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See detailBismuth-based perovskites as multiferroics
Guennou, Mael; Viret, Michel; Kreisel, Jens UL

in Comptes Rendus Physique (2015), 16(2), 182-192

This review devoted to multiferroic properties of bismuth-based perovskites is divided into two parts. The first one focuses on BiFeO3and summarizes the recent progress made in the studies of its ... [more ▼]

This review devoted to multiferroic properties of bismuth-based perovskites is divided into two parts. The first one focuses on BiFeO3and summarizes the recent progress made in the studies of its pressure–temperature phase diagram and magnetoelectric coupling phenomena. The second part discusses in a more general way the issue of polar—and multiferroic—phases in BiBO3perovskites and the competition between ferroelectricity and other structural instabilities, from an inventory of recently synthesized compounds. [less ▲]

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See detailMaterials science in Luxembourg
Kreisel, Jens UL; Wirtz, Ludger UL; Schiltz, Marc

in Nature Materials (2014), 13

With its strategic location and firm commitment to investing in research, Luxembourg has ambitious plans to become a significant player in the international research arena.

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See detailJahn-Teller, Polarity, and Insulator-to-Metal Transition in BiMnO3 at High Pressure
Guennou, Mael; Bouvier, Pierre; Toulemonde, Pierre et al

in Physical Review Letters (2014), 112

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained ... [more ▼]

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60 GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases with a great variety of structures and properties, including a metallic phase above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase that allows ferroelectricity, which contradicts the traditional expectation that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma structure remains remarkably stable but shows a reduction of the Jahn-Teller distortion in a way that differs from the behavior observed in the archetypal orthorhombic Jahn-Teller distorted perovskite LaMnO3. [less ▲]

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See detailPrediction of giant elastocaloric strength and stress-mediated electrocaloric effect in BaTiO3 single crystals
Liu, Yang; Wei, Jie; Janolin, Pierre-Eymeric et al

in Physical Review B (2014), 90(10),

An applied stress field σ3 can reversibly change the temperature of an elastocaloric material under adiabatic conditions, and the temperature change Tσ3 is usually maximized near phase transitions.Using a ... [more ▼]

An applied stress field σ3 can reversibly change the temperature of an elastocaloric material under adiabatic conditions, and the temperature change Tσ3 is usually maximized near phase transitions.Using a thermodynamic approach, we demonstrate that an elastocaloric strength α = | Tσ3 |/|σ3| of 0.016 K/MPa can be achieved benefiting from the full first-order phase transition in BaTiO3 single crystals, which is comparable with typical elastocaloric materials reported in the literature. The elastocaloric temperature change is found to be giant (3.2 K) under a stress of 200 MPa with a temperature span of over 50 K, which can be significantly larger than its electrocaloric counterpart (∼1 K). Moreover, it is found that the elastocaloric strength can be remarkably enhanced (2.32 K/MPa) as long as the phase transition is triggered even by a modest stress near the sharp first-order phase transition, which is two orders of magnitude larger than those accomplished by full transition. Therefore, even a low stress (<30 MPa) can induce a modest elastocaloric effect (1.3 K) comparable with the electrocaloric counterpart, which is accompanied by a reduction of the working temperature span. In addition, it is found that the electrocaloric peak under tensile stresses moves towards higher temperatures with its magnitude slightly enhanced. Hopefully, our study will stimulate further investigations on elastocaloric and stress-mediated electrocaloric effects in ferroelectrics. [less ▲]

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See detailFirst-principles study of PbTiO3 under uniaxial strains and stresses
Sharma, Henu; Kreisel, Jens UL; Ghosez, Philippe

in Physical Review B (2014), 90(21),

The behavior of PbTiO3 under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical ... [more ▼]

The behavior of PbTiO3 under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground state, with the polarization aligned either along the constraint direction (FEz phase) or along one of the pseudocubic axes perpendicular to it (FEx phase). This contrasts with the cases of isotropic and biaxialmechanical constraints for which novel phases combining ferroelectric and antiferrodistortive motions have been previously reported. Under uniaxial strain, PbTiO3 switched from an FEx ground state under compressive strain to an FEz ground state under tensile strain beyond a critical strain ηc zz ≈ +1%. Under uniaxial stress, PbTiO3 exhibits either an FEx ground state under compression (σzz < 0) or an FEz ground state under tension (σzz > 0). Here, however, an abrupt jump of the structural parameters is also predicted under both compressive and tensile stresses at critical values σzz ≈ +2 and −8 GPa. This behavior appears to be similar to that predicted under negative isotropic pressure and might turn out to be practically useful for enhancing the piezoelectric response in nanodevices. [less ▲]

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See detailOrder-parameter symmetries of domain walls in ferroelectrics and ferroelastics
Toledano, Pierre; Guennou, Mael; Kreisel, Jens UL

in Physical Review B (2014), 89(13),

The symmetry of boundaries between ferroelectric, ferroelastic, and antiphase domains is a key element for a theoretical understanding of their properties. Here, we derive this symmetry from their organic ... [more ▼]

The symmetry of boundaries between ferroelectric, ferroelastic, and antiphase domains is a key element for a theoretical understanding of their properties. Here, we derive this symmetry from their organic relation to the symmetry of the primary transition order parameters. The domain wall symmetries are shown to coincide with directions of the order-parameter n-dimensional vector space, corresponding to sum of the vectors associated with adjacent domain states. This property is illustrated by the determination of the maximal symmetries of domain walls in BaTiO3, LaAlO3, SrTiO3, and Gd2(MoO4)3. Besides, the domain pattern in YMnO3 is interpreted as resulting from an annihilation-creation process, the annihilation of the antiphase domain walls creating six ferroelectric domain walls merging at a single point. [less ▲]

