The Debye Temperature for Hydrothermally Grown ThO 2 Single Crystals
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The Debye Temperature for Hydrothermally Grown ThO2 Single Crystals Tony D. Kelly1, James C. Petrosky1, John W. McClory1, Timothy Zens1, David Turner1, J. Matthew Mann2, Joseph W. Kolis3, Juan A. Colón Santana4, and Peter A. Dowben4 1
Department of Engineering Physics, Air Force Institute of Technology, 2950 Hobson Way, WPAFB, OH 45433, U.S.A. 2 Sensors Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, U.S.A. 3 Department of Chemistry and Center for Optical Materials Science and Engineering Technologies (COMSET), Clemson University, Clemson, SC 29634-0973, U.S.A. 4 Dept. of Physics and Astronomy, Theodore Jorgensen Hall, 855 North 16th Street, University of Nebraska-Lincoln, Lincoln, NE 68588-0299, U.S.A. ABSTRACT The electronic properties of ThO2 single crystals were studied using x-ray photoemission spectroscopy (XPS). The XPS results show that the Th 4f core level is in an oxidation state that is consistent with that expected for Th in ThO2. The effective Debye temperature is estimated from the temperature dependent photoemission intensities of the Th 4f core level over the temperature range of 290 to 360 K. A Debye temperature of 468±32 K has been determined. INTRODUCTION Actinides and their oxides are important for the nuclear fuel cycle and other new energy sources [1]. In particular, there is growing interest in a thorium fuel cycle due to thorium’s natural abundance, refractory nature, and potentially limited radioactive waste formation [2]. While refractory suggests high temperature stability, a low Debye temperature translates into low barriers to surface and grain boundary segregation of impurities, as well as a possible propensity for phase separation, as in the case of Gd-Ni [3]. A medium Debye temperature, say in the region of 600 K or more, generally implies that surface segregation must be thermally activated, or that significant radiation damage and vacancy creation must occur prior to facile surface and grain boundary segregation of impurities. While the technique of hydrothermal crystal growth is not new, hydrothermally grown single crystals of ThO2 are an excellent route for obtaining large single crystals of actinide oxides, sufficient for an accurate determination of the effective Debye temperature as well as possible differences in the surface and the bulk Debye temperatures [4]. The XPS determined Debye temperature is also useful in elemental “specific” studies in order to help ascertain if dopants occupying similar sites within a host matrix form similar types of bonds [5]. Additionally, the changes in the effective Debye temperature are a useful signature in quantifying and identifying phonon mediated transitions [6] from temperature dependent photoemission studies [7]. The investigations here are aimed at addressing whether ThO2 is robust as a single crystal, especially with regard to impurity and vacancy diffusion, in spite of prior measurements that suggest that the Debye temperature of ThO2 is in fact quite low, in the region of 259 to 290 K [8].
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