ORIGINAL PAPER Venkataraman Thangadurai á Robert A. Huggins Werner Weppner Mixed ionic-electronic conductivity in phases in the praseodymium oxide system Received: 12 May 2000 /Accepted: 28 October 2000 / Published online: 13 June 2001 Ó Springer-Verlag 2001 Abstract The praseodymium oxide system contains a series of related phases with very narrow compositional width and unique extended defect structures, rather than the more common isolated point defects. The electrical conductivity of three of these phases has been measured bytheuseofACcompleximpedanceandDCmethodsin the temperature range 75±400 °C. The beta phase, Pr 6 O 11 , exhibits a total conductivity of 6.77´10 ±2 S/cm at 400 °C, with an activation energy of 0.52 eV/atom. The conductivity of the epsilon phase, Pr 5 O 9 ,isslightlylower, with an activation energy of 0.51 eV/atom. The iota phase, Pr 7 O 12 , has a very low conductivity. The activa- tion energies for electrical transport in the beta and epsilonphasesareinthegeneralrangefoundinanumber of mixed conductors based upon LSGM oxides. Keywords Praseodymium oxides á Oxide ion conduction á Mixed conduction á Intermediate temperature conductor á Electrical properties Introduction Rare earth oxides have been of continuous interest ow- ing to their important electrical, optical and magnetic properties. The praseodymium oxides are particularly intriguing, as they are among the group of binary oxides that show a wide range of stoichiometry, but in which the deviation from the ideal stoichiometry is not continuous, as is expected from the classical model of randomly distributed isolated point defects. Instead, there are a number of phases of unique structures and stoichiometries with no appreciable concentrations of isolated point defects, and thus essentially no compositional width. These structures are closely related to each other, and to a simple ``mother'' structure. Discussions of this general class of materials have been published [1, 2]. In the case of the praseodymium oxide system, these phases have been shown to have cation sublattices es- sentially identical to that of ¯uorite-type PrO 2 . The stoichiometric dierences between them are accommo- dated by the presence of topotactically ordered arrangements of large localized concentrations of oxide ion vacancies that divide the structure up into micro- domains. There are six sub-oxide phases in addition to the mother phase PrO 2 . They form a homologous series whose compositions can be written as Pr n O 2n±2 . The compositions of these phases are listed in Table 1. Sev- eral investigators have studied the structures and ther- modynamic properties of materials in this binary system [3, 4, 5, 6]. The phase diagram of this system, projected onto the temperature-composition plane, is shown in Fig. 1. Be- cause these are oxides, their ranges of equilibrium pressure are dierent. Thus they cannot all be prepared at an ambient pressure of one atmosphere. The oxygen partial pressure-temperature relationship for the stabil- ity of the various phases is shown in Fig. 2. The pra- seodymium oxide system is unique in that the phase Pr 6 O 11 or PrO 1.83 ), rather than the mother phase PrO 2 , is stable at ambient temperatures in air. Transitions between these phases upon heating and cooling, as well as upon changes in the oxygen activity of their environment, occur very rapidly, indicating un- usually high oxygen mobility. This is also indicated by the fact that it was found dicult to cool fast enough to freeze-in equilibrium compositions above about 350± 400 °C [3]. Because of the large concentrations of ordered oxy- gen vacancies in the sub-oxide phases, they can absorb signi®cant amounts of water, even at ambient tempera- ture, apparently by the electrically neutral mechanism proposed by Stotz and Wagner [7, 8]. This involves the J Solid State Electrochem 2001) 5: 531±537 DOI 10.1007/s100080000187 V. Thangadurai á R.A. Huggins &) á W. Weppner Faculty of Engineering, Christian Albrechts University, 24143 Kiel, Germany E-mail: rhg@techfak.uni-kiel.de Tel.: +49-0431-880-6212 Fax: +49-0431-880-6203