Electronic transport in Ce 0.8 Sm 0.2 O 1.9d ceramics under reducing conditions J.C.C. Abrantes a , D. Pe ´rez-Coll b , P. Nu ´n ˜ez b , J.R. Frade c, * a ESTG, Instituto Polite ´cnico de Viana do Castelo, 4900 Viana do Castelo, Portugal b Department Quı ´mica Inorga ´nica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain c Department Engenharia Cera ˆmica e do Vidro, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal Received 20 January 2003; received in revised form 6 May 2003; accepted 15 May 2003 Abstract Ce 0.8 Sm 0.2 O 1.9d powders were prepared by a freeze drying method and used to obtain ceramic disks. These samples were used to study the electronic transport properties of this material. A Hebb  /Wagner method was used to obtain the electronic conductivity under ion blocking conditions. Typical values of electronic conductivity measured for this material at 800 8C were about 0.37 S m 1 at Po 2 /10 16 atm and 0.58 S m 1 at P O 2 /10 18 atm. These values are significantly lower than results reported for ceria-based materials with different trivalent additives. A coulometric titration method was used to estimate the charge carrier concentrations, and the mobility of carriers was obtained on combining the results of conductivity and concentration. Typical values of mobility show weak temperature dependence and decrease with increasing oxygen deficiency, suggesting a limiting value of about 0.5 /10 7 m 2 .V 1 s 1 for relatively high d . # 2003 Elsevier Ltd. All rights reserved. Keywords: Ceria /samaria; Electronic conductivity; Oxygen nonstoichiometry; Polaron mobility 1. Introduction Ceria based materials have attracted great interest as potential electrolyte materials for intermediate tempera- ture solid oxide fuel cells [1  /5], with higher ionic conductivity than the standard yttria stabilised zirconia electrolyte. One of the main limitations of ceria based electrolytes is the onset of electronic conductivity under reducing conditions. Yet, this disadvantage of potential electrolyte materials is an advantage for alternative potential applications as components of SOFC anodes [6  /10]. This shows the importance of an accurate characterisation of the electronic conductivity of ceria based materials both in order to establish the limits of the electrolytic domains, and/or to optimise the mixed conducting properties for prospective anode materials. The ionic conductivity of ceria based materials is promoted by suitable trivalent additives such as Gd 3 , Sm 3 or Y 3 . Some authors reported the highest values of ionic conductivity for ceria  /gadolinia [5], and this has also been supported by computer atomistic model- ling [11]. These simulations predicted a combination of coulombic interactions and lattice relaxation, which vary with the size of the trivalent additive. The lowest overall interaction was predicted for Gd 3 . However, the values of trapping energy are also dependent on the fraction of the trivalent additive [4,12]. The ionic conductivity goes through a maximum at intermediate contents of the additive. Other authors [1,2] thus reported higher ionic conductivity values for ceria  / samaria materials. Materials with mixed dopants (e.g. Ce 1xy Sm x Y y O 2(xy )/2d ) may also be promising [13]. Some discrepancies between different authors can also be related to differences in sample preparation, and the corresponding effects on the relative role of resistive grain boundaries, especially at relatively low or inter- mediate temperatures [14 /17]. One may thus expect some improvements by optimizing the powder and ceramic preparation methods. * Corresponding author. Tel.:  /351-370-254; fax:  /351-2344- 25300. E-mail address: jfrade@cv.ua.pt (J.R. Frade). Electrochimica Acta 48 (2003) 2761  /2766 www.elsevier.com/locate/electacta 0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00395-5