High temperature Raman study of UO 2 :A possible tool for in situ estimation of irradiation-induced heating G. Guimbretière, a * A. Canizarès, a N. Raimboux, a J. Joseph, a P. Desgardin, a L. Desgranges, b C. Jegou c and P. Simon a Recently, in situ Raman setups under irradiation become available, and uranium dioxide is one of the most studied compounds. This paper reports the temperature dependence of the only Raman-active mode in fluorine UO 2 . Besides the interest in terms of phonon anharmonicity knowledge, this can be used to estimate the heating induced by irradiation: The frequency shift and width of the T 2g peak can be used as an internal indirect probe of the local temperature in the 20–590 °C range. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: in situ; High temperature; Uranium dioxide; irradiation; nuclear materials Introduction In the scientific community dealing with the physics and chemis- try of materials exposed to irradiation, the recent period has viewed a strong increase of the use of Raman spectroscopy. [1–9] Moreover, in situ setups allowing a real-time probing of the struc- tural evolution of sample under irradiation are now available. [10–13] In irradiation experiments, one open question is often the sam- ple temperature, and more precisely, the temperature increase due to irradiation itself. Indeed, in the study of damage-induced processes, an estimation of the annealing through increase of temperature induced by the irradiation itself is mandatory. The most often one applies the desired temperature through a temperature-controlled sample holder, assuming the irradiation heating will induce a change in the temperature control parameters. [14] Due to the specific access conditions to the sam- ple, it is complicated to conceive ways of temperature measure- ments able to spatially resolve the heat gradient due to dispersion of the irradiation beam. Here, we propose a route based on in situ Raman spectroscopy where the temperature shift due to phonon anharmonicity of a Raman-active vibration mode should be used to calibrate temperature during irradiation. This phonon anharmonicity (presumably third-order one, i.e., the first one after the quadratic dependence of energy upon position in the harmonic oscillator description) is responsible for lattice parameter dilatation upon heating, and consequently on the fre- quency shift of vibration modes upon external strains such as temperature or hydrostatic pressure. Contrary to a Stokes/anti- Stokes Raman intensities ratio approach, [15] this very simple cali- bration can be implemented with a majority of standard com- mercial Raman spectrometers. This is applied to uranium dioxide UO 2 , obviously one of the most important materials the behavior under irradiation must be known. Moreover, UO 2 is one of the compounds studied in the first reported in situ under irradiation Raman characterizations. [10] Experimental The used setup is as follows: the sample is a (2 × 2 × 0.3 mm 3 ) part of non irradiated UO 2 ceramic used in Canizares et al. and Guimbretiere et al. [10,12] with annealing treatment insuring its oxygen stoichiometry. The UO 2 sample was heated in a TS 1500 Linkam optical micro-oven device allowing a fine temperature regulation (±1 °C). In order to avoid any oxidation during annealing, the atmosphere was controlled to be 95%Ar/5%H 2 gas. A 24h gas purge was completed before starting Raman measurements. Spectra were collected using a Renishaw RA100 spectrometer equipped with a 633 nm laser, 1800 grooves/mm grating (4 cm À1 spectral resolution), and a Renishaw RP20 head. A long distance (20×) Olympus objective was used with numerical aperture of 0.25. Laser power is about 10mW on the sample. No laser heating was observed, as probed by tests with different laser powers. One spectrum was recorded every 10 °C from 20 to 590 °C. The procedure was the following: 10 °C/min ramp with a 2 min thermalization time. Finally, the Raman spectra set is made of 58 spectra of 2 min counting. * Correspondence to: G. Guimbretière, CNRS, CEMHTI UPR3079, University Orléans, CS 90055, F-45071 Orléans, France. E-mail: guillaume.guimbretiere@cnrs-orleans.fr a CNRS, CEMHTI UPR3079, University of Orléans, CS 90055, F-45071, Orléans, France b CEA/DEN/DEC Bat 352 Cadarache, F-13108, Saint Paul lez Durance, France c CEA/DEN/DTCD, Marcoule Research Center, F-30207, Bagnols-sur-Cèze, France J. Raman Spectrosc. (2015) Copyright © 2015 John Wiley & Sons, Ltd. Short communication Received: 9 October 2014 Revised: 12 January 2015 Accepted: 13 January 2015 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jrs.4661