Topotactic and reconstructive changes at high pressure and temperatures from Cs-natrolite to Cs-hexacelsian : potential nuclear waste disposal materials

Topotactic and reconstructive changes at high pressure and temperatures from Cs-natrolite to Cs-hexacelsian: potential nuclear waste disposal materials Hwang, H.1, Seoung, D.1, Gatta, G.D.2, Blom, D.A.3, Vogt, T.3, Lee, Y.1* 1Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea 2Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli, 23, I-20133 Milano, Italy 3NanoCenter & Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA Synchrotron X-ray powder diffraction experiments have been performed on dehydrated Cs-exchanged natrolite in order to systematically investigate successive transitions under high pressures and temperatures. At pressures above 0.5(1) GPa using water as a pressure transmitting fluid and after heating to 100°C, dehydrated Cs16Al16Si24O80 (deh-Cs-NAT) transforms to a hydrated phase Cs16Al16Si24O80∙16H2O (Cs-NAT-II), which has a ca. 13.9% larger unit-cell volume (Lee et al., 2011; Seoung et al., 2013). Further compression and heating to 1.5(1) GPa and 145 °C results in the transformation of Cs-NAT-II to Cs16Al16Si32O96 (anh-Cs-POL), a H2O-free pollucite-like triclinic phase with a 15.6% smaller unit-cell volume per 80 framework oxygen atoms (Of). At pressures and temperatures of 3.7(1) GPa and 180 °C, a new phase Cs1.547Al1.548Si6.452O16 (Cs-HEX) with a hexacelsian framework forms, which has a ca. 1.8% smaller unit-cell volume per 80Of . This phase can be recovered after pressure release. The structure of the recovered Cs-HEX has been refined in space group P63/mcm with a = 5.3731(2) Å and c = 16.6834(8) Å, and also been confirmed by HAADF-STEM real space imaging. Similar to the hexacelsian feldspar (i.e. BaAl2Si2O8), Cs-HEX contains Cs+ cations which act as bridges between the upper and lower layers composed of tetrahedra and are hexa-coordinated to the upper and lower 6-membered ring windows. These pressure- and temperature-induced reactions from a zeolite to a feldspar-like material are important constraints for the design of materials devoted to Cs+ immobilization. Lee Y., Lee Y. & Seoung D. 2010. Natrolite may not be a “soda-stone” anymore: Structural study of fully K-, Rb-, and Cs-exchanged natrolite. Am. Mineral., 95, 1636 – 1641. Seoung D., Lee Y., Kao C.-C., Vogt T. & Lee Y. 2013. Super-Hydrated Zeolites: Pressure-Induced Hydration in Natrolites. Chem. Eur. J., 19, 10876–10883.