A Study of Natural Metamict Yttrium Niobate as Analogue of Actinide Ceramic Waste form
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A Study of Natural Metamict Yttrium Niobate as Analogue of Actinide Ceramic Waste Form Cao Qiuxiang1, 2, Anton I. Isakov2, 3, Liu Xiaodong1, Sergey V. Krivovichev2, Boris E. Burakov3 1 East China Institute of Technology, Guanglan Road, 418, 330013, Nanchang, Jiangxi, China 2 Saint-Petersburg State University, Universitetskaya emb. 7/9, 199034, St.-Petersburg, Russia 3 V.G. Khlopin Radium Institute, 2-nd Murinskiy Ave. 28, 194021, St.-Petersburg, Russia ABSTRACT Natural metamict mineral found as large (1-3 cm in size) homogeneous grains (as assumed, former single crystals), was investigated by X-ray powder diffraction (pXRD), hightemperature pXRD, scanning electron microscopy (SEM) and electron microprobe analysis (EMPA). The average chemical composition obtained by EMPA is (wt. %): Nb2O5 – 42.6; Ta2O5 – 4.4; TiO2 – 9.2; UO3 – 4.4; ThO2 – 1.0; MnO – 1.3; FeO – 19.4; Y2O3 – 16.6. The untreated (original) sample is X-ray amorphous. The sample remained amorphous after annealing at 400 °C for 1 hour. The sample became almost fully crystalline after annealing ° at 700 C for 1 hour with an X-ray diffraction pattern similar to that of Fe-columbite (ICCD: 01074-7356). Further annealing at 1000 °C and higher temperatures caused changes in the phase composition of the sample. It was proposed that under self-irradiation a single-phase U-Thbearing solid solution, based on monocrystalline Y-niobate, became metamict but remained homogeneous without evidence of solid solution destruction. However, this metamict solid solution is unstable under thermal treatment and recrystallization. INTRODUCTION Since the term “metamict” was defined in 1893, the words “metamict” or “metamictization” have been increasingly synonymous for “amorphous” or “amorphization”. Minerals considered to be metamict were judged amorphous because of their conchoidal fracture and isotropic optical properties; however, well-developed crystal faces evidenced the prior crystalline state [1,2]. Metamict minerals may contain significant quantities of U and Th that lead to the transition from the crystalline to the aperiodic, amorphous state. Amorphization occurs mainly due to the effects of heavy-particle irradiation and their crystalline structures can be reconstituted by heating [3,4]. It was also surprising that the metastable, amorphous state could be preserved in a material for long periods of geological time (up to ~109 years) [1]. Consequently, metamict minerals can serve as natural analogues of ceramic nuclear waste-forms regarding their response to radiation damage [5-7]. The metamict state raised important fundamental questions concerning the stability of different structure types in a radiation field [8]. In this article, we report on the occurrence and study of a metamict mineral found in granite pegmatites from Karelia, Russia.
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EXPERIMENTAL DETAILS (1) EMPA analyses and secondary electron (SE) SEM imaging: Camscan electron microscope; 30 kV, 100 nA. Investigations were carried out in V.G. Khlopin Radium Institute. (2) XRD: Bruker D2 Phaser diffract
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