36 results found
Pilo J, Pruneda M, Bristowe NC, 2021, Structural and magnetic phase diagram of epitaxial La<inf>0.7</inf>Sr<inf>0.3</inf>MnO<inf>3</inf>from first principles, Electronic Structure, Vol: 3
ABO3perovskites host a huge range of symmetry lowering structural distortions, each of which can tune, or even switch on or off, different functional properties due to the strong coupling between the lattice, spin and charge degrees of freedom in these materials. The sheer number of different meta-stable structures present in perovskites creates a challenge for materials design via theory and simulation. Here, we tackle this issue using a first principles structure searching method on a prototypical half-metallic perovskite, La0.7Sr0.3MnO3, to predict how epitaxial strain can engineer structural and magnetic properties.We reveal a rich structural phase diagram through strain engineering in which the octahedral tilt pattern, and hence the crystal symmetry, is altered from the bulk.We show how the low-symmetry of the various phases in turn induces new structural modes, an increase in the magnetic anisotropy energy, and weak antiferromagnetic spin-canting.
Allen DJW, Bristowe NC, Goodwin AL, et al., 2021, Mechanisms for collective inversion-symmetry breaking in dabconium perovskite ferroelectrics, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 9, ISSN: 2050-7526
Lemal S, Bristowe NC, Ghosez P, 2020, Polarity-field driven conductivity in SrTiO3/LaAlO3: A hybrid functional study, PHYSICAL REVIEW B, Vol: 102, ISSN: 2469-9950
Pomiro F, Ablitt C, Bristowe NC, et al., 2020, From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilization, Physical Review B, Vol: 102, Pages: 014101 – 1-014101 – 8, ISSN: 2469-9950
Hybrid improper ferroelectrics (HIFs) have been studied intensively over the past few years to gain an understanding of their temperature-induced phase transitions and ferroelectric switching pathways. Here we report a switching from a first- to a second-order phase transition pathway for HIFs Ca3−xSrxTi2O7, which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions—such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others—is a crucial step in creating novel functional materials by design.
Peters JJP, Bristowe NC, Rusu D, et al., 2020, Polarisation screening mechanisms at La0.7Sr0.3MnO3-PbTiO3 interfaces, ACS Applied Materials and Interfaces, Vol: 12, Pages: 10657-10663, ISSN: 1944-8244
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 a strong dependence on the polarisation orientation. Density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.
Varignon J, Bristowe NC, Bousquet E, et al., 2020, Magneto-electric multiferroics: designing new materials from first-principles calculations, Physical Sciences Reviews, Vol: 5, Pages: 1-24, ISSN: 2365-659X
In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.
Ablitt C, McCay H, Craddock S, et al., 2019, Tolerance factor control of uniaxial negative thermal expansion in a layered perovskite, Chemistry of Materials, Vol: 32, Pages: 605-610, ISSN: 0897-4756
By tuning the tolerance factor, $t$, of the Ruddlesden--Popper oxide Ca$_2$MnO$_4$ through isovalent substitutions we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. High-resolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as $t$ decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-$t$ and low-$t$ limits of stability. First-principles calculations demonstrate that reducing $t$ enhances the contribution to thermal expansion from the lowest frequency phonons, which have the character of octahedral tilts and have negative mode Gr\"uneisen parameter components along the NTE axis. By tuning $t$ to the lower edge of the NTE phase stability window, we are hence able to maximise the amplitudes of these vibrations and thereby maximise NTE with a CLTE of -8.1~ppm/K at 125~K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.
Coles BD, Hillier AD, Coomer FC, et al., 2019, Spin interactions and magnetic order in the iron oxychalcogenides BaFe2Q2O(Q=Sand Se), Physical Review B, Vol: 100, ISSN: 2469-9950
The ability to tune the iron chalcogenides BaFe2Q3 from Mott insulators to metals and then superconductors with applied pressure has renewed interest in low-dimensional iron chalcogenides and oxychalcogenides. We report here a combined experimental and theoretical study on the iron oxychalcogenides BaFe2Q2O(Q=S,Se) and show that their magnetic behavior results from nearest-neighbor magnetic exchange interactions via oxide and selenide anions of similar strength, with properties consistent with more localized electronic structures than those of BaFe2Q3 systems.
