Journal of Electroceramics, 10, 153–164, 2003 C  2003 Kluwer Academic Publishers. Manufactured in The Netherlands. Mechanically-Activated Synthesis and Mixed Conductivity of TbMO 4−δ (M = Zr, Hf) Ceramics E.V. TSIPIS, 1 A.V. SHLYAKHTINA, 2 L.G. SHCHERBAKOVA, 2 I.V. KOLBANEV, 2 V.V. KHARTON, 1,3, ∗ N.P. VYSHATKO 1 & J.R. FRADE 1 1 Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal 2 Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kozigina Str., 119991 Moscow, Russia 3 Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus Abstract. Terbium hafnate and zirconate ceramics with submicron grain sizes were prepared via mechanically- activated synthesis. X-ray and electron diffraction and infrared (IR) absorption spectroscopy showed that TbZrO 4−δ has a disordered fluorite-type structure, while TbHfO 4−δ is partially ordered, containing pyrochlore microdomains. The oxygen ion transference numbers determined by the modified e.m.f. technique under oxygen/air gradient, vary in the range 0.08–0.26 at 873–1123 K, increasing with temperature. The activation energies for ionic and p-type electronic transport are 82–83 and 29–40 kJ/mol, respectively. The ionic conduction becomes domi- nant in reducing atmospheres, but tends to decrease at low p(O 2 ). Oxygen partial pressure dependencies of Seebeck coefficient can be described by a model common for oxide phases with mixed ionic and electron-hole conductivity. Due to partial cation ordering, terbium hafnate exhibits lower ionic and hole transport as com- pared to TbZrO 4−δ . The average thermal expansion coefficients of TbMO 4−δ (M = Zr, Hf) ceramics in air, calculated from dilatometric data, are (11.5–12.4) × 10 −6 K −1 at 600–1200 K and (18.4–20.3) × 10 −6 K −1 at 1200–1420 K. Keywords: terbium zirconate, terbium hafnate, fluorite, mechanical activation, mixed conductor Introduction Numerous oxide materials with pyrochlore- and fluorite-type crystal lattices possess a significant oxy- gen ionic conductivity and thus are of interest for high-temperature electrochemical applications, includ- ing solid oxide fuel cells (SOFCs), gas separation membranes and sensors [1–7]. For this group of solid electrolytes, the highest ionic transport is observed for Bi 2 O 3 − , CeO 2 - and ZrO 2 -based fluorite materials [5–7]. However, a substantially high level of ionic conduction is also characteristic of some pyrochlores, such as Gd 2 Zr 2 O 7 or Ca-doped Gd 2 Ti 2 O 7 [1–4]. The lattice of A 2 B 2 O 7 pyrochlore can be consid- ∗ To whom all correspondence should be addressed. E-mail: kharton@cv.ua.pt ered as a superstructure based on fluorite (A,B)O 2 , where the cations and anion vacancies are both or- dered [8, 9]. The pyrochlore structure contains one vacant oxygen site per formula unit, which would be occupied in case of fluorite; these unoccupied sites may provide pathways for fast oxygen transport [8, 9]. At elevated temperatures, typically around 1650– 2500 K, most pyrochlores disorder into fluorite poly- morphs [3, 9]. Decreasing A-site cation radius in- creases stability of fluorite modifications, thus favoring “pyrochlore → fluorite” transition [3, 4, 7–9]. When induced by such cation composition variations, this transformation is rather gradual and occurs via par- tial ordering of various types [3, 10]. In particular, for Tb 2 Zr 2 O 7 where the A/B cation radius ratio is close to the “pyrochlore-fluorite” boundary, X-ray diffrac- tion (XRD) analysis suggests a fluorite structure [3, 4],