arXiv subjects

Nicholas C. Bristowe

Publications and source records attributed to Nicholas C. Bristowe.

At least 19 recordsLinked to original sources

arXiv2026

Tilt-driven ferrielectricity in PbZrO$_3$

We reveal a tilt-driven mechanism for ferrielectricity in prototypical antiferroelectric PbZrO$_3$. Specifically, introducing an additional octahedral tilt into the antiferroelectric $Pbam$ phase breaks the symmetry constraint that enforces equal antiparallel dipoles, converting the compensated ``$\uparrow \uparrow \downarrow \downarrow$'' nonpolar configuration into an uncompensated ``$\uparrow \uparrow \downarrow \downarrow$'' polar $Pmc2_1$ phase. First-principles calculations show that the $Pmc2_1$ phase becomes stabilized under lattice contraction and gains increasing free-energy advantage over competing phases at finite temperatures. Atomic-scale imaging directly confirms the presence of $Pmc2_1$-like structures in thin films and single crystals. This work identifies a symmetry-governed, kinetically easily accessible pathway to ferrielectricity and establishes a form of ``competitive'' improper ferroelectricity, with broad implications for antiferroelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Near-room-temperature magnetoelectric coupling engineered through inversion-breaking tilts in a bulk perovskite polytype

Systematic strategies to design properties such as ferroelectricity or magnetoelectric coupling are well established in simple perovskite materials, but they remain scarce in more complex framework structures. Using a hexagonal polytype of the ternary manganite AMnO3 (A = Ba, Sr, Ca) as a model system, we introduce a symmetry-guided design principle in which an inversion-breaking rigid-unit mode (RUM) serves as a single structural instability generating both polar and ferromagnetic orders within a bulk material. Symmetry analysis and first-principles calculations reveal that co-operative tilts of the Mn2O9 bioctahedral dimers generate both a spontaneous polarization and a ferromagnetic moment. High-resolution diffraction and magnetic susceptibility measurements show the structural and magnetic orders persist as high as 450 K and 280 K, respectively, highlighting the untapped potential of framework structures which deviate from simple perovskite motifs to be designed to host useful ferroic properties. Our approach establishes a transferable symmetry-based framework to engineer ferroelectric and magnetoelectric states across chemically diverse framework architectures.

cond-mat.mtrl-sci↗

arXiv2026

Alternative origins of polarity in compressively strained SrTiO3-RENiO3 capacitors

Since its original prediction 25 years ago, room-temperature out-of-plane ferroelectricity in compressively strained SrTiO3 remains an ongoing pursuit. In this work, we investigate the structural, electrical and electromechanical properties of highly strained epitaxial SrTiO3 capacitors with rare earth nickelate electrodes. The SrTiO3 layers experience compressive strains up to -3% and exhibit pronounced tetragonality, comparable to that of bulk PbTiO3. Variable-temperature electrical measurements and room-temperature piezoresponse force microscopy reveal butterfly-shaped capacitance-voltage hysteresis and domain-like electromechanical response typical of ferroelectric materials. However, the overall behavior is inconsistent with a stable ferroelectric state. We therefore propose an alternative mechanism for the observed polarity in our samples based on spatially inhomogeneous internal fields. Our first-principles calculations show that such fields may arise from charge discontinuities between the formally charged NdNiO3 layers and charge-neutral SrTiO3 layers.

cond-mat.mtrl-sci↗

arXiv2025

The Microscopic Structure of Stacking Faults in Sr$_2$NaNb$_5$O$_{15}$

Stacking faults and other topological defects in ferroics can have a significant influence on the electronic and mechanical properties of the material. Here, regular stacking faults in the tetragonal tungsten bronze material Sr$_2$NaNb$_5$O$_{15}$ are investigated through transmission electron microscopy, symmetry mode analysis and machine-learned force-field calculations. It is shown that the faults, with a fault vector of $\frac{1}{4}[\bar{2}12]_o$, annihilate in sets of four in the material, owing to the $\frac{1}{4}$ unit cell displacement along the b-axis. The four resulting domains emerge as four possible directions of the S$_3$ order parameter, related to NbO$_6$ octahedral tilts in the material. Force-field calculations reveal that the stacking faults are likely placed at positions where the octahedra in neighbouring domains have similar magnitudes of rotation, and that the estimated stacking fault energy is 46 mJ/m$^2$. The investigation shows that the stacking faults have a significant local effect on the polar modes present in the structure, and therefore could affect the ferroelectric properties.