Warwick AR, Iniguez J, Haynes PD, et al., 2019, First-principles study of ferroelastic twins in halide perovskites, Journal of Physical Chemistry Letters, Vol: 10, Pages: 1416-1421, ISSN: 1948-7185
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.
Mostofi A, Ablitt C, Bristowe N, et al., 2019, Negative thermal expansion in high pressure layered perovskite Ca2GeO4, Chemical Communications, Vol: 55, Pages: 2984-2987, ISSN: 1359-7345
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.
Beqiri D, Cascos V, Roberts-Watts J, et al., 2019, Tuning octahedral tilts and the polar nature of A-site deficient perovskites Electronic supplementary information (ESI) available., CHEMICAL COMMUNICATIONS, Vol: 55, Pages: 2609-2612, ISSN: 1359-7345
Lee JH, Deng Z, Bristowe NC, et al., 2018, The competition between mechanical stability and charge carrier mobility in MA-based hybrid perovskites: insight from DFT, Journal of Materials Chemistry C, Vol: 6, Pages: 12252-12259, ISSN: 2050-7534
Hybrid organic-inorganic perovskites and their inorganic analogues, such as MAPbI3 (MA = methylammonium, CH3NH3) and CsPbI3, are currently under intense investigation due to their high-power conversion efficiencies and low cost for solar cell applications. Herein, we investigate the effect of methylammonium and the inorganic A-cations on the elastic and related transport properties of halide perovskites using van der Waals (vdW) corrected density functional theory (DFT) calculations. For inorganic halide perovskites we find that the bonding within the inorganic framework is mainly responsible for their elastic behavior. However, our DFT calculations show that when a MA cation is substituted into the structure the combined effects of stericity (conformation) and hydrogen-framework interactions improve the material's resistance to deformation. For example, the orientationally-averaged Young's modulus of orthorhombic MAPbI3 increases by about 19% compared to the equivalent inorganic series of structures. We also find that, within the carrier-acoustic phonon scattering regime, the electron and hole carrier mobilities of hybrid halide perovskites are lowered by the hydrogen-bonding-induced tilting of the inorganic octahedra. Taken together, these results can help guide the optimization of the mechanical and transport properties of perovskite-based solar cell materials.
Mostofi AA, Ablitt C, Senn M, et al., 2018, Control of uniaxial negative thermal expansion in layered Perovskites by tuning layer thickness, Frontiers in Chemistry, Vol: 6, ISSN: 2296-2646
Uniaxial negative thermal expansion (NTE) is known to occur in low n members of the An+1BnO3n+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.
Saines PJ, Bristowe NC, 2018, Probing magnetic interactions in metal-organic frameworks and coordination polymers microscopically, DALTON TRANSACTIONS, Vol: 47, ISSN: 1477-9226
Senn MS, Bristowe NC, 2018, A group-theoretical approach to enumerating magnetoelectric and multiferroic couplings in perovskites, ACTA CRYSTALLOGRAPHICA A-FOUNDATION AND ADVANCES, Vol: 74, Pages: 308-321, ISSN: 2053-2733
Ablitt C, Craddock S, Senn MS, et al., 2017, The origin of uniaxial negative thermal expansion in layered perovskites, npj Computational Materials, Vol: 3, ISSN: 2057-3960
Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion (NTE) over a wide temperature range, whereas layered perovskites of the same chemical family often do? It is generally accepted that there are two key ingredients that determine the extent of NTE: the presence of soft phonon modes that drive contraction (have negative Grüneisen parameters); and anisotropic elastic compliance that predisposes the material to the deformations required for NTE along a specific axis. This difference in thermal expansion properties is surprising since both ABO3 and layered perovskites often possess these ingredients in equal measure in their high-symmetry phases. Using first principles calculations and symmetry analysis, we show that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and condensed rotations of oxygen octahedra. This feature, unique to layered perovskites of certain symmetry, is what allows uniaxial NTE to persist over a large temperature range. This fundamental insight means that symmetry and the elastic tensor can be used as descriptors in high-throughput screening and to direct materials design.
