The Interplay of Electronic Configuration and Anion Ordering on the Magnetic Behavior of Hydroxyfluoride Diaspores.

We report a new nickel hydroxyfluoride diaspore Ni(OH)F prepared using hydrothermal synthesis from NiCl2·6H2O and NaF. Magnetic characterization reveals that, contrary to other reported transition-metal hydroxyfluoride diaspores, Ni(OH)F displays weak ferromagnetism below the magnetic ordering temperature. To understand this difference, neutron diffraction is used to determine the long-range magnetic structure. The magnetic structure is found to be distinct from those reported for other hydroxyfluoride diaspores and shows an antiferromagnetic spin ordering in which ferromagnetic canting is allowed by symmetry. Furthermore, neutron powder diffraction on a deuterated sample, Ni(OD)F, reveals partial anion ordering that is distinctive to what has previously been reported for Co(OH)F and Fe(OH)F. Density functional theory calculations show that OH/F ordering can have a directing influence on the lowest energy magnetic ground state. Our results point toward a subtle interplay between the sign of magnetic exchange interactions, the electronic configuration, and anion disordering.

First-principles investigation of the magnetoelectric properties of Ba7Mn4O15.

Type-II multiferroics, in which the magnetic order breaks inversion symmetry, are appealing for both fundamental and applied research due their intrinsic coupling between magnetic and electrical orders. Using first-principles calculations we study the ground state magnetic behaviour of Ba7Mn4O15which has been classified as a type-II multiferroic in recent experiments. Our constrained moment calculations with the proposed experimental magnetic structure shows the spontaneous emergence of a polar mode giving rise to an electrical polarisation comparable to other known type-II multiferroics. When the constraints on the magnetic moments are removed, the spins self-consistently relax into a canted antiferromagnetic ground state configuration where two magnetic modes transforming as distinct irreducible representations coexist. While the dominant magnetic mode matches well with the previous experimental observations, the second mode is found to possess a different character resulting in a non-polar ground state. Interestingly, the non-polar magnetic ground state exhibits a significantly strong linear magnetoelectric (ME) coupling comparable to the well-known multiferroic BiFeO3, suggesting strategies to design new linear MEs.

Quantum Spin-1/2 Dimers in a Low-Dimensional Tetrabromocuprate Magnet.

This work describes a homometallic spin- 1 / 2 tetrabromocuprate adopting a bilayer structure. Magnetic-susceptibility measurements show a broad maximum centred near 70 K, with fits to this data using a Heisenberg model consistent with strong antiferromagnetic coupling between neighbouring copper atoms in different layers of the bilayer. There are further weak intralayer ferromagnetic interactions between copper cations in neighbouring dimers. First-principles calculations are consistent with this, but suggest there is only significant magnetic coupling within one direction of a layer; this would suggest the presence of a spin ladder within the bilayer with antiferromagnetic rung and weaker ferromagnetic rail couplings.

Striping of orbital-order with charge-disorder in optimally doped manganites.

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the [Formula: see text] doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn[Formula: see text]Mn[Formula: see text]O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.

Mechanisms for collective inversion-symmetry breaking in dabconium perovskite ferroelectrics.

Dabconium hybrid perovskites include a number of recently-discovered ferroelectric phases with large spontaneous polarisations. The origin of ferroelectric response has been rationalised in general terms in the context of hydrogen bonding, covalency, and strain coupling. Here we use a combination of simple theory, Monte Carlo simulations, and density functional theory calculations to assess the ability of these microscopic ingredients-together with the always-present through-space dipolar coupling-to account for the emergence of polarisation in these particular systems whilst not in other hybrid perovskites. Our key result is that the combination of A-site polarity, preferred orientation along 〈111〉 directions, and ferroelastic strain coupling drives precisely the ferroelectric transition observed experimentally. We rationalise the absence of polarisation in many hybrid perovskites, and arrive at a set of design rules for generating FE examples beyond the dabconium family alone.

Polarization Screening Mechanisms at La0.7Sr0.3MnO3-PbTiO3 Interfaces.

