| Summary: | Five new cerium (IV) fluoride and fluoride–arsenate framework structures have been synthesised through hydrothermal techniques using CeF4 as a fluoride source. Cs[CeIV2F8[F.H2O]] (I) consists of layers, formed from linked Ce(F,O)n polyhedra cross-linked by hydrogen bonding that defines channels containing the caesium ions. [(NH4)5(H2O)2][CeIV4(AsO4)6(H2O)F3] (II), has an open framework structure with large channels filled with NH4+ cations and H2O molecules. CeIV[AsO4]F (III) and CeIV[AsO4]F[H2O] (IV) exhibit two types of bridging (Ce–O–Ce and Ce–F–Ce) bonds between Ce(O,F)n polyhedra, and (NH4)[CeIVF2(AsO4)] (V), is isostructural with the previously reported fluoride-phosphate (NH4)[CeIVF2(PO4)]. The use of metal tetra-fluorides (MF4; M = Zr, Hf) in hydrothermal reactions, has led to the production of new structures that incorporate either phosphate or arsenate oxotetrahedra linking metal oxo-fluoro polyhedra. This new synthetic pathway has provided a safer route to formation that does not require the addition of the dangerous HF reagent. Seven new zirconium and four hafnium arsenate or phosphate materials have been identified including ZrIV(HAsO4)(AsO4)(NH4) (VI) a three-dimensional framework with NH4+ within the channels, ZrIV(AsO4)F(H2O) (VII) exhibits Zr-F-Zr linkages, ZrIV(HAsO4)2 a novel three-dimensional framework (VIII) and ZrIV2(AsO4)3(NH4)(H2O) (IX) analogous to the mineral Langbeinite. Organic amines included in the reaction mixture can template the frameworks and form cavities, as observed within the channels of HfIV2F(HAsO4)(AsO4)2[Temp-Hn]1/n(H2O)1.5 (XIII) (Temp = ethylenediamine/diethylenetriamine). The substitution of fluoride ions for oxygen in the metal polyhedral unit has produced variations in the dimensionality of the materials formed as fluoride is more frequently incorporated as terminating species, reducing framework dimensionality. The controlled addition of these fluoride species, has provided a route to the rational design of one, two and three-dimensional framework structures. Hexagonal lanthanide phosphates (LnIIIPO4.nH2O) with the mineral Rhabdophane structure type were synthesised through hydrothermal techniques and analysed upon heating where a distinct increase in the a-axis lattice length was observed upon dehydration of the structure. Variable-temperature X-ray diffraction, EDX, TGA and fluorescence studies were undertaken to illustrate the interaction between the lanthanide cations and the water molecules that reside within the channels of the framework.
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