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See detailDynamic and structural properties of orthorhombic rare-earth manganites under high pressure
Mota, D. A.; Almeida, A.; Rodrigues, V. H. et al

in Physical Review B (2014), 90(5),

We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb, and Dy) and synchrotron x-ray diffraction (XRD), for A = Pr, Sm, Eu, and Dy ... [more ▼]

We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb, and Dy) and synchrotron x-ray diffraction (XRD), for A = Pr, Sm, Eu, and Dy. In all cases, a phase transition was evidenced by the disappearance of the Raman signal at a critical pressure that depends on the A cation. For the compounds with A = Pr, Sm, and Dy, XRD confirms the presence of a corresponding structural transition to a noncubic phase, so that the disappearance of the Raman spectrum can be interpreted as an insulator-to-metal transition. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the ac plane, and the Raman bands associated with out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smaller A cations considered in this study (Dy and Tb), the compression is nearly isotropic in the ac plane, with only small evolutions of the tilt angles and cooperative Jahn-Teller distortion. As the radius of the A cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced tilting of octahedra chains becomes conversely less pronounced. We finally discuss our results in light of the notion of chemical pressure and show that the analogy with hydrostatic pressure works quite well for manganites with the smaller A cations considered in this paper but can be misleading with large A cations. [less ▲]

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See detailInterplay of chemical structure and magnetic order coupling at the interface between Cr2O3 and Fe3O4 in hybrid nanocomposites
Otero-Lorenzo, Ruth; Weber, Mads C.; Thomas, Pamela A. et al

in Physical Chemistry Chemical Physics (2014), 16(40), 22337-22342

Hybrid nanocomposites based on ferrimagnetic (FiM) Fe3O4 and magnetoelectric antiferromagnetic (AFM) Cr2O3 nanocrystals were synthesized to offer a particular three-dimensional (3D) interface between the ... [more ▼]

Hybrid nanocomposites based on ferrimagnetic (FiM) Fe3O4 and magnetoelectric antiferromagnetic (AFM) Cr2O3 nanocrystals were synthesized to offer a particular three-dimensional (3D) interface between the two oxides. This interface favours an intermixing process (demonstrated by combining Raman spectroscopy and magnetization measurements) that determines the final magnetic behavior. [less ▲]

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See detailSingle-crystalline BiMnO3 studied by temperature-dependent x-ray diffraction and Raman spectroscopy
Toulemonde, P.; Bordet, P.; Bouvier, P. et al

in Physical Review B (2014), 89(22),

We report on the temperature dependence of the phonons and crystallographic parameters in BiMnO3 single crystals grown under high pressure and high temperature. The crystallographic structure of the ... [more ▼]

We report on the temperature dependence of the phonons and crystallographic parameters in BiMnO3 single crystals grown under high pressure and high temperature. The crystallographic structure of the sample was refined from room temperature to liquid helium temperature in the centrosymmetric C2/c space group, i.e., a group which does not allow ferroelectricity. In addition, the lattice dynamics was probed by Raman spectroscopy down to liquid nitrogen temperature, i.e., below the ferromagnetic transition at TC = 100 ± 2 K. Both crystallographic and Raman data indicate the absence of a structural phase transition at the ferromagnetic ordering or any other temperature. The Raman signature around TC shows a significant spin-phonon coupling for the high-frequency bands. [less ▲]

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See detailStructures and Magnetism of the Rare-Earth Orthochromite Perovskite Solid Solution LaxSm1−xCrO3
Daniels, Luke M.; Weber, Mads C.; Lees, Martin R. et al

in Inorganic Chemistry (2013), 52

A new mixed rare-earth orthochromite series, LaxSm1−xCrO3, prepared through single-step hydrothermal synthesis is reported. Solid solutions (x = 0, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0) were prepared by ... [more ▼]

A new mixed rare-earth orthochromite series, LaxSm1−xCrO3, prepared through single-step hydrothermal synthesis is reported. Solid solutions (x = 0, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0) were prepared by the hydrothermal treatment of amorphous mixed-metal hydroxides at 370 °C for 48 h. Transmission electron microscopy (TEM) reveals the formation of highly crystalline particles with dendritic-like morphologies. Rietveld refinements against high-resolution powder X-ray diffraction (PXRD) data show that the distorted perovskite structures are described by the orthorhombic space group Pnma over the full composition range. Unit cell volumes and Cr−O−Cr bond angles decrease monotonically with increasing samarium content, consistent with the presence of the smaller lanthanide in the structure. Raman spectroscopy confirms the formation of solid solutions, the degree of their structural distortion. With the aid of shell-model calculations the complex mixing of Raman modes below 250 cm−1 is clarified. Magnetometry as a function of temperature reveals the onset of low-temperature antiferromagnetic ordering of Cr3+ spins with weak ferromagnetic component at Néel temperatures (TN) that scale linearly with unit cell volume and structural distortion. Coupling effects between Cr3+ and Sm3+ ions are examined with enhanced susceptibilities below TN due to polarization of Sm3+ moments. At low temperatures the Cr3+ sublattice is shown to undergo a second-order spin reorientation observed as a rapid decrease of susceptibility. [less ▲]

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