Browsing by Author "Narayanan, N"

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    18O isotope substitution on the multiferroic compound DyMnO3
    (Australian Institute of Physics, 2013-02-06) Narayanan, N; Li, F; Hutchison, WD; Reynolds, NM; Rovillain, P; Ulrich, C; Hester, JR; McIntyre, GJ; Mulders, AM
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    Collective nonlinear electric polarization via defect-driven local symmetry breaking
    (Royal Society of Chemistry, 2019-05-17) Dong, W; Cortie, DL; Lu, T; Sun, QB; Narayanan, N; Hu, WB; Jacob, L; Li, Q; Yu, DH; Chen, H; Chen, AP; Wei, XY; Wang, G; Humphrey, MG; Frankcombe, TJ; Liu, Y
    In this work, we report the defect-mediated, abnormal non-linear polarization behavior observed in centrosymmetric rutile TiO2 where less than 1 at% of sterically mismatched Mg2+ ions are introduced to create ferroelectric-like polarization hysteresis loops. It is found that the Image ID:c9mh00516a-t2.gif defect cluster produces a dipole moment exceeding 6 Debye, with a rotatable component. Such a polarization is further enhanced by the displacement of neighboring Ti4+ ions. The coupling between such defect-driven symmetry-breaking regions generates a collective nonlinear electrical polarization state that persists to high temperatures. More importantly, an observation of abnormal bias shift of polarization hysteresis suggests an antiparallel alignment of certain dipoles frozen relative to the external poling electric field, which is associated with oxygen vacancy hopping. This result challenges the long-standing notion of parallel alignment of dipoles with the external electric field in ferroelectrics. This work also reveals an unexpected new form of non-linear dielectric polarization (non-ferroelectricity) in solid-state materials. © Royal Society of Chemistry 2024
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    Comparison of the magnetic and crystal field excitations in orthorhombically distorted vanadates and multiferroic manganites
    (Australian Institute of Nuclear Science and Engineering, 2012-11-15) Reynolds, N; Rovillain, P; Narayanan, N; Fujioka, F; Tokura, Y; Danilkin, SA; Mulders, AM; McIntyre, GJ; Ulrich, C
    Magnetism and ferroelectricity are both exciting physical properties and are used in everyday life in sensors and data storage. In multiferroic materials both properties coexist. They offer a great potential for future technological applications like the increase of data storage capacity or in novel senor applications. We have performed a comparative inelastic neutron scattering (INS) investigation on a series of vanadates, in particularly TbV0{sub 3} DyV0{sub 3}, PrV0{sub 3}, and CeV0{sub 3}, with their multiferroic Mn-counterparts. The Vanadates are isostructural to the multiferroic materials TbMnO{sub 3} and DyMn0{sub 3}, but posses a collinear antiferromagnetic spin arrangement below TN ≈110 K instead of a cycloidal spin structure below TFE 28 ≈K. By using inelastic neutron scattering we have obtained the spin wave dispersion relation and the crystal field excitations of the V-sublattice and the rare earth ions, respectively. The data will be compared with previously obtained INS data of D. Senff on TbMnO{sub 3} and our INS data on DyMnO{sub 3} with the intention of uncovering information about the complex interplay between the magnetic moments of the rare earth ions its role in the formation of the multiferroic phase.
