Flexibility of the zeolite RHO framework: Effect of dehydration on the crystal structure of the beryllophosphate mineral, pahasapaite

Abstract

Pahasapaite, a hydrated beryllophosphate mineral (Ca 5.5 Li 3.6 K 1.2 Na 0.2 □ 13.5 )Li 8 Be 24 P 24 O 96 ·38 H 2 O, has a distorted cubic, zeolite RHO-type structure with a = 13.781 A. On dehydration, the structure distorts further to give a cell with a = 13.100(3) A. As with the aluminosilicate RHO framework, Li + is associated with the 6-ring sites and Ca 2+ is sited in the double 8-ring sites, leading to severe distortion of the double 8-rings. The double 8-rings consist of elliptically distorted single 8-rings orthogonal to each other and the degree of their distortion can be described by the Δ/a parameter, where Δ is one-half the difference between the major and minor axes of these elliptical 8-rings and a is the cell edge. This parameter, Δ/a, varies from 0 to 0.178 for the aluminosilicate system, but is 0.201 for the dehydrated pahasapaite mineral. This is clearly the greatest distortion yet observed for this framework. This combination of new framework chemistries (Be/P/O vs. Si/Al/O) and the increased distortion is being examined by theory and experiment with respect to the effect on guest molecules.