Overstepping Löwenstein's Rule-A Route to Unique Aluminophosphate Frameworks with Three-Dimensional Salt-Inclusion...

The synthesis of four non-Löwenstein uranyl aluminophosphates, [Cs₁₃Cl₅][(UO₂)₃Al₂O(PO₄)₆], Rb₇[Al₂O(PO₄)₃][(UO₂)₆O₄(PO₄)₂], Cs₃[Al₂O(PO₄)₃][(UO₂)₃O₂], and Rb₃[Al₂O(PO₄)₃][(UO₂)₃O₂], the first uranyl phosphate salt-inclusion material [Cs₄Cs₄Cl][(UO₂)₄(PO₄)₅], and a related structure Cs₄[UO₂Al₂(PO₄)₄], all prepared by molten flux methods, is reported. All compounds are discussed from the point of view of their structural features favoring, in some cases, ion-exchange properties. Löwenstein's rule, well known in the realm of zeolites, aluminosilicate, and aluminophosphate minerals, describes the tendency of tetrahedra (Al, P, Si, and Ge) linked by an oxygen bridge to be of two different elements resulting in the avoidance of Al-O-Al bonds. Zeolites and related aluminosilicate/aluminophosphate minerals are traditionally formed under relatively mild temperatures, where zeolites are synthesized using the hydrothermal synthetic technique. Few exceptions to Löwenstein's rule are known among aluminophosphates, and four of the five exceptions are synthesized under either high temperature or high pressure methods. For that reason, the high-temperature flux synthesis of four new non-Löwenstein uranyl aluminophosphates realizes a unique synthetic approach to forming the new pyroaluminate-based building block, [Al₂O(PO₄)₆]^14-, that can be easily obtained and employed for the construction of new porous structures.