cond-mat.mtrl-sci↗

arXiv2025

Dielectric softening in the halide double perovskites $A_2$Au$_2X_6$ ($A$: Cs, Rb; $X$: Cl, Br, I) via a strain-mediated pseudotriggered mechanism

Halide perovskites have emerged as promising candidates for next generation photovoltaic applications, attracting significant attention in recent years. Through first-principles calculations combined with group-theoretical analyses, we investigate the structural phase diagram of Pb-free Jahn-Teller-active $A_2$Au$_2X_6$ ($A$: Cs, Rb; $X$: Cl, Br, I) double perovskites. Our study identifies a previously unreported ferroelectric phase, where the softening of the polar mode$, $key to ferroelectricity, is driven by an unconventional and indirect coupling with improper strains originating from Jahn-Teller distortions. The proposed strain mediated \textit{pseudo}triggered mechanism offers an alternative pathway to enhance the static dielectric constant or even promote (photo-)ferroelectricity, addressing challenges such as defects, excitons, and charge scattering that hinder photovoltaic efficiency. More broadly, this unique mechanism could be extended to oxide double perovskites and opens up a new type of ferroelectric phase transition worthy of future investigation.

cond-mat.mtrl-sci↗

arXiv2025

First principles study of [111]-oriented epitaxially strained Rare-Earth Nickelate NdNiO$_3$

Density functional theory is used to investigate the effect of biaxial strain on the structural, electronic and magnetic properties of [111]-oriented NdNiO$_3$, as a representative of the rare-earth perovskites that undergo metal-to-insulator transitions. We find that this constraint on the system induces unique structural phase transitions not previously observed under the well-studied bulk or [001]-oriented strained systems. We also report unique electronic behaviour, including amplification of the electronic band-gap with tensile strain, and insulating, charge-ordered phases with non-orthorhombic tilt patterns. To provide clarity to the trends we observe, we also investigate the coupling between the breathing mode and strain, where we observe certain strains to directly favour and disfavour the creation of the breathing mode (and thus the associated charge-ordering). The amplification of the band gap with strain is understood in terms of a cooperative coupling between the elastic constraint and octahedral breathing, which expands on the previously reported triggered mechanism mediated by octahedral tilting.

cond-mat.mtrl-sci↗

arXiv2025

Prediction of Room Temperature Electric Field Reversal of Magnetization in the Family of $A_4B_3\rm{O}_9$ Layered Oxides

The promise of a strong magnetoelectric coupling in a multiferroic material is not only of fundamental interest, but also forms the basis of next generation memory devices where the direction of magnetization can be reversed by an external electric field. Using group-theory led first-principles calculations, we have identified a hitherto unknown polar phase of the $A_4B_3\rm{O}_9$ layered oxides, where the polar mode couples to the magnetic modes through a rare $Γ$-point magnetoelectric-multiferroic coupling scheme such that the net magnetization can be directly reversed by an electric field switching of the polar mode. Furthermore, in agreement with previous experimental observations, we predict room temperature magnetism in $A_4B_3\rm{O}_9$ oxides which indicates the promising practical applications of these compounds in the next generation memory devices.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-induced orbital reordering in Na$_2$CuF$_4$

