Structural transition in rare earth doped zirconium oxide: A positron annihilation study Keka Chakraborty a , Abhijit Bisoi b , Bichitra Nandi Ganguly b, *, Vinita Grover a , Farheen Nasir Sayed a , A.K. Tyagi a a Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India 1. Introduction Materials having A 2 B 2 O 7 composition (where A represents a rare earth element, with oxidation state +3, B denotes a fourth group transition metallic element, with oxidation state +4), with a pyrochlore lattice structure, has been the subject of intensive interest due to its wide applicability, especially as solid oxide fuel cells [1–3]. Such complex oxide compounds could be synthesized by chemical route [4] involving gel combustion method and could be characterized by powdered XRD method [5]. These complex oxides are considered potentially useful as optoelectronic devices [6] owing to the large band gap inorganic lattices activated with rare earth ions. They are also considered structurally viable material for high level rad-waste disposal [7] as their structural stability and resistance toward amorphization by irradiation is correlated by the propensity of cation-antisite defects. It was suggested that compounds with similar cationic radii would behave more robustly in a radiation environment. However, this radiation response cannot be solely dependent on the cationic radius ratio (A:B) alone, but short range covalent interactions and long range ionic forces [8] could also play a vital role together with the topological freedom of the lattice structure [9]. An interplay of the chemical properties has been tested by changing the composition of A- and B-site cations in the pyrochlore lattice to correlate the electronic structure and resistance against amorphization. For example, zirconates (A 2 Zr 2 O 7 ) with strong ionic character and electronic structure was found to be reasonably the fittest candidate against strong radiation environment [1,10]. It has also been conjectured, that a transformation from pure pyrochlore to defect fluorite structure would accommodate a better and suited condition for a radiation resistive character against amorphization [7,11]. Further in this work, an attempt has been made to vary the ‘A’ site cations to investigate the structural changes leading to order– disorder transitions between the said lattice configurations. It is known that Sm 2 Zr 2 O 7 is an ordered pyrochlore compound, while Dy 2 Zr 2 O 7 is reported to be defect fluorite structure [12] and similar compounds were studied recently [13,14]. This pyrochlore compound was gradually substituted with heavier rare earth element, Dy, yielding a compound of the type Sm 2x Dy x Zr 2 O 7 , and such type of doped rare earth oxide compounds are eventually found to be potential ionic conductor and a photo catalyst [15]. While the X-ray crystallographic study reports a retention of single phasic structure of the compound up to about 40 mole% doping of Dy 3+ , an order–disorder phase transition beyond this is evident, resulting ultimately in a defect fluorite structure as in Dy 2 Zr 2 O 7 [12]. Such a transformation results in randomized defect formation in the lattice which can be explored and the phase transition can be detected through suitable and sensitive microstructural probe such as positron annihilation spectroscopy (PAS) which has gained its importance in gauzing very subtle effects in electronic microenvironment [16,17]. Positrons as Materials Research Bulletin 47 (2012) 3660–3664 A R T I C L E I N F O Article history: Received 21 October 2011 Received in revised form 24 April 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: A. Ceramics, Inorganic compound, C. Positron annihilation spectroscopy, X-ray diffraction, D. Crystal structure, Defects A B S T R A C T A series of compounds with the general composition Sm 2x Dy x Zr 2 O 7 (where 0  x  2.0) were synthesized by chemical route and characterized by powder X-ray diffraction (XRD) analysis. The rare earth ion namely Sm +3 in the compound was gradually replaced with another smaller and heavier ion, Dy +3 of the 4f series, there by resulting in order–disorder structural transition, which has been studied by positron annihilation lifetime and Doppler broadening spectroscopy. This study reveals the subtle electronic micro environmental changes in the pyrochlore lattice (prevalent due to the oxygen vacancy in anti-site defect structure of the compound) toward its transformation to defect fluorite structure as found in Dy 2 Zr 2 O 7 . A comparison of the changes perceived with PAS as compared to XRD analysis is critically assayed. ß 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +91 33 23375345; fax: +91 33 23374637. E-mail address: bichitra.ganguly@saha.ac.in (B.N. Ganguly). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.044