Room-Temperature Synthesis and Electrical Properties of La, Nd and Gd Apatite-Type Silicates
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Room-Temperature Synthesis and Electrical Properties of La, Nd and Gd Apatite-Type Silicates Antonio F. Fuentes1, Luis G. Martinez-Gonzalez1, Karla J. Moreno1, Evelyn RodriguezReyna1, and Ulises Amador2 1 Unidad Saltillo, Cinvestav, Carretera Saltillo-Monterrey Km. 13, Ramos Arizpe, Coahuila, 25900, Mexico 2 Departamento de Quimica, Facultad de Farmacia, Universidad San Pablo-CEU, Boadilla del Monte, Madrid, 28668, Spain ABSTRACT We show in this paper the possibility of using mechanical milling to prepare apatitetype La, Nd and Gd silicates starting from stoichiometric mixtures of the constituent oxides. XRD patterns collected after grinding the starting mixtures for 9 hours contain only the characteristic reflections of the target materials with no other phase apparently present. Electrical conductivity data were successfully fitted to a Jonscher-type empirical expression with fractional exponent n included in the 0.35-0.75 range. Activation energies for oxygen migration were found to decrease as the size of the rare-earth cation increases. Therefore, the highest conductivity values were found for the apatite-type lanthanum silicate.
INTRODUCTION Apatite-type rare earth silicates, RE10-x(SiO4)6O3-1.5x, have recently gained considerable attention in the field of solid oxide ion conductors because of their high ionic conductivity at low temperatures (e.g. 0.01 S·cm-1 at 700ºC), higher even than that obtained with yttria stabilized zirconia (YSZ) (for a recent review on apatite-type ion conductors see ref 1). The crystal structure of these compounds is built up of isolated SiO4 tetrahedra with the extra oxide ions responsible for the ionic conduction, occupying the center of one dimensional channel running through the structure along the c-axis at [0,0,z]. In addition, there are two RE3+ sites, one fully occupied on the periphery of the same tunnel containing the mobile oxygen atoms and another one, partially occupied, in the centre of a smaller tunnel running also parallel to the c-axis at [1/3,2/3,z]. While rare earth atoms located in the first site are coordinated to 7 oxygen atoms, those on the second one are 9-coordinated The presence of cation vacancies on the smaller channel allows the SiO4 tetrahedra substructure to relax towards the empty site enhancing oxide ion conduction along the larger channel. Therefore, ionic conductivity in single crystal apatite-type silicates have been found to be highly anisotropic with that parallel to the c-axis in the hexagonal lattice about one order of magnitude higher than that perpendicular to the same axis. Studied initially by Nakayama and co-workers (2,3) and despite their promising performance, their application has been hindered by the high temperatures and long firing cycles needed for their synthesis and sintering (>1400ºC). In this work, we will show that is possible to prepare apatite-type La, Nd and Gd silicates at room-temperature, by mechanically milling mixtures of the corresponding oxides. We will also analyze the electrical properties of as-prep
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