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Metal–Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal–Organic Materials

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

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Physics of thin-film ferroelectric oxides

This review (over 380 references) summarizes metal–organic frameworks (MOFs), Materials Institute Lavoisier (MILs), iso-reticular metal–organic frameworks (IR-MOFs), porous coordination networks (PCNs), zeolitic metal–organic frameworks (ZMOFs) and porous coordination polymers (PCPs) with selected examples of their structures, concepts for linkers, syntheses, post-synthesis modifications, metal nanoparticle formations in MOFs, porosity and zeolitic behavior for applications in gas storage for hydrogen, carbon dioxide, methane and applications in conductivity, luminescence and catalysis.

MOFs, MILs and more: concepts, properties and applications for porous coordination networks (PCNs)

A short review is given of the recent work on single-phase crystals that are ferroelectric and ferromagnetic at or near room temperature. BiFeO3 is mentioned only briefly, because it has been reviewed in detail elsewhere very recently; emphasis instead is on copper oxide, perovskite oxides with mixed B-site occupancy (such as Pb(Fe1/2Ta1/2)x(ZryTi1−y)1−xO3 and related Nb and W compounds), Fe-, Co-, and Mn-based Aurivilius-phase oxides and hexaferrites. Seven years ago, Wilma Eerenstein, Neil Mathur and Jim Scott published a Venn diagram (above) showing the overlap of piezoelectricity, ferroelectricity (green circle), ferromagnetism (black circle), and magnetoelectricity (blue hatched center circle); and soon thereafter Manuel Bibes put into each sector the crystals which were thought to belong. The overlap region between green and black are multiferroic ferromagnetic-ferroelectrics. Not all were correct; BiMnO3 is NOT ferroelectric. Of these materials, only Cr2O3 and BiFeO3 function at room temperature (or are magnetoelectric, and the former is neither a ferromagnet nor ferroelectric). Today’s review is an update on the new multiferroic and/or magnetoelectric materials that function at or near ambient temperatures and pressures: Cupric oxide (not actually magnetoelectric), iron magnesium hexaferrites, and perovskite oxides based upon PbTiO3.
 Multiferroics combine two properties not typically found together: ferroelectricity and ferromagnetism. With complex structures and properties, such materials are undeniably intriguing. James F. Scott reviews single-phase multiferroics that exhibit the magnetoelectric effect — whereby magnetic polarization is induced by applying an external electric field or, conversely, an electric polarization is induced through a magnetic field — at or near room temperature. In particular, four classes of materials — copper oxides, perovskite oxides, ‘Aurivillius-phase’ layered oxides and manganese hexaferrites — are discussed. Multiferroic magnetoelectrics have potential data storage applications and should ideally combine properties desirable for both magnetic and ferroelectric random-access memory. Various limitations such as low operational temperatures, small polarizations, and possibly slow switching speeds (still unmeasured for most new materials) had hindered their use, but these are likely to be overcome in future. The materials reviewed here seem very promising, advances in processing may improve the properties of multiferroic magnetoelectrics, and these compounds could also prove suitable for other memory devices.

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Room-temperature multiferroic magnetoelectrics

ABO3 perovskites have fascinated solid-state chemists and physicists for decades because they display a seemingly inexhaustible variety of chemical and physical properties. However, despite the diversity of properties found among perovskites, very few of these materials are ferroelectric, or even polar, in bulk. In this Perspective, we highlight recent theoretical and experimental studies that have shown how a combination of non-polar structural distortions, commonly tilts or rotations of the BO6 octahedra, can give rise to polar structures or ferroelectricity in several families of layered perovskites. We discuss the crystal chemical origin of the polarization in each of these families – which emerges through a so-called ‘trilinear coupling’ or ‘hybrid improper’ mechanism – and emphasize areas in which further theoretical and experimental investigation is needed. We also consider how this mechanism may provide a generic route for designing not only new ferroelectrics, but also materials with various other multifunctionalities, such as magnetoelectrics and electric field-controllable metal-insulator transitions.