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    Defect structure and property consequence when small Li+ ions meet BaTiO3
    (American Physical Society, 2020-08-31) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, Y
    In the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped BaTiO3. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance (7Li-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the BaTiO3 host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in BaTiO3 could not be determined by NPD, 7Li-NMR revealed one broad “disordered” and multiple “ordered” peaks. Local structure models with different defect types involving Li+ were modeled and the corresponding chemical shifts (δ) were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with Ti4+ being replaced by four Li+ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion Ti3+ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of Li+. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped BaTiO3. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides. ©2020 American Physical Society
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    Defect structure-property correlations in Li doped BaTiO3
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, Y; Li, G
    In the present work we investigate the important issue of the structure and dynamics of smaller ions in oxides and the resulting impact on its functional properties. For this purpose, we selected a 7% Li-doped BaTiO3. Li is a vital ingredient in novel energy storage technologies such as Li-ion batteries. The smaller Li-ion can influence the structural stability, homogeneity, local environment, and dynamic behavior of the host lattice, affecting and optimizing the dielectric and multiferroic properties of novel polar functional materials [1-2]. However, the Li-ion positions and dynamics in functional materials are not completely understood, controversially discussed and are the subject of extensive ongoing research [3]. Furthermore, sample inhomogeneity due to Li migration to the grain boundary and/or development of multiple phases complicates the elucidation of the structure-property correlations that may lead to incorrect interpretations [4]. The selection of BaTiO3 as the host lattice is due to materials based on this being considered as the alternative to the piezoelectric lead zirconate titanate, citing environmental issues [5]. BaTiO3 also crystallizes in a simple perovskite structure and Li ions can be effectively doped into it at lower doping levels. Very recently, field-dependent electric polarization measurements on BaTiO3 exhibited a polarization–electric field double hysteresis loop upon Li doping [4]. These drastic changes to the electric polarization, related to the doping poses a good test case for the investigation of the Li induced defect structure model and its influence on the functional properties. To elucidate the above structure-property correlations, we combined several techniques, such as neutron powder diffraction electron microprobe associated with the wavelength-dispersive spectroscopy, 7Li nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance (EPR), electric polarization measurement, and theoretical calculations based on density functional theory [6].
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    The effect of oxygen isotopes substitution on magnetism in multiferroic CaMn7O12
    (Australian Institute of Nuclear Science and Engineering, 2012-11-15) Li, F; Narayanan, N; Hutchison, WD; Ulrich, C; McIntyre, GJ
    Multiferroic materials, where ferroelectricity and ferromagnetism coexist and interact and one property can be used to drive the other, can find potential applications in spintronics and information technology and form the basis for four-state memory. However the details of coupling between these two orders are not yet understood. Competing theories of inherent electronic structure and ionic displacement are proposed to explain this coupling, but no experimental evidence currently exists to differentiate these models. To investigate the interaction between magnetic moments and electric dipoles on a fundamental level, this study will extend the isotopic pure oxygen substitution, a widely used technique for the investigation of high temperature superconductors, to multiferroics. Single crystal CaMn{sub 7}0{sub 12} showing the largest magnetically induced electric polarization measured to date is chosen as a test material and synthesized by flux method. The preliminary results show that single crystals of a size —100x100x100 μm can be obtained however a small amount of CaMn 3 0 6 impurity phase is also detected in XRD. Efforts on growing a larger single crystal are under way.
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    Effects of 18O isotope substitution in multiferroic RMnO3 (R = Tb, Dy)
    (Australian Institute of Physics, 2016-02-05) Graham, PJ; Narayanan, N; McIntyre, GJ; Hutchison, WD; Ulrich, C; Reynolds, N; Rovillain, P; Hester, JR; Kimpton, JA; Yethiraj, M; Pomjakushina, E; Condor, K; Kenzelmann, M
    Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO3 (R=Tb, Dy) systems. We have performed Raman spectroscopy on 16O and 18O-substituted TbMnO3 single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn3+ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on 16O and 18O-substituted DyMnO3 powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm in the b direction. The Pbnm space group is centrosymmetric and therefore does not allow ferroelectricity via atomic displacements, however our Reitveld analysis for the subgroup P21 shows significant displacements and polarisation along b that is comparable to the experimental value, making it the most promising candidate for ionic displacement induced polarisation in DyMnO3. These combined results demonstrate that structure is an important consideration in the emergence of ferroelectricity in these materials.
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    Effects of 18O isotope substitution in multiferroic RMnO3 (R=Tb, Dy)
    (Australian Institute of Physics, 2015-02-02) Graham, PJ; Narayanan, N; Reynolds, NM; Li, F; Rovillain, P; Bartkowiak, M; Hester, JR; Kimpton, JA; Yethiraj, M; Pomjakushina, E; Conder, K; Kenzelmann, M; McIntyre, GJ; Hutchison, WD; Ulrich, C
    Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO3 (R=Tb, Dy) systems. We have performed Raman spectroscopy on 16O and 18O-substituted TbMnO3 single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn3+ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions are seen. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on 16O and 18O-substituted DyMnO3 powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm. These results indicate that the structure is actively involved in the emergence of ferroelectricity in these materials.
