Rules and mechanisms governing octahedral tilts in perovskites under pressure

in Physical Review. B (2017), 96(5),

The rotation of octahedra (octahedral tilting) is common in ABO(3) perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to ... [more ▼]

The rotation of octahedra (octahedral tilting) is common in ABO(3) perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this paper we discover several new mechanisms and formulate a set of simple rules that allow us to understand how pressure affects oxygen octahedral tiltings via the use and analysis of first-principles results for a variety of compounds. Besides the known A-O interactions, we reveal that the interactions between specific B ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure (dR/dP) usually decreases with both the tolerance factor and the ionization state of the A ion by illustrating the key role of A-O interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered, namely that (i) the octahedral rotations in ABO(3) perovskites with empty low-lying d states on the B site are greatly enhanced by pressure, in order to lower the electronic kinetic energy; (ii) dR/dP is enhanced when the system possesses weak tilt instabilities, and (iii) for the most common phase exhibited by perovskites-the orthorhombic Pbnm state-the in-phase and antiphase octahedral rotations are not automatically both suppressed or both enhanced by the application of pressure because of a trilinear coupling between these two rotation types and an antipolar mode involving the A ions. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure by indirectly controlling the amplitude of octahedral rotations. [less ▲]

Conductivity and Local Structure of LaNiO3 Thin Films

in Advanced Materials (2017), 29(18),

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with ... [more ▼]

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6-11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth. [less ▲]

Evolution of defect signatures at ferroelectric domain walls in Mg-doped LiNbO3

in Physica Status Solidi. Rapid Research Letters (2016), 10(3), 222-226

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is ... [more ▼]

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is studied by analysing frequency shifts at domain walls and their variations with dopant concentration and annealing conditions. It is shown that defects are stabilized at domain walls and that changes in the defect structures with Mg concentration can be probed by the shift of Raman modes. We show that the signatures of polar defects in the bulk and at the domain walls differ. (C) 2016 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim [less ▲]

Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate

in SCIENTIFIC REPORTS (2016), 6

The understanding of domain structures, specifically domain walls currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in ... [more ▼]

The understanding of domain structures, specifically domain walls currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM - electrons reflected) to Low Energy Electron Microscopy (LEEM - electrons backscattered) gives rise to a robust contrast between domains with upwards (P-up) and downwards (P-down) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field. [less ▲]

Raman spectroscopy of rare-earth orthoferrites RFeO3 (R=La, Sm, Eu, Gd Tb, Dy)

in Physical Review. B (2016), 94(21),

We report a Raman scattering study of six rare-earth orthoferrites La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of SmFeO3 and first-principles calculations enable the assignment ... [more ▼]

We report a Raman scattering study of six rare-earth orthoferrites La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of SmFeO3 and first-principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allow correlating the phonon modes with the orthorhombic structural distortions of RFeO3 perovskites. In particular the positions of two specific A(g) modes scale linearly with the two FeO6 octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective A(g) modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and associated phonon-mode assignment provide reference data for structural investigations of the whole series of orthoferrites. [less ▲]

Multiple strain-induced phase transitions in LaNiO3 thin films

in Physical Review. B (2016), 94(1),

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or ... [more ▼]

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully relaxed films, appear under both compressive and tensile strains indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations. [less ▲]

Materials science in Luxembourg

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.

Order-parameter symmetries of domain walls in ferroelectrics and ferroelastics

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 ▲]

Single-crystalline BiMnO3 studied by temperature-dependent x-ray diffraction and Raman spectroscopy

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 ▲]

Prediction of giant elastocaloric strength and stress-mediated electrocaloric effect in BaTiO3 single crystals

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 ▲]