Miao N, Xu B, Bristowe NC, et al., 2017, Tunable Magnetism and Extraordinary Sunlight Absorbance in Indium Triphosphide Monolayer, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 139, Pages: 11125-11131, ISSN: 0002-7863
Lee J-H, Bristowe N, Lee JH, et al., 2016, Resolving the physical origin of octahedral tilting in halide perovskites, Chemistry of Materials, Vol: 28, Pages: 4259-4266, ISSN: 1520-5002
Hybrid perovskites are currently the fastest growing photovoltaic technology, having reached a solar cell efficiency of over 20%. One possible strategy to further improve the efficiency of perovskite solar cells is to tune the degree of octahedral tilting of the halide frame, since this in turn affects the optical band gap and carrier effective masses. It is commonly accepted that the ion sizes are the main control parameter influencing the degree of tilting in perovskites. Here we re-examine the origin of octahedral tilts in halide perovskites from systematic first principles calculations. We find that whilst steric effects dominate the tilt magnitude in inorganic halides, hydrogen bonding between an organic A-cation and the halide frame plays a significant role in hybrids. For example, in the case of MAPbI3, our calculations suggest that without the contribution from hydrogen bonding the octahedra would not tilt at all. These results demonstrate that tuning the degree of hydrogen bonding can be used as an additional control parameter to optimise the photovoltaic properties of perovskites.
Miao N, Xu B, Bristowe N, et al., 2016, First-Principles Study of the Thermoelectric Properties of SrRuO3, Journal of Physical Chemistry C, Vol: 120, Pages: 9112-9121, ISSN: 1932-7455
The Seebeck coe!cient, thermoelectric power factor, electrical conductivity, andelectronic thermal conductivity of the orthorhombic P bnm phase of SrRuO3 are studiedcomprehensively by combining first-principles density functional calculations andBoltzmann transport theory. The influence of exchange-correlation functional on theSeebeck coe!cient is carefully investigated. We show that the best agreement withexperimental data is achieved when SrRuO3 is described as being at the limit of ahalf-metal. Furthermore, we analyse the role of individual symmetry-adapted atomicdistortions on the Seebeck coe!cient, highlighting a particularly strong sensitivity toR+4 oxygen rotational motions, which may shed light on how to manipulate the Seebeckcoe!cient. We confirm that the power factor of SrRuO3 can only be slightlyimproved by carrier doping. Our results provide a complete understanding of the thermoelectricproperties of SrRuO3 and an interesting insight on the relationship betweenexchange-correlation functionals, atomic motions, and thermoelectric quantities.
Senn MS, Murray C, Luo X, et al., 2016, Symmetry Switching of Negative Thermal Expansion by Chemical Control, Journal of the American Chemical Society, Vol: 138, Pages: 5479-5482, ISSN: 1520-5126
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 maximised 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.
Bristowe N, Varignon J, Ghosez P, 2016, Electric field control of Jahn-Teller distortions in bulk perovskites, Physical Review Letters, Vol: 116, ISSN: 1079-7114
The Jahn-Teller distortion, by its very nature, is often at the heart of the various electronicproperties displayed by perovskites and related materials. Despite the Jahn-Teller mode beingnon-polar, we devise and demonstrate in the present letter an electric field control of Jahn-Tellerdistortions in bulk perovskites. The electric field control is enabled through an anharmonic latticemode coupling between the Jahn-Teller distortion and a polar mode. We confirm this couplingand quantify it through first-principles calculations. The coupling will always exist within theP b21m space group, which is found to be the favoured ground state for various perovskites undersufficient tensile epitaxial strain. Intriguingly, the calculations reveal that this mechanism is notonly restricted to Jahn-Teller active systems, promising a general route to tune or induce novelelectronic functionality in perovskites as a whole.