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    Electromagnons in multiferroics probed by Raman light scattering comparison to neutron scattering investigations
    (Australian Institute of Nuclear Science and Engineering, 2012-11-15) Rovillain, P; Graham, PJ; Reynolds, N; Narayanan, N; Gallis, Y; Sacuto, A; Measson, MA; Sakata, H; McIntyre, GJ; Mulders, AM; Ulrich, C; Cazayous, M
    In multiferroic materials the two antagonistic effects, magnetic and ferroelectric orders, exist simultaneously. The switching of these orders is known as magnetoelectric coupling. Thereby, magnetoelectric materials can potentially be used to control spins or electric polarization with the application of an external electric or magnetic field, respectively. This makes them promising candidates for applications in spintronics or magnonics that use magnetic excitations for information processing. BiFe03, is the rare case where both orders coexist at room temperature. Using Raman scattering, we show that in BiFe03 the spin-wave energy can be tuned electrically by over 30%, in a non-volatile way with virtually no power dissipation. In TbMnO3 (and RMn2O5) the coupling of the orders gives rise to a hybrid excitation: the electromagnon. Electromagnons are spin wave excitations which possess an electric dipole. We have identified the magnetic excitation underneath the electromagnon by comparison with neutron measurement and further the phonon mode at the origin of the dipole activity. We have extended our investigations to Raman scattering and inelastic neutron scattering on DyMn03. The combination of both techniques offers the opportunity to obtain more information on the electromagnetic interaction in this type of multiferroic material.
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    Lead-free (Ag,K)NbO3 materials forhigh-performance explosive energy conversion
    (Science Advances, 2020-05-20) Liu, Z; Lu, T; Xue, F; Nie, HC; Withers, RL; Studer, AJ; Kremer, F; Narayanan, N; Dong, XL; Yu, DH; Chen, LQ; Liu, Y; Wang, GS
    Explosive energy conversion materials with extremely rapid response times have broad and growing applications in energy, medical, defense, and mining areas. Research into the underlying mechanisms and the search for new candidate materials in this field are so limited that environment-unfriendly Pb(Zr,Ti)O3 still dominates after half a century. Here, we report the discovery of a previously undiscovered, lead-free (Ag0.935K0.065)NbO3 material, which possesses a record-high energy storage density of 5.401 J/g, enabling a pulse current ~ 22 A within 1.8 microseconds. It also exhibits excellent temperature stability up to 150°C. Various in situ experimental and theoretical investiga-tions reveal the mechanism underlying this explosive energy conversion can be attributed to a pressure-induced octahedral tilt change from a−a−c+ to a−a−c−/a−a−c+, in accordance with an irreversible pressure-driven ferroelectric-antiferroelectric phase transition. This work provides a high performance alternative to Pb(Zr,Ti)O3 and also guidance for the further development of new materials and devices for explosive energy conversion. Copyright © 2020 The Authors. CC-By 4.0 licence
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    Lead-free (Ag,K)NbO3materials for high-performance explosive energy conversion
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Liu, Z; Lu, T; Xue, F; Withers, RL; Studer, AJ; Narayanan, N; Dong, XL; Yu, D; Chen, L; Wang, G; Liu, Y
    Explosive energy conversion materials with extremely rapid response times have a diverse and growing range of applications in energy, medical, and mining areas. Research into the underlying mechanisms and the search for new candidate materials is so limited that Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 is still the dominant material after half a century. In this work, we report the discovery of a new, lead-free ferroelectric material, (Ag0.935K0.065)NbO3 for explosive energy conversion applications. This material not only possesses a record-high energy storage density of 5.401 J/g, but also exhibits excellent temperature stability (up to a disruptive ferroelectric to ferroelectric phase transition at 150oC) by comparison with Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (which exhibits the ferroelectric to ferroelectric phase transition but at the much lower temperature of 41~70oC). (Ag0.935K0.065)NbO3 enables extremely high power, energy conversion within 1.8 microseconds, generating a pulse with e.g. a current ~ 22 A. Furthermore, pressure-dependent physical characterization, together with transmission electron microscopy, in-situ neutron diffraction analysis and theoretical modelling, reveals the mechanism underlying the observed explosive energy conversion behavior. It is found that the fast release of the stored energy can be attributed to a pressure-induced octahedral tilt change from a-a-c+ to AgNbO3-type a-a-c-/a-a-c+, in accordance with an irreversible pressure driven FE-AFE phase transition. This work provides not only an alternative (with significantly better performance) to the current commercially-employed lead-containing materials, but also provides guidance for the further development of new materials and devices for explosive energy conversion applications. Copyright © 2020 The Authors.