The structural, electronic, and magnetic properties of interfaces between epitaxial La0.7Sr0.3MnO3 and PbTiO3 have been explored via atomic resolution transmission electron microscopy of a functional multiferroic tunnel junction. Measurements of the polar displacements and octahedral tilting show the competition between the two distortions at the interface and demonstrate strong dependence on the polarization orientation. The density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.

First-Principles Study of Ferroelastic Twins in Halide Perovskites.

We present an ab initio simulation of 90° ferroelastic twins that were recently observed in methylammonium lead iodide. There are two inequivalent types of 90° walls that we calculate to act as either electron or hole sinks, which leads us to propose a mechanism for enhancing charge carrier separation in photovoltaic devices. Despite separating nonpolar domains, we show these walls to have a substantial in-plane polarization of ∼6 µC cm-2, due in part to flexoelectricity. We suggest this in turn could allow for the photoferroic effect and create efficient pathways for photocurrents within the wall.

Negative thermal expansion in high pressure layered perovskite Ca2GeO4.

We report the high pressure synthesis of a layered perovskite Ca2GeO4 which is found to have the Ruddlesden-Popper structure with I41/acd symmetry. Consonant with our recent predictions [Ablitt et al., npj Comput. Mater., 2017, 3, 44], the phase displays pronounced uniaxial negative thermal expansion over a large temperature range. Negative thermal expansion that persists over a large temperature range is very unusual in the perovskite structure, and its occurrence in this instance can be understood to arise due to both soft lattice vibrations associated with a phase competition and the unusually compliant nature of this structure, which effectively couples thermal expansion in the layer plane to lattice contractions perpendicular to the layering direction via a "corkscrew" mechanism.

Tuning octahedral tilts and the polar nature of A-site deficient perovskites.

Herein we highlight the ability to tune the structural chemistry of A-site deficient perovskite materials Ln1/3NbO3. Computational studies explore the balance between proper and hybrid-improper mechanisms for polar behaviour in these systems.

Control of Uniaxial Negative Thermal Expansion in Layered Perovskites by Tuning Layer Thickness.

Uniaxial negative thermal expansion (NTE) is known to occur in low n members of the A n+1B n O3n+1 Ruddlesden-Popper (RP) layered perovskite series with a frozen rotation of BO6 octahedra about the layering axis. Previous work has shown that this NTE arises due to the combined effects of a close proximity to a transition to a competing phase, so called "symmetry trapping", and highly anisotropic elastic compliance specific to the symmetry of the NTE phase. We extend this analysis to the broader RP family (n = 1, 2, 3, 4, …, ∞), demonstrating that by changing the fraction of layer interface in the structure (i.e., the value of 1/n) one may control the anisotropic compliance that is necessary for the pronounced uniaxial NTE observed in these systems. More detailed analysis of how the components of the compliance matrix develop with 1/n allows us to identify different regimes, linking enhancements in compliance between these regimes to the crystallographic degrees of freedom in the structure. We further discuss how the perovskite layer thickness affects the frequencies of soft zone boundary modes with large negative Grüneisen parameters, associated with the aforementioned phase transition, that constitute the thermodynamic driving force for NTE. This new insight complements our previous work-showing that chemical control may be used to switch from positive to negative thermal expansion in these systems-since it makes the layer thickness, n, an additional design parameter that may be used to engineer layered perovskites with tuneable thermal expansion. In these respects, we predict that, with appropriate chemical substitution, the n = 1 phase will be the system in which the most pronounced NTE could be achieved.

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

Materials with magnetic interactions between their metal centres play a tremendous 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 enables them to exhibit multifunctional magnetic properties or unique magnetic states not found in the conventional magnetic materials, such as metal oxides. Techniques that enable the magnetic interactions in these materials to be probed at the atomic scale, long established to be key for developing 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 using 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 paper 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 highlights the information that can be obtained from such microscopic studies, including the strengths and limitations of these techniques. The article then concludes with a brief perspective on the future of this area.

