Studies of Pollucite
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STUDIES OF POLLUCITE E.R. VANCE, B.E. SCHEETZ, M.W. BARNES AND B.J. BODNAR Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 INTRODUCTION Pollucite (CsAlSi 2 06 ) has been suggested [1,2] as a suitable phase for the incorporation and immobilization of 1 3 7 Cs, on the basis of its refractory nature and leach resistance. The phase also occurs in tailored ceramic formulations designed for the disposal of commercial reprocessing waste [3,4] and zeolites which are heavily loaded with Cs [5,6]. Pollucite itself occurs in nature as a mineral. The Cs is held in 12-fold coordination in a zeolite-like aluminosilicate framework. Other alkalis may be present, though H2 0 molecules must be incorporated if Na or K is present [7]. The present work addresses several aspects which may be relevant to the use of pollucite in nuclear waste management and which have not been dealt with in much detail by other workers. These aspects are (i) possible range of stoichiometry at high temperatures, (ii) incorporation of cations other than Cs, Al or Si, (iii) compatibility with other phases occurring in tailored-ceramic formulations (iv) crystallization kinetics and possible sintering aids, (v) radiation damage response, (vi) transmutation stability and (vii) compatibility with cements. EXPERIMENTAL Ceramic preparations were made up by mixing stoichiometric amounts of metal nitrate solutions with an NH3 -stabilized sol of Si02 . These mixtures were dried at ^lO00 °C and then calcined at •60 0 *C. The calcines were cold-pressed at '\,200 MPa and then fired at elevated temperatures.
X-ray powder diffraction was
performed with standard diffractometers, using CuKQ radiation and graphite monochromators. RESULTS AND DISCUSSION Stoichiometry range Cern:F [7] suggests that pollucite exists as a slightly off-stoichiometric material, with a deficiency of alkali and an excess of Si. If so, it might reasonably be expected that high-temperature treatment might lead to a range of stoichiometry, with the materials being characterized by different unit cell parameters. Ten separate ceramic preparations of CsxAlySi20O6+0.5(y-x) stoichiometries in which x and y were varied independently between 0.8 and 1.2 were fired in air for various periods in the II00-1400 0 C range. Within the experimental accuracy of about 1 part in 2000, variations in unit-cell parameters of the pollucite phases were not observed. Incorporation of other cations Fe3+ can completely replace the A13 + in pollucite [8], although there is a strong preference for Al [9]. Preparations of CsRSi 2 06 (R = Mn, Co or Ni) stoichiometry gave a pollucite phase, together with RO, on crystallization at I000-12000 C. No pollucite phase was observed for R = Cr. The respective unit-cell parameters for Mn, Co, Ni, Fe and Al pollucites were found as 13.76, 13.69, 13.68, 13.85 and 13.67 X. Crystallization in a high pressure of 02 should lead to single phase pollucites for R = Mn, Co and Ni and these pollucite phases should have a strong preference for Al, as
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