Journal of Solid State Chemistry 178 (2005) 2301–2305 Structural and thermal investigation of gadolinium gallium mixed oxides obtained by coprecipitation: Observation of a new metastable phase Marco Bazzoni a , Marco Bettinelli b , Matteo Daldosso b , Stefano Enzo a,Ã , Filomena Serra a , Adolfo Speghini b a Dipartimento di Chimica, Universita` di Sassari, via Vienna n. 2, I-07100 Sassari, Italy b Dipartimento Scientifico e Tecnologico, Universita` di Verona, Ca’ Vignal 1, strada le Grazie 15, I-37134 Verona, Italy Received 11 March 2005; received in revised form 6 May 2005; accepted 10 May 2005 Abstract Polycrystalline gadolinium gallium mixed oxides were prepared by coprecipitation and annealing at various temperatures below 1000 1C. The oxide materials appear to be X-ray amorphous after a heat treatment at 500 1C for 30 h, but after 30 h at 800 and 900 1C a major, unreported, hexagonal phase, isostructural with TAlO 3 compounds (where T ¼ Y, Eu, Gd, Tb, Dy, Ho, Er) appears to crystallize. On the other hand, a highly energetic mechanical treatment of the amorphous powder previously annealed at 500 1C changes considerably the shape and position of exothermal events occurring in the range from 700 up to 900 1C. Subsequent annealing at 900 1C of the mechanically treated powder gives rise to the complete formation of the Gd 3 Ga 5 O 12 garnet structure at the expense of the hexagonal phase and of the minor Gd 4 Ga 2 O 9 oxide phase. However, a 7.0 wt% contamination is found to be due to tetragonal zirconia coming from vials and balls colliding media. The garnet phase may have strong deviations from the nominal stoichiometry of the garnet, as suggested by the refined lattice parameter obtained from the powder diffraction patterns and by the remarkable absence of intensity relative to the (220) Bragg peak position. r 2005 Elsevier Inc. All rights reserved. Keywords: Gadolinium galilium oxide system; Metastable state; Powder diffraction; Coprecipitation synthesis; Thermogravimetry; Mechanical treatment; Rietveld refinement 1. Introduction Materials belonging to the Gd 2 O 3 –Ga 2 O 3 system find important technological applications. In particular, single crystals of gadolinium gallium garnet (Gd 3 Ga 5 O 12 , GGG), mainly grown by the Czochralski technique, have been employed as substrates for magnetic bubbles [1–3] and, when doped with trivalent lanthanide ions (Ln 3+ ), are important laser crystals [4,5]. Moreover, it has been recently shown that nanocrystalline powders of Ln 3+ doped GGG can give rise to efficient IR-to-visible upconversion [6–9]. For these reasons, a considerable effort has been carried out, in order to obtain a detailed understanding of the Gd 2 O 3 –Ga 2 O 3 binary system. The equilibrium phase diagram of the Gd 2 O 3 –Ga 2 O 3 system was determined by Nicolas et al. [10], after substantial contributions on the subject by DiGiuseppe et al. [11], while Allibert et al. [12] and Brandle and Barns [13] pointed out the existence of non-equilibrium phases and off-stoichiometric compounds. In addition to this, Allibert et al. [12] adopted the ‘‘citric complex’’ method for the solid-state synthesis of oxides, which is very similar to the ‘‘modern’’ methods of synthesis by co- precipitation. Thermal properties of GGG were studied by Daudin et al. [14] with a view to their magnetic refrigeration applications. Following this, magnetic ARTICLE IN PRESS www.elsevier.com/locate/jssc 0022-4596/$-see front matter r 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2005.05.017 Ã Corresponding author. Fax: +39079229559. E-mail address: enzo@uniss.it (S. Enzo).