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    Magnetic ordering and spin dynamics in the S = 5/2 staggered triangular lattice antiferromagnet Ba2MnTeO6
    (American Physical Society, 2020-09-09) Li, L; Narayanan, N; Jin, SJ; Yu, J; Liu, ZJ; Sun, HL; Wang, CW; Peterson, VK; Liu, Y; Danilkin, SA; Yao, DX; Yu, DH; Wang, M
    We report studies of the magnetic properties of a staggered stacked triangular lattice Ba2MnTeO6 using magnetic susceptibility, specific heat, neutron powder diffraction, inelastic neutron scattering measurements, and first-principles density functional theory calculations. Neutron diffraction measurements reveal Ba2MnTeO6 to be antiferromagnetically ordered with a propagation vector k=(0.5,0.5,0) and Néel transition temperature of TN≈20 K. The dominant interaction derived from the Curie-Weiss fitting to the inverse DC susceptibility is antiferromagnetic. Modeling of the inelastic neutron scattering data with linear spin wave theory yielded magnetic exchange interactions for the nearest intralayer, nearest interlayer, and next-nearest interlayer J1=0.27(3), meV J2=0.27(3) meV, and J3=−0.05(1) meV, respectively, and a small value of easy-axis anisotropy of Dzz=−0.01 meV. We derive a magnetic phase diagram that reveals a collinear stripe-type antiferromagnetic order that is stabilized by the competition between J1, J2, and J3. ©2020 American Physical Society
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    Magnetic structure and spin correlations in magnetoelectric honeycomb Mn4Ta2O9
    (American Physical Society, 2018-10-22) Narayanan, N; Senyshyn, A; Mikhailova, D; Faske, T; Lu, T; Liu, Z; Weise, B; Ehrenberg, H; Mole, RA; Hutchison, WD; Fuess, H; McIntyre, GJ; Liu, Y; Yu, DH
    We elucidate the magnetic interactions and the role of spin (electron) correlation in determining the ground state of the honeycomb compound Mn4Ta2O9, by neutron powder diffraction, inelastic neutron scattering (INS), specific-heat (CP) measurements, and electronic-structure calculations. The antiferromagnetic long-range order with moments along c occurs at 102 K with strong exchange striction and small anisotropy. It is escribed by the three-dimensional Ising model. Diffuse magnetic scattering has been observed above TN, which is attributed to the two-dimensional spin correlations within the Mn2+ honeycombs. This is confirmed by the calculated exchange constants. INS experiments and spin-wave simulations together with CP measurements reveal two gapped modes on the ab plane, originating from the rotation of the spins away from the easy axis c. The magnetic anisotropy is mainly determined by an electron-correlation-assisted dipole-dipole interaction. This work provides insight into the competing origins of the magnetic anisotropy, which leads to different magnetic ground states in the family of honeycomb compounds. ©2018 American Physical Society
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    Magnetically driven electric polarization in frustrated magnetic oxide multiferroics
    (Australian Institute of Physics, 2014-02-06) Narayanan, N; Reynolds, NM; Li, F; Mulders, AM; Rovillain, P; Ulrich, C; Bartkowiak, M; Hester, JR; McIntyre, GJ; Hutchison, WD
    In multiferroics more than one ferroic order can coexist and in the present case we are interested in systems which exhibit simultaneous magnetic ordering and electric polarization (EP). Of particular interest are frustrated magnetic materials that exhibit an electric polarization that is strongly coupled to the magnetism [1]. Examples of such multiferroics are RMnO3 (R= Tb, Dy), Ni3V2O8, and RbFe(MoO4)2 [2-4]. This coupling can be utilized in applications such as magnetoelectric random access memory. Although technically relevant, the coupling mechanism between these two orders is complicated [1]. Whereas the magnetic ordering results from exchange interaction of unpaired spins, origins of EP coupled to the magnetic ordering depends on the interplay between lattice, orbital, spin and charge degrees of freedom. Several mechanisms such as the inverse Dzyaloshinskii - Moriya interaction, magnetostriction and coupling of the chirality to the crystal structure or a combination of them are currently discussed depending on the compound [2-5]. Additionally EP has ionic and electronic contributions. In the present work we investigate the coupling of magnetism to EP involving all three above mechanisms, in orthorhombic DyMnO3 (DMO), Cu3Nb2O8 and Ba3NiNb2O9 with neutron powder diffraction (NPD), magnetization and heat capacity measurements focusing on the magnetic and multiferroic phase transitions. In order to investigate the role of the lattice distortion or equivalently the role of oxygen, isotope substitution of 16O with 18O was performed on DMO. All samples are prepared as single phases via the solid state route and NPD experiments are carried out at Wombat and at Echidna at OPAL