Bristowe N, Varignon J, Bousquet E, et al., 2015, Coupling and electrical control of structural, orbital and magnetic orders in perovskites, Scientific Reports, Vol: 5, ISSN: 2045-2322
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.
Bristowe NC, Varignon J, Fontaine D, et al., 2015, Ferromagnetism induced by entangled charge and orbital orderings in ferroelectric titanate perovskites, Nature Communications, Vol: 6, ISSN: 2041-1723
In magnetic materials, the Pauli exclusion principle typically drives anti-alignment betweenelectron spins on neighbouring species resulting in antiferromagnetic behaviour.Ferromagnetism exhibiting spontaneous spin alignment is a fairly rare behaviour, but oncematerialized is often associated with itinerant electrons in metals. Here we predict andrationalize robust ferromagnetism in an insulating oxide perovskite structure based on thepopular titanate series. In half-doped layered titanates, the combination of Jahn–Teller andoxygen breathing motions opens a band gap and creates an unusual charge and orbitalordering of the Ti d electrons. It is argued that this intriguingly intricate electronic networkfavours the elusive inter-site ferromagnetic (FM) ordering, on the basis of intra-site Hund’srules. Finally, we find that the layered oxides are also ferroelectric with a spontaneouspolarization approaching that of BaTiO3. The concepts are general and design principles of thetechnologically desirable FM ferroelectric multiferroics are presented.
Aguado-Puente P, Bristowe NC, Yin B, et al., 2015, Model of two-dimensional electron gas formation at ferroelectric interfaces, Physical Review B, Vol: 92, ISSN: 1550-235X
The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here we study under what circumstances a similar processes can also take place underneath ferroelectric thin films. We use a simple Landau model to demonstrate that in the absence of extrinsic screening mechanisms a monodomain phase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO3/SrTiO3 heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first principles simulations of free standing slabs of PbTiO3. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a non-volatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces.
Lee J-H, Bristowe NC, Bristowe PD, et al., 2015, Role of hydrogen-bonding and its interplay with octahedral tilting in CH3NH3PbI3, Chemical Communications, Vol: 51, Pages: 6434-6437, ISSN: 1364-548X
First principles calculations on the hybrid perovskite CH3NH3PbI3predict strong hydrogen-bonding which influences the structureand dynamics of the methylammonium cation and reveal its interactionwith the tilting of the PbI6 octahedra. The calculated atomiccoordinates are in excellent agreement with neutron diffractionresults.
Varignon J, Bristowe NC, Bousquet É, et al., 2015, Novel magneto-electric multiferroics from first-principles calculations, Comptes Rendus Physique, Vol: 16, Pages: 153-167, ISSN: 1631-0705
Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed behaviours, the trend has evolved and, nowadays, density functional theory calculations have become also an essential predicting tool for identifying original rules to achieve multiferroism and design new magneto-electric compounds. This chapter aims at highlighting the key advances in the field of multiferroics, to which first-principles methods have contributed significantly. The essential theoretical developments that made this research possible are also briefly presented.
Bristowe NC, Ghosez P, Littlewood PB, et al., 2014, The origin of two-dimensional electron gases at oxide interfaces: insights from theory, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 26, ISSN: 0953-8984
Miao N, Bristowe NC, Xu B, et al., 2014, First-principles study of the lattice dynamical properties of strontium ruthenate, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 26, ISSN: 0953-8984
Bristowe NC, Stengel M, Littlewood PB, et al., 2013, One-dimensional half-metallic interfaces of two-dimensional honeycomb insulators, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950
Kumar A, Arruda TM, Kim Y, et al., 2012, Probing Surface and Bulk Electrochemical Processes on the LaAlO3-SrTiO3 Interface, ACS NANO, Vol: 6, Pages: 3841-3852, ISSN: 1936-0851
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