Journal of Alloys and Compounds 390 (2005) 230–235 Direct synthesis of A 2 (Ti (1 - y) Zr y ) 2 O 7 (A = Gd 3+ ,Y 3+ ) solid solutions by ball milling constituent oxides Karla J. Moreno a , R. Silva Rodrigo b , Antonio F. Fuentes a,∗ a CINVESTAV-IPN Unidad Saltillo, Carretera Saltillo-Monterrey Km 13, Apartado Postal 663, 25000 Saltillo, Coahuila, M´ exico b Divisi´ on de Estudios de Postgrado e Investigaci´ on del Instituto Tecnol´ ogico de Ciudad Madero (ITCM), 89440-Ciudad Madero, Tamaulipas, M´ exico Received 27 May 2004; accepted 13 July 2004 Abstract Different compositions in two solid solutions, A 2 (Ti (1 - y) Zr y ) 2 O 7 (A = Gd 3+ ,Y 3+ ), with high oxygen ion conductivity, have been successfully prepared at room temperature via mechano-chemical synthesis. Stoichiometric mixtures of the constituent oxides were milled in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by using X-ray diffraction showing that in all cases and after milling for 19 h, the powders consisted of a single phase. Powders were also examined by scanning electron microscopy (SEM) finding out that they basically consist of sub-micron size agglomerates and aggregates of nanoparticles. © 2004 Elsevier B.V. All rights reserved. Keywords: Disordered systems; Mechano-chemical synthesis; X-ray diffraction; Ionic conduction 1. Introduction Oxides A 2 B 2 O 7 , with a pyrochlore type of structure, are well known ionic conductors with some of them showing at high temperatures, conductivities of the same order of magni- tude than that of yttria stabilized zirconia (YSZ) [1]. The fully ordered pyrochlore presents cubic symmetry (S.G.: Fd ¯ 3m) and can be described in terms of a superstructure of the ideal defect fluorite structure (cubic, S.G.: Fm ¯ 3m) with twice the cell constant, a ≈ 10 ˚ A [2]. Oxygen mobility in these com- pounds is closely related to structural disorder and while fully ordered pyrochlores such as Gd 2 Ti 2 O 7 , are poor ionic con- ductors, disordered oxides such as Gd 2 Zr 2 O 7 , present high oxygen conductivity. Interesting enough, different degrees of disorder can be reached in systems of solid solutions by using the appropriate substitutions on the A and B sites. Thus, in- creasing disorder is obtained as the ratio of the average radii of cations A and B (R A /R B ) decreases, leading when the ratio approaches unity (complete disorder), to phases with a defect ∗ Corresponding author. Fax: +52 844 4389630. E-mail address: afernandez@saltillo.cinvestav.mx (A.F. Fuentes). fluorite type of structure [3]. In this work, we have focused our attention to the mechano-chemical synthesis of two solid solutions A 2 (Ti (1 - y) Zr y ) 2 O 7 (A = Gd 3+ and Y 3+ ), with a py- rochlore type of structure and with potential application in many areas of technological interest because of their high oxygen conductivities [4,5]. Since preparing them by tradi- tional solid state reactions requires prolonged firing at tem- peratures well above 1000 ◦ C, they are normally prepared by the Pechini ‘liquid mix’ technique [6]. This chemical method starts with the dissolution of metal compounds in a mixture of citric acid and ethylene glycol and implies several heating steps to promote polymerization of metal citrates, eliminate organic constituents and obtain the final products, which is normally achieved at temperatures between 700 and 800 ◦ C. In this paper, we report a direct method for preparing them at room temperature by ball milling constituent oxides. 2. Experimental Several compositions of general formula A 2 (Ti (1 - y) Zr y ) 2 O 7 (A = Gd 3+ and Y 3+ ;0 ≤ y ≤ 1) were 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.07.059