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    Magnetically driven electric polarization in frustrated magnetic oxide multiferroics
    (Australian Institute of Physics, 2014-02-04) Narayanan, N; Reynolds, NM; Li, F; Mulders, AM; Rovillian, P; Ulrich, C; Bartkowiak, M; Hester, JR; McIntyre, GJ; Hutchinson, WD
    In multiferroics more than one ferroic order can coexist and in the present case we are interested in systems which exhibit simultaneous magnetic ordering and electric polarization (EP). Of particular interest are frustrated magnetic materials that exhibit an electric polarization that is strongly coupled to the magnetism [1]. Examples of such multiferroics are RMnO3 (R= Tb, Dy), Ni3V2O8, and RbFe(MoO4)2 [2-4]. This coupling can be utilized in applications such as magnetoelectric random access memory. Although technically relevant, the coupling mechanism between these two orders is complicated [1]. Whereas the magnetic ordering results from exchange interaction of unpaired spins, origins of EP coupled to the magnetic ordering depends on the interplay between lattice, orbital, spin and charge degrees of freedom. Several mechanisms such as the inverse Dzyaloshinskii-Moriya interaction, magnetostriction and coupling of the chirality to the crystal structure or a combination of them are currently discussed depending on the compound [2-5]. Additionally EP has ionic and electronic contributions. In the present work we investigate the coupling of magnetism to EP involving all three above mechanisms, in orthorhombic DyMnO3 (DMO), Cu3Nb2O8 and Ba3NiNb2O9 with neutron powder diffraction (NPD), magnetization and heat capacity measurements focusing on the magnetic and multiferroic phase transitions. In order to investigate the role of the lattice distortion or equivalently the role of oxygen, isotope substitution of 16O with 18O was performed on DMO. All samples are prepared as single phases via the solid state route and NPD experiments are carried out at Wombat and at Echidna at OPAL.
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    Neutron powder diffraction experiments on multiferroic DyMnO 3
    (Australian Institute of Nuclear Science and Engineering, 2012-11-15) Narayanan, N; Li, F; Hutchison, WD; Mulders, AM; Reynolds, N; Rovillan, P; Ulrich, C; Hester, JR; McIntyre, GJ
    Multiferroic materials of particular interest are the frustrated magnetic compounds that exhibit a strongly coupled electric polarization (EP). One such compound is DyMnO{sub 3} which exists in an orthorhombic (o-DMO) and hexagonal modification depending on the synthesis conditions. In the present work the o-DMO is investigated by means of neutron powder diffraction focusing on the magnetic phase transitions and the behavior of the structural parameters in different magnetic and multiferroic phases. Below T{sub n,mn} =39 K, the Mn moments order sinusoidally with no EP. Then T{sub l=}16 K the Mn moments order in a spin spiral structure with an induced Dy moment and EP and finally below T{sub N,DY}=9 K a collinear ordering of the Dy moments takes place that reduces the EP significantly. Single phase samples are prepared via the solid state route and neutron diffraction (ND) experiments are carried out at the high flux ND beamline Wombat and at the high resolution ND beamline Echidna at OPAL. 0-DMO crystallizes in the space group Pbnm. All three magnetic phase transitions are identified and are in good agreement with. Below T{sub N,MN} an increased rotation of the rigid Mn06 octahedra in the ab plane, likely due to the competition between nearest neighbour aid next nearest neighbour superexchange interactions takes place. However below T{sub l} the MnO{sub 6} octahedra are significantly distorted along the c axis, the direction of the EP. Therefore the correlation between significant changes in the Mn-O bonds and the spontaneous EP are evident below T{sub l}. The reduction of EP below T{sub N.DY} on the other hand correlates with the rapid increase in the orbital ordering angle towards the 120° corresponding to the 3x{sup 2}-r{sup 2}/3y{sup 2}-r{sup 2} character of the orbitals.
