Pressure-dependent phase transitions in hybrid improper ferroelectric Ruddlesden-Popper oxides

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a combined high-pressure powder synchrotron x-ray diffraction experiment and abinitio study on the bilayered Ruddlesden-Popper phases Ca3Mn2O7 and Ca3Ti2O7. In both compounds we observe a first-order phase transition, that in combination with our density functional theory calculations, we can confidently assign as being between polar A21am and nonpolar Acaa structures. Interestingly, we show that while the application of pressure ultimately favors a nonpolar phase, as is commonly observed for proper ferroelectrics, regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability

Pseudo-proper two-dimensional electron gas formation

In spite of the interest in the two-dimensional electron gases (2DEGs) experimentally found at surfaces and interfaces, important uncertainties remain about the observed insulator–metal transitions (IMTs). Here we show how an explicit pseudo-proper coupling of carrier sources with a relevant soft mode significantly affects the transition. The analysis presented here for 2DEGs at polar interfaces is based on group theory, Landau-Ginzburg theory, and illustrated with first-principles calculations for the prototypical case of the LaAlO3⁄SrTiO3 interface, for which such a structural transition has recently been observed. This direct coupling implies that the appearance of the soft mode is always accompanied by carriers. For sufficiently strong coupling an avalanchelike first-order IMT is predicted

Large dynamic scissoring mode displacements coupled to band gap opening in the cubic phase of the methylammonium lead halide perovskites.

Hybrid perovskites are a rapidly growing research area, having reached photovoltaic power conversion efficiencies of over 25 %. There is a increasing consensus that the structures of these materials, and hence their electronic structures, can not be understood purely from the time and space averaged crystal structures observable by conventional methods. We apply a symmetry-motivated analysis method to analyse X-ray pair distribution function data of the cubic phases of the hybrid perovskites MAPb$X_3$ ($X$ = I, Br, Cl). We demonstrate that, even in the cubic phase, the local structure of the inorganic components of MAPb$X_3$ ($X$ = I, Br, Cl), are dominated by scissoring type deformations of the Pb$X_6$ octahedra. We find these modes to have a larger amplitude than equivalent distortions in the $A$-site deficient perovskite ScF$_3$ and demonstrate that they show a significant departure from the harmonic approximation. Calculations performed on an inorganic perovskite analogue, FrPbBr show that the large amplitudes of the scissoring modes are coupled to a dynamic opening of the electronic band gap. Finally, we use density functional theory calculations to show that the organic MA cations reorientate to accommodate the large amplitude scissoring modes. [Abstract copyright: © 2024 IOP Publishing Ltd.

Structural origins of dielectric anomalies in the filled tetragonal tungsten bronze, Sr2NaNb5O15

The tetragonal tungsten bronze, Sr2NaNb5O15, shows promise for application in high-temperature high-efficiency capacitors vital for the sustainable energy revolution. Previously, the structural complexity of this and related materials has obscured the mechanisms underpinning two large anomalies in relative permittivity (εr) which give rise to their exceptionally broad dielectric response. We comprehensively investigate the structural evolution from −173 to 627 °C, combining electron, X-ray and neutron diffraction, electron microscopy, and first principles electronic structure calculations to unambiguously identify the structural origins of both anomalies. The peak in εr at 305 °C is associated with a polar-nonpolar phase transition, wherein cations displace along the c-axis. Guided by DFT, we identify a further transition upon cooling, associated with the second peak at −14 °C, linked to the softening of an in-plane polar distortion with a correlation length limited by ferroelastic nano-domains arising from rigid-unit-like tilting of NbO6 octahedra at high temperature, imparting relaxor-like behaviour. Thus, the two dielectric anomalies in Sr2NaNb5O15 are associated with two distinct crystallographic phase transitions and their interplay with a microstructure that arises from a third, non-polar structural distortion. Chemical control of these will enable development of tuneable materials with dielectric properties suitable for high-temperature energy storage applications

Symmetry-informed design of magnetoelectric coupling in the manganite perovskite CeBaMn2O6

Magnetoelectric multiferroics hold great promise for the development of new sustainable memory devices. However, practical applications of many existing multiferroic materials are infeasible due to the weak nature of the coupling between the magnetic and electrical orderings, meaning new magnetoelectric multiferroics featuring intrinsic coupling between their component orderings are sought instead. Here, we apply a symmetry-informed design approach to identify and realize the new manganite perovskite CeBaMn2O6 in which magnetoelectric coupling can be achieved via an intermediary non-polar structural distortion. Through first-principles calculations, we demonstrate that our chosen prototype system contains the required ingredients to achieve the desired magnetoelectric coupling. Using high-pressure/high-temperature synthesis conditions, we have been able to synthesize the CeBaMn2O6 perovskite system for the first time. Our subsequent neutron and electron diffraction measurements reveal that the desired symmetry-breaking ingredients exist in this system on a nanoscopic length scale, enabling magnetoelectric nanoregions to emerge within the material. Through this work, we showcase the potential of the new CeBaMn2O6 perovskite material as a promising system in which to realize strong magnetoelectric coupling, highlighting the potential of our symmetry-informed design approach in the pursuit of new magnetoelectric multiferroics for next-generation memory devices

