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Title: Structural chemistry of transition metal dicyanometallates
Author: Hill, Joshua A.
ISNI:       0000 0004 6496 1310
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2016
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Transition metal dicyanometallates are a family of coordination polymers characterised by their structural diversity and nanoscale separation between transition metal centres. This thesis addresses the relationships between molecular extra-framework cations and their host dicyanometallate frameworks as determinants of structure type and phase transitions. It also addresses the new tilt degrees of freedom accessible to perovskite-structured dicyanometallates and molecular perovskites more generally. A number of new dicyanometallate frameworks are reported which contain the extra-framework cations tetraethylammonium, [NEt4]+, tetra-n-butylammonium, [NBu4]+, and bis(triphenylphosphine)iminium, [PPN]+ = {[(C6H5)3P]2N}+. These new compounds are placed within the context of all known dicyanometallate frameworks of general formula AmB(L)Xn·G where A is an extra- framework cation; B is a metal cation; X is [Au(CN)2]-, [Ag(CN)2]-, or [Cu(CN)2]-; L is a neutral ligand bound to the node metal centre (i.e., not to the linker metal); and G is a non-coordinating guest molecule. They are organised according to their stoichiometry, AmBXn, showing the dual importance of size/shape of extra- framework cations and the coordinational preferences of the framework cation. The interdependence of degrees of freedom associated with extra-framework cations and with their host frameworks are explored in a diamondoid system and in a number of perovskite-like systems. In the diamondoid system [NEt4]Ag[Ag(CN)2]2, coupling between [NEt4]+ conformation and lattice modes results in a second-order phase transition to an incommensurately modulated phase at low temperatures. There are few reports of incommensurability in coordination polymers; coupling of 'molecular' degrees of freedom with phonons is of fundamental interest and has been implicated in the performance of lead halide perovskites in solar cells. Dicyanometallates with perovskite(-related) structures are here shown to undergo a variety of displacive and reconstructive phase transitions as a function of temperature. In each case, the extra-framework species play a key role in determining the nature of the transitions. In molecular perovskite analogues, tilt degrees of freedom can couple at arbitrary periodicities not accessible to oxides/halides. The 'tilt tensor' is developed here as a tool for describing these tilts. This understanding of unconventional tilts is finally applied to the Prussian Blue analogue (NH4)2SrFe(CN)6 in which such tilts can be switched on and off depending on hydration state.
Supervisor: Goodwin, Andrew Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available