Topotactic and reconstructive changes at high pressures and temperatures from Cs-natrolite to Cs-hexacelsian

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 H₂O as a pressure-transmitting medium 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. Further compression and heating to 1.5 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 (80O_f). At pressures and temperatures of 3.7 GPa and 180 °C, a new phase Cs_1.547Al_1.548Si_6.452O₁₆ (Cs-HEX) with a hexacelsian framework forms, which has a ca. 1.8% smaller unit-cell volume per 80O_f. 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 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⁺ immobilization in nuclear waste disposal.