Solid State Ionics 66 ( 1993) 85-95 North-Holland zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA SO LID zyxwvutsrqp STATE IONICS A high temperature lattice parameter and dilatometer study of the defect structure of nonstoichiometric cerium dioxide Huann-Wu Chiang, Robert N. Blumenthal and Raymond A. Foumelle zyxwvutsrqponmlkjihgfedcbaZY Department of Mechanical and Industrial Engineering, Marquette University, Milwaukee, W I 53233, USA Received 22 February 1993; accepted for publication 11 May 1993 The defect structure of CeO_x has been investigated over the composition range from Ce02 to CeOl.s at 800 and 900°C using the combined techniques of high temperature dilatometric length change and X-ray lattice parameter measurements. The oxygen partial pressure associated with the nonstoichiometric composition was controlled by passing different ratios of Hz/He mixtures through a constant temperature water bubbler and was monitored constantly with a YSZ oxygen sensor. The difference between the percent expansion for the dilatometric and X-ray lattice parameter measurements was zero, which indicated that the predom- inant defects of the Ce02_x are oxygen vacancies. 1. Introduction Several investigations indicate that nonstoichiom- etric cerium dioxide, CeOz+, may be classified as a metal excess n-type semiconductor [ l-3 1. Early in-’ vestigations led to the construction of the general form of the phase diagram in 1964 by Bevan and Kordis [ 41. Among the phases identified by these investigators, a cubic fluorite structure a phase was found above the miscibility gap, which possessed a large range of nonstoichiometry. The defect structure responsible for the nonsto- ichiometric behavior at high temperature in the sin- gle a phase region has been investigated by a number of different techniques including X-ray and neutron diffraction [ 5-7 1, dilatometry and X-ray measure- ments [8-l 11, electrical conductivity measurements [2,12-151, thermogravimetric measurements [ 15,161, oxygen self-diffusion studies [ 31 and ox- ygen dissociation measurements [ 171 .Except for the combined use of dilatometry and X-ray diffraction to study the expansion behavior as a function of nonstoichiometric composition, all of the techniques require extensive modeling of the defect structure in order to relate the temperature dependence of the measured quantities to the nature of the defect struc- ture. It has generally been agreed that the predom- inant defect in nonstoichiometric cerium dioxide is Elsevier Science Publishers B.V. the doubly ionized oxygen vacancy, unfortunately, an extensive dilatometry-X-ray study [ 9- 111 using CO/CO2 mixtures to control the oxygen partial pres- sure has shown that this may not be the case. How- ever, in that study there was some uncertainty as to the exact oxygen content of the nonstoichiometric cerium dioxide. This uncertainty was related to the fact that measurements of the oxygen partial pres- sure surrounding the specimens were not made. Therefore, the objective of the present investigation was to determine the nature of the defect structure of nonstoichiometric cerium dioxide using the com- bined techniques of dilatometry and X-ray diffrac- tion in such a way that the oxygen partial pressure was constantly monitored. To vary the oxygen par- tial pressure different ratios of hydrogen and helium gases were passed through a constant temperature water bubbler (water temperature was varied be- tween 0 to 20°C). Using this technique, a wide range of oxygen partial pressures corresponding to differ- ent nonstoichiometric compositions of cerium diox- ide, CeOz_X, ranging from x equal to 0 to about 0.2 could be obtained. An yttria-stabilized zirconia (YSZ) solid electrolyte was used as an oxygen sen- sor to monitor the oxygen partial pressure of the gas mixture after it excited the measurement chamber.