Crystal Chemistry of Some Organically Templated Metal Fluorides: The Crystal Structure of [C 2 N 2 H 10 ] 0.5 [Y 2 F 7 ]
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Crystal Chemistry of Some Organically Templated Metal Fluorides: The Crystal Structure of [C2N2H10]0.5[Y2F7] Philip Lightfoot and Nicholas F. Stephens School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK; e-mail [email protected] ABSTRACT As part of an ongoing program exploring the solvothermal synthesis and crystal chemistry of organically templated metal fluorides, a novel compound [C2N2H10]0.5[Y2F7] has been isolated. The structure consists of a continuous 3-D array of YF8 and YF9 polyhedra linked through corners, edges and faces to enclose small channels in which the template cations sit. The structure is stable up to 400ºC, at which point template loss causes structural collapse. This structure represents the first yttrium or rare-earth fluoride templated by organic cations, and suggests a wide variety of novel structure types will be amenable via such routes, possibly leading to novel families of luminescent materials.
INTRODUCTION The solvothermal chemistry of metal fluorides is relatively unexplored. In particular, the development of organically-templated metal fluorides is in its infancy, in contrast to the corresponding vast metal oxide and phosphate chemistry [1]. Our reasons for studying fluorides and oxyfluorides are two-fold: (i) lanthanide metal fluorides often have improved optical properties over their oxide analogues [2]. These arise, in general terms, from the increased electronegativity and decreased charge of the F- anion versus the O2- anion, viz. more ionic and less covalent M-X bonds (lack of π-backbonding in fluorides, ie. weaker ligand field strength), higher bandgap associated with the M-X bond, hence much increased transparency range, less polarisable, hence lower refractive index. (ii) metal oxyfluorides of early transition metals (generally d0 cations) often exhibit intrinsically polar local geometries (ie. octahedral units with characteristic ‘off-centre’ displacements of the cation); Poeppelmeier et al., in particular, have exploited this feature in the crystal engineering of novel non-linear optical materials [3]. Bentrup et al. [4] have reviewed the solution-based synthesis of transition metal fluorides templated by organic amines. Surprisingly, most of these materials consist of isolated [MF6] octahedral units, with very few extended-connectivity lattices produced, and no 3-D connected – M-F- frameworks. More recently, Albrecht-Schmitt [5], O’Hare [6] and Weller [7] have developed the corresponding zirconium, uranium and beryllofluoride chemistry, and several extended chain or layer-type structures have been discovered. We have followed up this work by exploring scandium and yttrium fluoride chemistry, and have discovered several new organically-templated scandium and yttrium fluorides, with extended lattice structures, and novel polyhedral connectivities [8]. In the present paper we describe the first organically-
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templated yttrium fluoride, which displays a 3-D connected framework. EXPERIMENTAL [C2N2H10]0.5[Y2F7] was prepared by hydroth
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