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Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments

Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Epitaxial (001)-, (118)-, and (104)-oriented Nd-doped Bi4Ti3O12 films have been grown by pulsed-laser deposition from a Bi4−xNdxTi3O12 (x=0.85) target on SrRuO3 coated single-crystal (100)-, (110)-, and (111)-oriented SrTiO3 substrates, respectively. X-ray diffraction illustrated a unique epitaxial relationship between film and substrate for all orientations. We observed a strong dependence of ferroelectric properties on the film orientation, with no ferroelectric activity in an (001)-oriented film; a remanent polarization 2Pr of 12 μC/cm2 and coercive field Ec of 120 kV/cm in a (118)-oriented film; and 2Pr=40 μC/cm2, Ec=50 kV/cm in a (104)-oriented film. The lack of ferroelectric activity along the c-axis is consistent with the orthorhombic nature of the crystal structure of the bulk material, as determined by powder neutron diffraction.

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Orientation dependence of ferroelectric properties of pulsed-laser-ablated Bi4−xNdxTi3O12 films

Two new metal–organic co-ordination frameworks have been prepared hydrothermally in mixed ligand systems and their crystal structures determined: [Co(phth)2(bipy)] is composed of a complex three-dimensional framework consisting of interlinked Co–phth–Co and Co–bipy–Co chains; [Co2(mal)2(bipy)(H2O)2] is also three-dimensional, being composed of Co–mal sheets which are pillared by bidentate bipy molecules (phth = phthalate, mal = malonate, bipy = 4,4′-bipyridine). Both frameworks enclose small void channels.

Metal–organic co-ordination frameworks based on mixed N- and O-donor ligands: crystal structures of [Co(phth)2(bipy)] and [Co2(mal)2(bipy)(H2O)2] (phth = phthalate, mal = malonate, bipy = 4,4′-bipyridine)†

Epitaxial (001)-, (118)-, and (104)-oriented Nd-doped Bi4Ti3O12 films have been grown by pulsed-laser deposition from a Bi4-xNdxTi3O12 (x=0.85) target on SrRuO3 coated single-crystal (100)-, (110)-, and (111)-oriented SrTiO3 substrates, respectively. X-ray diffraction illustrated a unique epitaxial relationship between film and substrate for all orientations. We observed a strong dependence of ferroelectric properties on the film orientation, with no ferroelectric activity in an (001)-oriented film; a remanent polarization, 2Pr, of 12 microC/cm2 and coercive field, Ec, of 120 kV/cm in a (118)-oriented film; and 2Pr = 40 microC/cm2, Ec = 50 kV/cm in a (104)-oriented film. The lack of ferroelectric activity along the c-axis is consistent with the orthorhombic nature of the crystal structure of the bulk material, as determined by powder neutron diffraction.

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Orientation Dependence of Ferroelectric Properties of Pulsed-Laser-Ablated Bi4-xNdxTi3O12 Films

Structural phase transitions in the Aurivillius phase ferroelectrics ${\mathrm{SrBi}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{Bi}}_{5}{\mathrm{Ti}}_{3}{\mathrm{FeO}}_{15},$ containing two and four perovskite layers, respectively, have been studied using powder neutron diffraction. At temperatures below the ferroelectric Curie temperature both materials crystallize in the polar orthorhombic space group ${A2}_{1}\mathrm{am}.$ On passing through ${T}_{C},$ both phases appear to transform directly to a tetragonal paraelectric phase, space group $I4/mmm.$ This behavior contrasts with that of the analogs ${\mathrm{Sr}}_{0.85}{\mathrm{Bi}}_{2.1}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{SrBi}}_{4}{\mathrm{Ti}}_{4}{\mathrm{O}}_{15},$ both of which have been shown unambiguously to transform via an intermediate paraelectric orthorhombic phase, space group Amam.

Alan Snedden

Papers

Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Orientation dependence of ferroelectric properties of pulsed-laser-ablated Bi4−xNdxTi3O12 films

Metal–organic co-ordination frameworks based on mixed N- and O-donor ligands: crystal structures of [Co(phth)2(bipy)] and [Co2(mal)2(bipy)(H2O)2] (phth = phthalate, mal = malonate, bipy = 4,4′-bipyridine)†

Orientation Dependence of Ferroelectric Properties of Pulsed-Laser-Ablated Bi4-xNdxTi3O12 Films

Ferroelectric phase transitions in SrBi 2 Nb 2 O 9 and Bi 5 Ti 3 FeO 15 : A powder neutron diffraction study