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    Role of a-site molecular ions dynamics in the polar functionality of perovskite metal-organic framework
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Lu, T; Cortie, DL; Li, Z; Narayanan, N; Liu, Z; Sun, QB; Frankcombe, TJ; McIntyre, GJ; Yu, DH; Liu, Y
    Recent studies on organic-inorganic hybrid perovskites (OIHPs) and ferroelectric metal-organic framework perovskites (MOFPs) reveal their superb performance as highly efficient photovoltaics and promising ferroelectrics. This has enabled a new generation of optic electronic-mechanical devices based on green chemistry. However, the ultimate strategies to optimize these polarization-related functionalities are not yet clear, leading to confused reports in the literature. In this work, we investigate a rationally selected series of molecular ions within Mg(HCOO)3– frameworks to form [CH3NH3]Mg(HCOO)3 (MAMOF),[(CH3)2NH2]Mg(HCOO)3 (DMAMOF), and [C(NH2)3]Mg(HCOO)3 (GUAMOF). Single-crystal X-ray diffraction, inelastic neutron spectroscopy and ab initio molecular dynamics are used to achieve detailed structural pictures of three MOFPs. Intriguingly, our study reveals that the alignments of protonated amines are highly dependent on the directional hydrogen bonds that link N-H units to the surrounding MgO6 octahedra. The alignments of different amines and their dynamics are therefore determined by the acceptor O provided by the distortive frameworks. We successfully assigned the alignments of the A-site ions associated with different polar behavior to the dielectric properties for three MOFPs and propose that the configuration of the A-site molecular ions and potential hydrogen bonds are critical to enable the design of polarization-related functionalities in both MOFPs and OIHPs.
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    Role of a-site molecular ions in the polar functionality of metal–organic framework perovskites
    (American Chemical Society (ACS), 2021-12-28) Lu, T; Cortie, DL; Li, ZX; Narayanan, N; Liu, Z; Sun, QB; Frankcombe, TJ; McIntyre, GJ; Yu, D; Liu, Y
    Recent studies on organic–inorganic hybrid perovskites (OIHPs) and ferroelectric metal–organic framework perovskites (MOFPs) reveal their superb performance as highly efficient materials for photovoltaics and ferroelectrics. This has enabled the development of a new generation of optic-electronic-mechanical devices based on green chemistry. However, the fundamental understanding of these polarization-related functionalities is not yet clear, which has hindered the progress in further designing and developing materials with expected properties. In this work, we investigate three MOFPs that have the same Mg(HCOO)3– frameworks with different molecular ions: [CH3NH3][Mg(HCOO)3] (MA-MOF), [(CH3)2NH2][Mg(HCOO)3] (DMA-MOF), and [C(NH2)3][Mg(HCOO)3] (GUA-MOF). Single-crystal and powder X-ray diffraction, inelastic neutron spectroscopy, and ab initio molecular dynamics simulations are combined to achieve a detailed description of the three MOFPs’ static and dynamic structures as a function of temperature. Intriguingly, our study reveals that the alignments and motions of the guest molecular ions are highly dependent on the directional hydrogen bonds that link N–H units to the surrounding MgO6 octahedra through the O acceptor from the frameworks. At the same time, the size, dynamic behavior, and alignments of the A-site molecular ions influence the distortive framework structures and their temperature-dependent deformation. Therefore, the mutual interaction between the guest and the framework determines the overall functionalities of the MOFPs. This study indicates that the configuration of the A-site molecular ions and the potential hydrogen bonds are critical to design the polar functionalities in both MOFPs and OIHPs. © 2021 American Chemical Society
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    Subpicometer-scale atomic displacements and magnetic properties in the oxygen-isotope substituted multiferroic DyMn O3