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

A group-theoretical approach is used to enumerate the possible couplings between magnetism and ferroelectric polarization in the parent Pm{\overline 3}m perovskite structure. It is shown that third-order magnetoelectric coupling terms must always involve magnetic ordering at the A and B sites which either transforms both as R-point or both as X-point time-odd irreducible representations (irreps). For fourth-order couplings it is demonstrated 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 odd with respect to inversion symmetry. It is possible to show that the time-even irreps in this triad must transform as one of: X1+, M3,5- or R5+, corresponding to A-site cation order, A-site antipolar displacements or anion rocksalt ordering, respectively. This greatly reduces the search space for type-II multiferroic perovskites. Similar arguments are used to demonstrate how weak ferromagnetism may be engineered and a variety of schemes are proposed for coupling this to ferroelectric polarization. The approach is illustrated with density functional theory calculations on magnetoelectric couplings and, by considering the literature, suggestions are given of which avenues of research are likely to be most promising in the design of novel magnetoelectric materials.

Tunable Magnetism and Extraordinary Sunlight Absorbance in Indium Triphosphide Monolayer.

Atomically thin two-dimensional (2D) materials have received considerable research interest due to their extraordinary properties and promising applications. Here we predict the monolayered indium triphosphide (InP3) as a new semiconducting 2D material with a range of favorable functional properties by means of ab initio calculations. The 2D InP3 crystal shows high stability and promise of experimental synthesis. It possesses an indirect band gap of 1.14 eV and a high electron mobility of 1919 cm2 V-1 s-1, which can be strongly manipulated with applied strain. Remarkably, the InP3 monolayer suggests tunable magnetism and half-metallicity under hole doping or defect engineering, which is attributed to the novel Mexican-hat-like bands and van Hove singularities in its electronic structure. A semiconductor-metal transition is also revealed by doping 2D InP3 with electrons. Furthermore, monolayered InP3 exhibits extraordinary optical absorption with significant excitonic effects in the entire range of the visible light spectrum. All these desired properties render 2D InP3 a promising candidate for future applications in a wide variety of technologies, in particular for electronic, spintronic, and photovoltaic devices.

Symmetry Switching of Negative Thermal Expansion by Chemical Control.

The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.

Electric Field Control of Jahn-Teller Distortions in Bulk Perovskites.

The Jahn-Teller distortion, by its very nature, is often at the heart of the various electronic properties displayed by perovskites and related materials. Despite the Jahn-Teller mode being nonpolar, we devise and demonstrate, in the present Letter, an electric field control of Jahn-Teller distortions in bulk perovskites. The electric field control is enabled through an anharmonic lattice mode coupling between the Jahn-Teller distortion and a polar mode. We confirm this coupling and quantify it through first-principles calculations. The coupling will always exist within the Pb2_{1}m space group, which is found to be the favored ground state for various perovskites under sufficient tensile epitaxial strain. Intriguingly, the calculations reveal that this mechanism is not only restricted to Jahn-Teller active systems, promising a general route to tune or induce novel electronic functionality in perovskites as a whole.

Coupling and electrical control of structural, orbital and magnetic orders in perovskites.

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from first-principles calculations. The proposed mechanism is completely general, however for illustrative purposes, we demonstrate the concept on vanadium based perovskites where we reveal an unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks to the intimate coupling between Jahn-Teller distortions and electronic degrees of freedom, the electric field control of Jahn-Teller distortions is of general relevance and may find broad interest in various functional devices.

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

First 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.

First-principles study of the lattice dynamical properties of strontium ruthenate.

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO3 show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. Our systematic analysis confirms previous expectations and contributes to rationalizing better why many ABO3 perovskites, including metallic compounds, exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties.

Probing surface and bulk electrochemical processes on the LaAlO3-SrTiO3 interface.

Local electrochemical phenomena on the surfaces of the LaAlO(3)-SrTiO(3) heterostructure are explored using unipolar and bipolar dynamic electrochemical strain microscopy (D-ESM). The D-ESM suggests the presence of at least two distinct electrochemical processes, including fast reversible low-voltage process and slow high-voltage process. The latter process is associated with static surface deformations in the sub-nanometer regime. These behaviors are compared with Kelvin probe force microscopy hysteresis data. The possible origins of observed phenomena are discussed, and these studies suggest that charge-writing behavior in LAO-STO includes a strong surface/bulk electrochemical component and is more complicated than simple screening by surface adsorbates.

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.