GARCÍA

Ginebra

Congreso; 3rd Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions; 2010

The family of compounds PbTixHf1-xO3 (PHT) presents an important interest due to its ferroelectric properties. Its chemical formula is a particular occurrence of the perovskite-type general one ABO3, in this case ABB’O3, due to the partial substitution of the B cation by other isovalent B’. Regarding the pure constituent oxides, PbTiO3 (PTO) is ferroelectric at ambient temperature with a tetragonal P4mm structure,[8,9] while PbHfO3 (PHO) is antiferroelectric with an orthorhombic Pbam structure. [8,10,11] In the PHT the mix of the primary oxides PTO and PHO produces at room pressure a new structure that is not present in them.[12] The phase diagram of PHT have been previously studied.[12,13] For its statement, different experimental techniques were employed (Impedance Spectroscopy, Differential Scanning Calorimetric, Thermo Gravimetric Analysis, X-ray Diffraction, Neutron Diffraction).[12-17] Between them there are some discrepancies in the temperature at which the phase transitions occur. The different results presented were obtained with different experimental conditions and thermal histories. Moreover, the different methods of sample preparation can affect the grain size, its distribution, and the corresponding phase stability. The Electric Field Gradient tensor (EFG) is a very sensible physical magnitude that strongly depends on the charge distribution in the near neighbour of the probe. Thus, the determination of the variations of the hyperfine parameters as a function of temperature can detect very tiny changes of the surrounding charge distribution at a microscopic level. So, the use of PAC spectroscopy have been demonstrated to be a suitable technique to sense the presence of vacancies, disorder and charge transfer processes, together to allow a precise determination of phase transition temperatures. In an ideal cubic perovskite structure, the EFG at a B site should be null. However, in real compounds there were detected two different kinds of perturbations due to the hyperfine fields: quadrupolar static asymmetric and disordered interactions [21, 22] and nuclear spin relaxation.[25] In a recent work there was explained the origin of the static fields as a result of the presence of oxygen vacancies and the different degree of covalence B-O and B’-O.[22] On the other hand the origin of the nuclear spin relaxation processes in pure ABO3 compounds seems to be related with other different complex dynamic processes, which still were no definitely understood. One way to study the effects of the B->B’ cation substitution in the charge distribution is by means of the hyperfine interactions. As the PHT materials are nonmagnetic, the charge density is manifested through the Electric Field Gradient tensor (EFG) measured at the B site. PAC spectroscopy can accomplish this study if one succeeds in placing a suitable probe at this site. PAC measurements have already been performed for PbZrxTi1-xO3, and cubic BaTixHf1-xO3 as a function of temperature using 181Ta as probes.[18,23] For all these compounds the hyperfine interactions were characterized with static electric quadrupolar interaction, while for BaHfO3 there was detected a spin relaxation interaction.[25] In this contribution we present the electric hyperfine interaction measured by PAC spectroscopy at 181Ta probes in polycrystalline samples of PbHfxTi1-xO3 for x= 0.25, 0.50 and 0.75. The choice and preparation of 181Hf probes via neutron capture warrants B-site location of the daughter 181Ta in the matrix. This opens the possibility to follow the phase transitions at a well-defined lattice site. By measuring the temperature dependence of the EFG we also hoped to gain information on the samples defect structure, due to such structures often lead to additional contributions to the PAC perturbation function. Poster presentation