Epr of Fe 3+ and Cr 3+ Ions in NZP Ceramics
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A.Y. TROOLE*, S.V. STEFANOVSKY*, L.D. BOGOMOLOVA** *SIA "Radon", 7 th Rostovskii per., 2/14, Moscow 119121 Russia' **Institute of Nuclear Physics of Moscow State University, Vorobyovy gory, Moscow, Russia.
ABSTRACT A new method to estimate the thermodynamic stability of crystalline host phases for radwaste is based on the determination of the degree of coordination polyhedra distortion. Analysis of the stability of sodium zirconium phosphate (NZP) structure containing Fe and Cr has been completed. The NZP structure can incorporate a limited amount of iron (-2 mol. % Fe20 3). From the EPR study, Fe" and Cr" ions 4 occupy two different structural positions substituting for Zr + and Na'. Moreover, these ions can enter extra phases. INTRODUCTION One of the principal requirements of crystalline waste forms is long-term stability involving thcrmodynamic stability and resistance to alteration under geological conditions. The correct determination of the long-term stability of a specified crystalline waste form is problematic. Usually, this problem is solved by means of numerous material syntheses experiments under various conditions followed by testing to determine the boundaries of stability. This can require many experiments and much time. A simple and effective method for the determination of thermodynamic stability of single crystalline matrices can be based on analysis of the coordination polyhedra distortion. Information about the degree of distortion max be obtained by electron paramagnetic resonance (EPR) spectra analysis of paramagnetic ions incorporated into the crystalline matrix. Sodium zirconium phosphate NaZr 2(PO4) 3 (NZP) has been chosen as the object for this investigation because it is considered a suitable host phase for the incorporation of all radioactive waste constituents I1]. NZP is a well-known member of a large family of chemical compounds and solid solutions extcnsively studied for over 30 years [1-7]. The NZP structure has a number of important properties such as supcrionic conductivity 12,31, a thermal expansion coefficient close to zero 14,51, and a wide range of cationic substitutions maintaining an unaltered crystalline structure [6]. The capability of the NZP structure to incorporate cations with various valences from +1 to +6 [7] stimulated consideration of this phase as an appropriate single phase for radioactive waste immobilization. This single phase is able to incorporate all of the waste elements and has great advantages over multiphase matrices because this removes the comiplicated question of thermodynamic stability of a multiphase ceramic as a phase assemblage. Therefore, if the NZP structure is thermodynamically stable, it would be a top candidate to be used as a host phase for all types of radioactive wastes. THEORETICAL CONSIDERATIONS In general, the problem of establishing the thermodynamic stability of this crystalline phase is uncertain. A specific issue is the thermodynamic stability of a phase which contains radionuclides as impurities. As waste ions are not "parent" for
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