    (American Physical Society, 2017-02-27) Narayanan, N; Graham, PJ; Reynolds, N; Li, F; Rovillain, P; Hester, JR; Kimpton, JA; Yethiraj, M; McIntyre, GJ; Hutchison, WD; Ulrich, C
    We have investigated DyMn16O3 and its isotopically substituted counterpart DyMn18O3 by neutron powder diffraction, x-ray diffraction, and heat capacity measurements to investigate the mechanism leading to its magnetically induced electric polarization. 18O isotope substitution does not influence the magnetic ordering temperature of the Mn ions TN,Mn or the multiferroic ordering temperature Tl coinciding with the onset of the spin spiral phase; however, it does reduce the ordering temperature of Dy into its incommensurate magnetic state TN,Dy from 7.0(1) K to 5.9(1) K. The temperature dependence of the magnetic propagation vector, qIC, changes with 18O substitution, while Tl remains almost constant, independent of qIC. Pronounced changes in the lattice parameters occur at the various phase transitions. Furthermore, distinct subpicometer-scale distortions of the MnO6 octahedra and displacements of the Dy ions are observed below the ferroelectric phase transition at Tl in both samples, pointing toward the mechanism for electric polarization and its coupling to the orbital degrees of freedom. ©2017 American Physical Society
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    Symmetry analysis of the ferroic transitions in the coupled honeycomb system (Fe, Co, Mn)4Ta2O9
    (Australian Institute of Physics, 2020-02-04) Narayanan, N; Faske, T; Lu, T; Liu, Z; Brennan, M; Hester, JR; Avdeev, M; Senyshyn, A; Mikhailova, D; Ehrenberg, H; Hutchison, WD; Mole, RA; Fuess, H; McIntyre, GJ; Liu, Y; Yu, DH
    Exotic phenomena such as spin liquid, spin-orbital entities, magnetic order induced multiferroicity (type ii) or quantum criticality have recently triggered extensive research on the ground state properties of frustrated magnetic systems. The ground states of these compounds are determined by the coupling of the spin to the orbital, charge and lattice degrees of freedom. One of the extensively investigated lattices is the honeycomb lattice due to the development of the Kitaev model for quantum spin liquids [1-2]. In this work, we are interested in the coupled honeycomb system M4A2O9 (M=Fe, Co and Mn and A=Nb, Ta). All members have two crystallographically distinct M sites, which are in the distorted octahedral oxygen cages. These cages form edge-shared coplanar and corner-shared buckled honeycombs respectively which are interconnected in the perpendicular direction leading to competing exchange paths. The M=Co and Mn members were magnetoelectrics, whereas Fe2Ta2O9 was reported to exhibit both magnetoelectric and (type ii) multiferroic phases depending on the temperature [3-4]. Magnetoelectrics and multiferroics are technically highly relevant with a variety of applications such as MRAMs and field sensors. However, the coupling mechanism is very complicated [5]. Furthermore, due to the group properties of the symmetry analysis methods such as representation analysis and magnetic space groups, the magnetic structure of the Nb counterpart Co4Nb2O9 is controversially discussed. It is therefore apparent that the above discussed diversities of the properties are determined by the magnetic structure and the closely related electronic structure. These can be elucidated by investigating the structure and dynamics of these compounds, which will help to understand the emergence of different ground states and the diverse phase transitions in this family of materials In this work, we systematically investigate the magnetic and electronic structure of the (Fe, Co, Mn)4Ta2O9 system. We combined several different techniques of neutron powder diffraction, inelastic neutron scattering, heat capacity, electronic band structure calculations and spin wave modeling based on linear spin wave theory.