The high-pressure behaviour of Na$_2$CuF$_4$ is explored by powder neutron diffraction and density functional theory (DFT) calculations. A first-order phase transition is observed to take place between 2.4 - 2.9 GPa, involving a reorientation of the Jahn-Teller (JT) long axes of the (CuF6) octahedra (and therefore the d$_{z^2}$ Cu orbitals), in agreement with our DFT calculations which suggest a transition at 2.8 GPa. The transition can be described as being between a state of ferro-orbital order and one of A-type antiferro-orbital order, reflecting a shift in the associated electronic instability from being in the zone-center to zone boundary of the first Brillouin zone of the parent structure, with pressure. This change results in a decoupling of magnitude of the associated Jahn-Teller distortion of the Cu-F bond lengths from the lattice strain. This scenario is supported by our observations that the compressibility of the pre-transition phase is highly anisotropic, whilst in the post-transition phase it becomes almost isotropic, and that we observed no further decrease of the magnitude the JT distortion up to 5 GPa, or melting of the OO in our DFT calculations up to at least 5 GPa.

cond-mat.str-el↗

arXiv2025

Pressure induced enhancement of polar distortions in a metal, and implications on the Rashba spin-splitting

Polar metals are an intriguing class of materials in which electric polarisation and metallicity can coexist within a single phase. The unique properties of polar metals challenge expectations, making way for the exploration of exotic phenomena such as unconventional magnetism, hyperferroelectric multiferroicity and developing multifunctional devices that can leverage both the materials electric polarization and its asymmetry in the spin conductivity, that arises due to the Rashba effect. Here, via a high pressure single crystal diffraction study, we report the pressure-induced enhancement of polar distortions in such a metal, Ca$_3$Ru$_2$O$_7$. Our DFT calculations highlight that naive assumptions about the linear dependency between polar distortion amplitudes and the magnitude of the Rashba spin splitting may not be generally valid.

cond-mat.str-el↗

arXiv2024

Universal Polar Instability In Highly Orthorhombic Perovskites

The design of novel multiferroic ABO$_3$ perovskites is complicated by the presence of necessary magnetic cations and ubiquitous antiferrodistortive modes, both of which suppress polar distortions. Using first-principles simulations, we observe that the existence of quadlinear and trilinear invariants in the free energy, coupling tilts and antipolar motions of the A and B sites to the polar mode, drives an avalanche-like transition to a non-centrosymmetric $Pna2_1$ symmetry in a wide range of magnetic perovskites with small tolerance factors - overcoming the above restrictions. We find that the $Pna2_1$ phase is especially favoured with tensile epitaxial strain, leading to an unexpected but technologically useful out-of-plane polarization. We use this mechanism to predict various novel multiferroics displaying interesting magnetoelectric properties with small polarization switching barriers.

cond-mat.mtrl-sci↗

arXiv2023

First-principles investigation of the magnetoelectric properties of Ba$_7$Mn$_4$O$_{15}$

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 Ba$_7$Mn$_4$O$_{15}$ which 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 coupling comparable to the well-known multiferroic BiFeO$_3$, suggesting strategies to design new linear magnetoelectrics.

cond-mat.mtrl-sci↗

arXiv2023

The origin of strain-induced stabilisation of superconductivity in the lanthanum cuprates

Suppression of superconductivity in favour of a striped phase, and its coincidence with a structural transition from a low-temperature orthorhombic (LTO) to a low-temperature tetragonal (LTT) phase, is a ubiquitous feature of hole-doped lanthanum cuprates. We study the effect of anisotropic strain on this transition using density-functional theory on both La$_2$CuO$_4$ and the recently-synthesised surrogate La$_2$MgO$_4$ to decouple electronic and structural effects. Strikingly, we find that compressive strain applied diagonally to the in-plane metal-oxygen bonds dramatically stabilises the LTO phase. Given the mutual exclusivity of 3D superconductivity and long-range static stripe order, we thereby suggest a structural mechanism for understanding experimentally-observed trends in the superconducting $T_{\mathrm{c}}$ under uniaxial pressure, and suggest principles for tuning it.

