Palladium-based ferroelectrics and multiferroics : theory and experiment

Palladium normally does not easily substitute for Ti or Zr in perovskite oxides. Moreover, Pd is not normally magnetic (but becomes ferromagnetic under applied uniaxial stress or electric fields). Despite these two great obstacles, we have succeeded in fabricating lead zirconate titanate with 30% Pd substitution. For 20:80 Zr:Ti the ceramics are generally single-phase perovskite (>99%), but sometimes exhibit 1% PbPdO2, which is magnetic below T=90K. The resulting material is multiferroic (ferroelectric-ferromagnet) at room temperature. The processing is slightly unusual (>8 hrs in high-energy ball-milling in Zr balls), and the density functional theory provided shows that it occurs because of Pd+4 in the oversized Pb+2 site; if all Pd+4 were to go into the Ti+4 perovskite B-site, no magnetism would result.PostprintPeer reviewe

Synthesis and Properties of a Compositional Series of MIL-53(Al) Metal-Organic Framework Crystal-Glass Composites

Metal-organic framework crystal-glass composites (MOF-CGCs) are materials in which a crystalline MOF is dispersed within a MOF glass. In this work, we explore the room temperature stabilization of the open-pore form of MIL-53(Al), usually observed at high-temperature, which occurs upon encapsulation within a ZIF-62(Zn) MOF glass matrix. A series of MOF-CGCs containing different loadings of MIL-53(Al) were synthesized and characterized using X-ray diffraction and nuclear magnetic resonance spectroscopy. An upper limit of MIL-53(Al) that can be stabilized in the composite was determined for the first time. The nanostructure of the composites was probed using pair distribution function analysis and scanning transmission electron microscopy. Notably, the distribution and integrity of the crystalline compo-nent in a sample series was determined, and these findings related to the MOF-CGC gas adsorption capacity in order to identify the optimal loading necessary for maximum CO2 sorption capacity.TDB would like to thank both the Royal Society for a University Research Fellowship (UF150021) and the Royal Society for a Research Grant (RG94426). CWA would like to thank the Royal Society for a PhD studentship (RG160498), and the Commonwealth Scientific and Industrial Research Council for additional support (C2017/3108). Both JH and TDB gratefully acknowledge the EPSRC (EP/R015481/1). AFS acknowledges EPSRC for a studentship award under the Doctoral Training Programme. AMB acknowledges the Royal Society for funding (RGF\EA\180092), as well as the Cambridge Trust for a Vice Chancellor’s Award (304253100). We extend our gratitude to Diamond Light Source, Rutherford Appleton Laboratory, UK, for access to Beamline I15-1 (EE20038-1) and access and support in the use of the electron Physical Science Imaging Centre (EM20195). SMC acknowledges the Henslow Research Fellowship at Girton College, Cambridge. PAM thanks the EPSRC for financial support under grant number EP/R025517/1

Modulator-controlled synthesis of microporous STA-26, an interpenetrated 8,3-connected zirconium MOF with the the-i topology, and its reversible lattice shift

The authors acknowledge the support of the EPSRC/St Andrews Criticat CDT (RRRP, PAW) and the European Community Seventh Framework Program (FP7/2007-2013) number 608490 (project M4CO2) (KKC, MYM, KIH, PAW). SEA would like to thank the Royal Society and Wolfson Foundation for a merit award. This research made use of the Balena High Performance Computing (HPC) Service at the University of Bath. The research data (and/or materials) supporting this publication can be accessed at DOI: http://dx.doi.org/10.17630/6ffeed8a-e75f-4648-968f-3ed32a94e9a0.A fully interpenetrated 8,3-connected zirconium MOF with the the-i topology type, STA-26 (St Andrews porous material-26), has been prepared using the 4,4',4"-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate (TMTB) tritopic linker with formic acid as a modulating agent. In the as-prepared form STA-26 possesses Im-3m symmetry compared with the Pm-3m symmetry of the non-interpenetrated analogue, NU-1200, prepared using benzoic acid as a modulator. Upon removal of residual solvent there is a shift between the interpenetrating lattices and a resultant symmetry change to Cmcm which is fully reversible. This is observed by X-ray diffraction and 13C MAS NMR is also found to be remarkably sensitive to the structural transition. Furthermore, heating STA-26(Zr) in vacuum dehydroxylates the Zr6 nodes leaving coordinatively unsaturated Zr4+ sites, as shown by IR spectroscopy using CO and CD3CN as probe molecules. Nitrogen adsorption at 77 K together with grand canonical Monte Carlo simulations confirms a microporous, fully interpenetrated, structure with pore volume 0.53 cm3 g−1 while CO2 adsorption at 196 K reaches 300 cm3 STP g−1 at 1 bar. While the pore volume is smaller than that of its non-interpenetrated mesoporous analogue, interpenetration makes the structure more stable to moisture adsorption and introduces shape selectivity in adsorption.PostprintPeer reviewe

