Metastable microporous lanthanide silicates – Light emitters capable of 3D-2D-3D transformations

The synthesis of zeolites and porous heteropolyhedral silicates is usually accompanied by crystal phase transformations. Typically, this process is irreversible and characterized by a conversion from a metastable phase to a stable one. Here we demonstrate a reversible transformation between porous (K3LnSi3O9·nH2O) and layered (K3LnSi3O8(OH)2) structures exemplified by a dual synthesis (hydrothermal and solid-state transformation). We discuss structural and photoluminescent properties of the obtained microporous lanthanide silicates MS-4 (Minho-Sofia, solid number 4; K3LnSi3O9·nH2O, Ln = Y, Dy; orthorhombic, Pmc21). These materials are metastable, transforming from 3D porous to a 2D layered structure after prolonged crystallization time at hydrothermal conditions and returning to the same but distorted (≈ 2 % smaller lattice volume) porous 3D framework by calcination of 2D structure. The mechanism of 3D-2D structure transformation is driven by interzeolite-like transformation, while the rearrangement of common structural fragments controls 2D-to-3D conversion. We also show how the avoidance of Dy and preference for Y ions during MS-4 hydrothermal synthesis may be overcome by a 2D-3D conversion, allowing the production of white light emitters with controllable photoluminescence.