A group-theoretical approach to enumerating magnetoelectric and multiferroic couplings in perovskites

We use a group theoretical approach to enumerate the possible couplings between magnetism and ferroelectric polarisation in the parent $Pm\bar{3}m$ perovskite structure. We show that third order magnetoelectric coupling terms must always involve magnetic ordering at the A and B-site which either transforms both as R-point or both as X-point time odd irreducible representations (irreps). For fourth order couplings we demonstrate that this criterion may be relaxed allowing couplings involving irreps at X, M and R-points which collectively conserve crystal momentum, producing a magnetoelectric effect arising from only B-site magnetic order. In this case, exactly two of the three irreps entering the order parameter must be time-odd irreps and either one or all must be even with respect to inversion symmetry. We are able to show that the time-even irreps in this triad must transform as one of: X$_{1}^{-}$, M$_{3,5}^{-}$ and R$_{5}^{+}$, corresponding to A-site cation order, A-site anti-polar displacements or anion rock-salt ordering. This greatly reduces the search-space for (type-II multiferroic) perovskites. We use similar arguments to demonstrate how weak ferromagnetism may be engineered, and we propose a variety of schemes for coupling this to ferroelectric polarisation. We illustrate our approach with DFT calculations on magnetoelectric couplings, and by considering the literature we suggest which avenues of research are likely to be most promising in the design of novel magnetoelectric materials

Probing magnetic interactions in metal-organic frameworks and coordination polymers microscopically

Materials with magnetic interactions between their metal centres play both a tremedous role in modern technologies and can exhibit unique physical phenomena. In recent years magnetic metal-organic frameworks and coordination polymers have attracted significant attention because their unique structural flexibility enable them to exhibit multifunctional magnetic properties or unique magnetic states not found in conventional magnetic materials, such as metal oxides. The techniques that enable the magnetic interactions in these materials to be probed at the atomic scale, as long established as key for devloping other magnetic materials, are not well established for studying metal-organic frameworks and coordination polymers. This review focuses on studies where metal-organic frameworks and coordination polymers have been examined by such microscopic probes, with a particular focus on neutron scattering and density-functional theory, the most-well established experimental and computational techniques for understanding magnetic materials in detail. This builds on a brief introduction to these techniques to describe how such probes have been applied to a variety of magnetic materials starting with select historical examples before discussing multifunctional, low dimensional and frustrated magnets. This review highlight the information that can be obtained from such micrscopic studies, including the strengths and limitations of these techniques. The article then concludes with a brief perspective on the future of this area

Role of hydrogen-bonding and its interplay with octahedral tilting in CH3NH3PbI3

This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C5CC00979KFirst principles calculations on the hybrid perovskite CH3NH3PbI3 predict strong hydrogen-bonding which influences the structure and dynamics of the methylammonium cation and reveal its interaction with the tilting of the PbI6 octahedra. The calculated atomic coordinates are in excellent agreement with neutron diffraction results. [Image - see article]Funding from the Winton Programme for the Physics of Sustainability at the University of Cambridge is gratefully acknowledged. NCB acknowledges financial support from the Royal Commission for the Exhibition of 1851 for a fellowship at Imperial College London. The calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service. Access to the latter was obtained via the UKCP consortium and funded by EPSRC Grant No. EP/K014560/1

Van der Waals forces in the perfluorinated metal-organic framework zinc 1,2-bis(4-pyridyl)ethane tetrafluoroterephthalate

Traditional density functional theory (DFT) and dispersion-corrected DFT calculations are performed to investigate the metal-organic framework zinc 1,2-bis(4-pyridyl)ethane tetrafluoroterephthalate (Znbpetpa). Without dispersion correction, straightening of the zigzag C-O-Zn chain connecting the secondary building units across the diagonal of the unit cell is observed, accompanied by a large anisotropic expansion of the structure along one cell parameter. The results show that van der Waals dispersion forces and specifically Zn-C equatorial interactions and the resulting effects on the zigzag chain play an important role in maintaining key structural features which match with experimental observations. It is suggested that the pore volume of the framework could be controlled by substituting the Zn metal centre with another transition element of different polarizability, while maintaining functional linkers