The Effect of Framework Structure and Chemical Functionality on Melting in Zeolitic Imidazolate Frameworks

Interest in the amorphous phases of metal–organic frameworks (MOFs) has increased in recent years. Special consideration has been given to melt-quenched MOF glasses: the first new category of glass discovered in 50 years. Zeolitic imidazolate frameworks (ZIFs) are the most common MOF family that have been found to undergo melt-quenching. They are composed of tetrahedrally coordinated metal ions connected to imidazolate linkers. The dynamic nature of the melting mechanism in ZIFs has been demonstrated, with melting occurring via de-coordination and re-coordination of the imidazolate linkers at high temperatures. A wide variety of ZIF crystal structures have been reported to date. However, at present, the number of ZIFs that can undergo melt-quenching remains limited. This thesis aims to provide a better understanding of melting in ZIFs as well as the different factors that control the melting process so that, ultimately, novel melt-quenched MOF glasses can be prepared. Initially, four closely related ZIFs were studied to systematically investigate how linker chemistry and framework structure influence the melting process. Importantly, dense framework structures — specifically those displaying the cag network topology — were found to be crucial for melting. Moreover, their presence could initiate melting in more open framework structures. As dense frameworks were found to be essential for melting, the thermal behaviour of ZIFs displaying ultra-high framework densities, specifically those exhibiting the zni network topology, were investigated. Melting in these ZIFs was found to occur at higher temperatures than in cag topology systems. Furthermore, melting was found to be highly sensitive to chemical composition, with a 0.25% change in linker composition capable of eliciting a 7 °C change in melting temperature. We then demonstrate, for the first time, the possibility of further altering the chemistry in a ZIF glass by post-synthetic modification (PSM). A novel amine-functionalised ZIF glass was prepared that would be an ideal candidate for PSM. As a proof of concept, this amine-functionalised ZIF glass was reacted with octyl isocyanate, resulting in a urea-functionalised glass surface and a change in its surface wetting behaviour from hydrophilic to hydrophobic. Finally, we further expand the possible chemistries that can be incorporated in ZIF glasses by the inclusion of purine in a novel ZIF structure. The resulting glass forming ZIF was found to have one of the lowest melting temperatures reported for any ZIF. This represents a reduction in the melting temperature of over 250 °C compared to some of the early reports of ZIF melting. Evidently, judicious control of both linker chemistry and framework structure can be utilised to alter the thermal behaviour of ZIFs and to prepare novel melt-quenched glasses.The Royal Society, Cambridge Trus

Mechanochemical Synthesis of Mixed Metal, Mixed Linker GlassForming Metal–Organic Frameworks

Current methodologies to produce glass forming metal–organic frameworks (MOFs) rely on non-scalable solvothermal syntheses which have high energy requirements, relatively low yields and large tetratogenic solvent usage. Here we use a mechanochemical method to produce glass-forming MOFs, ZIF-62 and ZIF-UC-5, in 30 minutes at room temperature, using microlitre quantities of solvent and stoichiometric amounts of organic linkers. This method facilitates the accurate synthesis of ZIF-62 structures containing both Co and Zn, allowing the effect of metal-ion dopant upon melting temperature to be studied for the first time. Further to this, we present variable organic linker ratio series of IF-62 and of ZIF-UC-5. The specific composition of the materials in the series is made possible by the mechanochemical method. We also present a greener solvothermal method to form ZIF-62, which is capable of producing crystalline materials of suffcient quality for single crystal diffraction experiments.<br /

Multivariate Analysis of Disorder in Metal–Organic Frameworks

The rational design of disordered frameworks is an appealing route to target functional materials. However, intentional realisation of such materials relies on our ability to readily characterise and quantify structural disorder. Here, we use multivariate analysis of pair distribution functions to fingerprint and quantify the disorder within a series of compositionally identical metal–organic frameworks, possessing different crystalline, disordered, and amorphous structures. We find this approach can provide powerful insight into the kinetics and mechanism of structural collapse that links these materials. Our methodology is also extended to a very different system, namely the melting of a zeolitic imidazolate framework, to demonstrate the potential generality of this approach across many areas of disordered structural chemistry

Multivariate Analysis of Disorder in Metal–Organic Frameworks

The rational design of disordered frameworks is an appealing route to target functional materials. However, intentional realisation of such materials relies on our ability to readily characterise and quantify structural disorder. Here, we use multivariate analysis of pair distribution functions to fingerprint and quantify the disorder within a series of compositionally identical metal–organic frameworks, possessing different crystalline, disordered, and amorphous structures. We find this approach can provide powerful insight into the kinetics and mechanism of structural collapse that links these materials. Our methodology is also extended to a very different system, namely the melting of a zeolitic imidazolate framework, to demonstrate the potential generality of this approach across many areas of disordered structural chemistry.JM11106 EPSRC iCASE Fundin