Synchrotron X-ray powder diffraction experiments have been performed on dehydrated Csexchanged natrolite to systematically investigate successive transitions under high pressures and temperatures. At pressures above 0.5(1) GPa using H<inf>2</inf>O as a pressure-transmitting medium and after heating to 100 °C, dehydrated Cs<inf>16</inf>Al<inf>16</inf>Si<inf>24</inf>O<inf>80</inf> (deh-Cs-NAT) transforms to a hydrated phase Cs<inf>16</inf>Al<inf>16</inf>Si<inf>24</inf>O<inf>80</inf>·16H<inf>2</inf>O (Cs-NAT-II), which has a ca. 13.9% larger unit-cell volume. Further compression and heating to 1.5 GPa and 145 °C results in the transformation of Cs-NAT-II to Cs<inf>16</inf>Al<inf>16</inf>Si<inf>32</inf>O<inf>96</inf> (anh-Cs-POL), a H<inf>2</inf>O-free pollucite-like triclinic phase with a 15.6% smaller unit-cell volume per 80 framework oxygen atoms (80O<inf>f</inf>). At pressures and temperatures of 3.7 GPa and 180 °C, a new phase Cs<inf>1.547</inf>Al<inf>1.548</inf>Si<inf>6.452</inf>O<inf>16</inf> (Cs-HEX) with a hexacelsian framework forms, which has a ca. 1.8% smaller unit-cell volume per 80O<inf>f</inf>. This phase can be recovered after pressure release. The structure of the recovered Cs-HEX has been refined in space group P6<inf>3</inf>/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., BaAl<inf>2</inf>Si<inf>2</inf>O<inf>8</inf>), Cs-HEX contains Cs<sup>+</sup> cations that 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 for Cs<sup>+</sup> immobilization in nuclear waste disposal.
|Titolo:||Topotactic and reconstructive changes at high pressures and temperatures from Cs-natrolite to Cs-hexacelsian|
|Parole Chiave:||hexacelsian; high pressure; Natrolite; phase transition; radioactive wastes; X-ray powder diffraction|
|Settore Scientifico Disciplinare:||Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul|
|Data di pubblicazione:||1-lug-2015|
|Digital Object Identifier (DOI):||10.2138/am-2015-5090|
|Appare nelle tipologie:||01 - Articolo su periodico|