Thermal evolution of La 3þ /ZrO 2 solid solutions obtained by mechanochemical activation M.C. Fuertes a , W. Salgueiro b , A. Somoza b,c, * , J.M. Porto L opez a a INTEMA (CONICET-UNMdP) Avda J.B. Justo 4302, B7608FDQ Mar del Plata, Argentina b Instituto de Fisica de Materiales Tandil (IFIMAT), Facultad de Ciencias Exactas, UNCentro, Pinto 399, B7000GHG Tandil, Argentina c Comisi on de Investigaciones Cient  ıficas de la Provincia de Buenos Aires, Argentina Received 6 June 2003; received in revised form 2 September 2003; accepted 17 September 2003 Abstract A study on the microstructural evolution with the temperature of La 3þ /ZrO 2 cubic solid solutions obtained by mechanochemical activation of mixtures ZrO 2 –La 2 O 3 is presented. The development of crystallinity with thermal treatments in the range 1073–1673 K was followed by means of positron lifetime measurements and X-ray diffraction as main experimental techniques. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Mechanochemistry; Zirconium oxide; Lanthanum oxide; Positron annihilation spectroscopy 1. Introduction Positron annihilation lifetime spectroscopy (PALS) is specially suited for the study of open volume defects and microstructural features in solids, due to the well-known sensitivity of positrons towards local electron density changes. This technique has been successfully used for the study and characterization of defects in condensed matter [1]. Different positron annihilation states can be obtained from the decomposition of the lifetime spectra and they are correlated with structural open volume defects in metals and semiconductors [2], alloys [3], as well as ceramics [4–7]. In the last case, the bibliography is scarce. Furthermore, PALS has been successfully used to study the formation of sub-nanometric holes in po- rous media, for instance in zeolites [8,9]. For this reason, its application to the study of solid solutions (SS) con- sisting in aliovalent cations introduced into inorganic oxides is of particular interest, since conventional ex- perimental techniques do not yield the information needed for a complete understanding of the involved processes. In the La 2 O 3 –ZrO 2 system, La 3þ introduction as substitutional solute has a significant effect on the ionic conduction properties of the oxide, in addition to sta- bilizing the high-temperature cubic structure. The con- ventional way for the synthesis of these SS involves thermal treatment of the oxides at high temperatures (>1673 K) during long times, followed by quenching to preserve the high-temperature form in metastable con- ditions at room temperature. The synthesis of the SS by mechanochemical activa- tion offers an alternative to these extreme experimental conditions [10]. However, several aspects of the physi- cochemical and thermal behavior of the SS, as well as their thermal evolution to form La 2 Zr 2 O 7 , are not completely elucidated, mainly from the point of view of the role of structural defects in determining that be- havior. A positron implanted in this type of materials can be annihilated by means of different processes: (i) A free positron annihilates with an electron in the bulk of the material and/or in open volume defects like vacancies, vacancy-clusters and voids (or mi- crovoids) and as a result different positron lifetimes are obtained, depending on the open volume associ- ated with the annihilation site. (ii) A positron associates with an electron of the me- dium in a free volume to form a bound state called * Corresponding author. Address: Instituto de Fisco de Materiales Tandil (IFIMAT), Facultad de Ciencias Exactas, UNCentro, Pinto 399, B7000GHG Tandil, Argentina. Tel.: +54-229344-2821; fax: +54- 2293-444190. E-mail address: asomoza@exa.unicen.edu.ar (A. Somoza). 1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.09.045 Scripta Materialia 50 (2004) 301–305 www.actamat-journals.com