Synthesis and structure of low-dimensional gallium fluorodiphosphates seen during the crystallization of the three-dimensional microporous gallium fluorophosphate ULM-3

Two one-dimensional gallium fluorodiphosphates Ga(P2O7)F·H3 N(CH2)3NH3·3H2O, 1, and Ga(P2O7) F·H3N(CH2) 3NH3·H2O, 2, have been synthesized under mild conditions (<100°C) from aqueous mixtures of Ga(NO3)3·6H2O, P2O5, 1,3-diaminopropane, and HF. Their structures consist of anionic chains of formula Ga(P2O7)F2- containing corner-sharing {GaO4F2} octahedra linked together via fluorine and P2O7 units, which are charge-balanced by propyldiammonium dications. During the hydrothermal synthesis of the open-framework three-dimensional ULM-3, compound 2 has been identified as the crystalline intermediate phase when phosphorus pentoxide is used as a starting material. This process has been followed in situ with time-resolved energy-dispersive X-ray diffraction (EDXRD), which reveals that dissolution of the one-dimensional phase 2 occurs before the rapid crystallization of ULM-3. If compound 2 is quenched from hydrothermal conditions, rapid and reversible conversion to compound 1 takes place. The two chain phases are thus solid precursors of the microporous ULM-3, and tracking the transformation process using in situ EXDRD shows that dissolution of 1 takes place before crystallization of 2 and then further transformation into the 3D phase, with the 1 and 2 acting as reservoirs of reactive species for ULM-3 crystallization