cond-mat.supr-con↗

arXiv2021

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 high-pressure powder synchrotron X-ray diffraction experiment and $ab~initio$ calculations on the bilayered Ruddlesden-Popper phases Ca$_{3}$Mn$_{2}$O$_{7}$ and Ca$_{3}$Ti$_{2}$O$_{7}$. In both compounds we observe a first-order phase transition between polar $A2_{1}am$ and non-polar $Acaa$ structures. Interestingly, we show that while the application of pressure ultimately favours a non-polar 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.

cond-mat.mtrl-sci↗

arXiv2021

Structural origins of the infamous "Low Temperature Orthorhombic" to "Low Temperature Tetragonal" phase transition in high-Tc cuprates

We undertake a detailed high-resolution diffraction study of a novel plain band insulator, La$_2$MgO$_4$, which may be viewed as a structural surrogate system of the undoped end-member of the high-T$_c$ superconductors, La$_{2-x-y}$A$^{2+}_x$RE$^{3+}_y$CuO$_{4}$ (A = Ba, Sr, RE= Rare Earth). We find that La$_2$MgO$_4$ exhibits the infamous low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase transition that has been linked to the suppression of superconductivity in a variety of underdoped cuprates, including the well known La$_{2-x}$Ba$_{x}$CuO$_4$ ($x=0.125$). Furthermore, we find that the LTO-to-LTT phase transition in La$_2$MgO$_4$ occurs for an octahedral tilt angle in the 4 $^{\circ}$ to 5 $^{\circ}$ range, similar to that which has previously been identified as a critical tipping point for superconductivity in these systems. We show that this phase transition, occurring in a system lacking spin correlations and competing electronic states such as charge-density waves and superconductivity, can be understood by simply navigating the density-functional theory ground-state energy landscape as a function of the order parameter amplitude. This result calls for a careful re-investigation of the origins of the phase transitions in high-T$_c$ superconductors based on the hole-doped, $n = 1$ Ruddelsden-Popper lanthanum cuprates.

cond-mat.supr-con↗

arXiv2021

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

The phase diagrams of LaMnO$_3$ perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the $\frac{3}{8}^{th}$ 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$_3^{A'}$Mn$_4^B$O$_{12}$), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn$^{3+}$ and charge disordered (CD) Mn$^{3.5+}$ 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 Mn$^{3.5+}$ with Mn$^{3+}$ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.

cond-mat.str-el↗

arXiv2021

Large dynamic scissoring mode displacements coupled to band gap opening in Hybrid Perovskites

Hybrid perovskites are a rapidly growing research area, having reached photovoltaic power conversion efficiencies of over 25 \%. 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 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 all-inorganic analogue to the hybrid perovskite, FrPbBr$_3$, show that the large amplitudes of the scissoring modes are coupled to an opening of the electronic band gap. Finally, we use density functional theory calculations to show that the organic MA cations reorientate to accomodate the large amplitude scissoring modes.

cond-mat.mtrl-sci↗

arXiv2021

Soft mode anisotropy in negative thermal expansion material ReO$_3$

We use a symmetry-motivated approach to analyse neutron pair distribution function data to investigate the mechanism of negative thermal expansion (NTE) in ReO$_3$. This analysis shows that the local structure of ReO$_3$ is dominated by an in-phase octahedral tilting mode and that the octahedral units are far less flexible to scissoring type deformations than the octahedra in the related compound ScF$_3$. These results support the idea that structural flexibility is an important factor in NTE materials, allowing the phonon modes that drive a volume contraction of the lattice to occupy a greater volume in reciprocal space. The lack of flexibility in ReO$_3$ restricts the NTE-driving phonons to a smaller region of reciprocal space, limiting the magnitude and temperature range of NTE. In addition, we investigate the thermal expansion properties of the material at high temperature and do not find the reported second NTE region. Finally, we show that the local fluctuations, even at elevated temperatures, respect the symmetry and order parameter direction of the observed $P4/mbm$ high pressure phase of ReO$_3$. The result indicates that the motions associated with rigid unit modes are highly anisotropic in these systems.

cond-mat.mtrl-sci↗

arXiv2020

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 $\langle111\rangle$ 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.

cond-mat.mtrl-sci↗