A single crystal of zeolite X was ion exchanged using a mixed aqueous solution, equinormal in Pb2+ and Tl+, with a total concentration of 0.05 M. It was then vacuum dehydrated at 380 degrees C and 2 x 10(-6) Torr, and its structure was determined by X-ray diffraction techniques in the cubic space group Fd (3) over bar at 21 degrees C (a=25.119(6) Angstrom). The structure, of formula Pb442+Pb45+Tl18+O172--Si100Al92O384 per unit cell, was refined to the final error indices R-1 = 0.057 and R-2 = 0.055 with 475 reflections for which I> 3 sigma(I). The reaction that occurred upon evacuation at elevated temperature may be written, per unit cell, as 34Pb(2+) + 24OH(-) -->4.25 [Pb4O4(Pb12/172+Pb5/174+)(4)](176/17 +) + 5H(2) + 7H(2)O. Lead ions occupy three nonequivalent sites. Five lead ions at site II are identified as Pb4+ by their coordination geometry: each is nearly in a six-ring plane where it coordinates to three framework oxygen atoms at 2.23(2) Angstrom and also to a nonframework oxygen at 2.11(7) Angstrom. There are also 12 Pb2+ ions occupying site II: each is 2.45(2) Angstrom from three framework oxygen atoms and 2.78(7) Angstrom from a nonframework oxygen. About 53% of the sodalite cages are filled by a Pb4O4(Pb2+,Pb(4+)mixed)(4) cluster whose neutral Pb4O4 core is nearly a cube with Pb2+ and O2- ions alternating. The four Pb2+ ions in the core are at site I', where each is 2.594(14) Angstrom from three framework oxygen atoms and 2.62(6) Angstrom from three nonframework oxygen atoms. There are 18 Tl+ ions located at two nonequivalent sites: 15 complete the filling of site II teach is 2.708(14) Angstrom from three framework oxygen atoms), and the remaining three are at III' teach is 2.52(8) Angstrom from one framework oxygen and 3.12(9) Angstrom from another). Upon dehydration, the zeolite retains only those nonframework oxygen atoms that can be accommodated in Pb8O4n+ clusters. (C) 1999 Elsevier Science B.V. All rights reserved.
