Lattice dynamics and Raman spectrum of supertetragonal PbVO3

in Journal of Physics and Chemistry of Solids (2023), 173(111092), 111092

Lead vanadate PbVO3 is a polar crystal with a P4mm space group under ambient conditions. PbVO3 is isostructural with the model soft mode-driven ferroelectric PbTiO3, but it differs due to the so-called ... [more ▼]

Lead vanadate PbVO3 is a polar crystal with a P4mm space group under ambient conditions. PbVO3 is isostructural with the model soft mode-driven ferroelectric PbTiO3, but it differs due to the so-called “supertetragonal” elongation of its unit cell. In this study, we investigated the lattice dynamics of PbVO3 based on Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical phonon modes were identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374, and 508 cm−1; A1 modes at 188, 429, and 874 cm−1, and B1 mode at 319 cm−1. The calculations confirmed the experimental symmetry assignment and allowed us to obtain the longitudinal optical phonon wavenumbers. In addition, we analyzed the mode eigenvectors in detail in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. Despite differences in chemistry and strain, the phonon eigenvectors were found to be highly comparable in both compounds. We investigated the position of the ferroelectric soft mode in PbVO3 compared with PbTiO3. Sizeable splitting of the B1+E modes appeared as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V–O bond frequency in PbVO3 was also addressed. [less ▲]

Stability of the tetragonal phase of BaZrO3 under high pressure

in PHYSICAL REVIEW B (2022), 106(6), 064105-10

In this paper, we revisit the high pressure behavior of BaZrO3 by a combination of first-principle calculations, Raman spectroscopy and x-ray diffraction under high pressure. We confirm experimentally the ... [more ▼]

In this paper, we revisit the high pressure behavior of BaZrO3 by a combination of first-principle calculations, Raman spectroscopy and x-ray diffraction under high pressure. We confirm experimentally the cubic-to -tetragonal transition at 10 GPa and find no evidence for any other phase transition up to 45 GPa, the highest pressures investigated, at variance with past reports. We reinvestigate phase stability with density functional theory considering not only the known tetragonal (I4/mcm) phase but also other potential antiferrodistortive candidates. This shows that the tetragonal phase becomes progressively more stable upon increasing pressure as compared to phases with more complex tilt systems. The possibility for a second transition to another tilted phase at higher pressures, and in particular to the very common orthorhombic Pnma structure, is therefore ruled out. [less ▲]

Emerging spin-phonon coupling through cross-talk of two magnetic sublattices

in NATURE COMMUNICATIONS (2022), 13(1), 443-7

Typically, magnetic phenomena result from the spontaneous order of the sublattices. Here, the cross-talk of two magnetic ions gives rise to an intrinsic, yet non-spontaneous ordering and manifests as ... [more ▼]

Typically, magnetic phenomena result from the spontaneous order of the sublattices. Here, the cross-talk of two magnetic ions gives rise to an intrinsic, yet non-spontaneous ordering and manifests as emergent strong spin-phonon coupling in SmFeO3. Many material properties such as superconductivity, magnetoresistance or magnetoelectricity emerge from the non-linear interactions of spins and lattice/phonons. Hence, an in-depth understanding of spin-phonon coupling is at the heart of these properties. While most examples deal with one magnetic lattice only, the simultaneous presence of multiple magnetic orderings yield potentially unknown properties. We demonstrate a strong spin-phonon coupling in SmFeO3 that emerges from the interaction of both, iron and samarium spins. We probe this coupling as a remarkably large shift of phonon frequencies and the appearance of new phonons. The spin-phonon coupling is absent for the magnetic ordering of iron alone but emerges with the additional ordering of the samarium spins. Intriguingly, this ordering is not spontaneous but induced by the iron magnetism. Our findings show an emergent phenomenon from the non-linear interaction by multiple orders, which do not need to occur spontaneously. This allows for a conceptually different approach in the search for yet unknown properties. [less ▲]

Crossover between distinct symmetries in solid solutions of rare earth nickelates

in APL MATERIALS (2021), 9(8),

A strong coupling of the lattice to functional properties is observed in many transition metal oxide systems, such as the ABO(3) perovskites. In the quest for tailor-made materials, it is essential to be ... [more ▼]

A strong coupling of the lattice to functional properties is observed in many transition metal oxide systems, such as the ABO(3) perovskites. In the quest for tailor-made materials, it is essential to be able to control the structural properties of the compound(s) of interest. Here thin film solid solutions that combine NdNiO3 and LaNiO3, two materials with the perovskite structure but distinct space groups, are analyzed. Raman spectroscopy and scanning transmission electron microscopy are combined in a synergistic approach to fully determine the mechanism of the structural crossover with chemical composition. It is found that the symmetry transition is achieved by phase coexistence in a way that depends on the substrate selected. These results carry implications for analog-tuning of physical properties in future functional materials based on these compounds. [less ▲]

Elucidating the growth mechanism of ZnO films by atomic layer deposition with oxygen gas via isotopic tracking

in JOURNAL OF MATERIALS CHEMISTRY C (2021), 9(12), 4307-4315

The growth process of zinc oxide (ZnO) thin films by atomic layer deposition (ALD) accompanied by the presence of oxygen gas pulsing is investigated by means of the isotopic tracking of oxygen O-18 from ... [more ▼]

The growth process of zinc oxide (ZnO) thin films by atomic layer deposition (ALD) accompanied by the presence of oxygen gas pulsing is investigated by means of the isotopic tracking of oxygen O-18 from the water precursor and oxygen O-16 from the gas. In a previous study [T. Nguyen et al., Results Mater., 2020, 6, 100088, DOI: 10.1016/j.rinma.2020.100088], by means of structural, electrical, and optical characterizations, we identified key growth parameters of this unusual ALD process. Unexpectedly, the influence of molecular oxygen on the crystallography, microstructure, and morphology of the hundred-nanometer- to micrometer-thick ZnO films was significant. In this study, we present an unprecedented methodology by combining isotopic tracers with mass spectrometry to elucidate the role of the two different sources of oxygen atoms during the evolution of the growth. Notably, the use of in situ quartz crystal microbalance (QCM) and Secondary Ion Mass Spectrometry (SIMS) reveals new insights into the reaction mechanism for ZnO thin film growth. On the one hand, the non-negative mass change during the ZnO growth without O-2 gas is attributed to the presence of bare zinc atoms on the surface due to the reaction between monoethyl zinc and hydroxyl groups of the water precursor after the diethyl zinc pulse. On the other hand, the detection of ZnxOyC2H5- ions by Time-of-Flight SIMS (TOF-SIMS) and the mass increase during the O-2 pulse suggest a new reaction mechanism for the ZnO thin film growth in the presence of gaseous O-2 where the ethyl ligand of the zinc precursor can react with O-2 to form ethylperoxy radicals. The formations of the ethylperoxy zinc and/or zinc atoms lead to more adsorption of water to form ethylhydroperoxide during the water pulse, inducing the positive mass change. The use of an isotopic substitution allowed us to unambiguously associate the mass gain with the gradual incorporation of gaseous oxygen throughout the growth process and thereby support the chemical reaction. [less ▲]

Controlling electrical and optical properties of zinc oxide thin films grown by thermal atomic layer deposition with oxygen gas

in Results in Materials (2020), 6

The preparation of ZnO thin films with controlled electrical resistivity and optical properties is often challenged by the presence of defects, such as oxygen vacancies or interstitial zinc. Here, we ... [more ▼]

The preparation of ZnO thin films with controlled electrical resistivity and optical properties is often challenged by the presence of defects, such as oxygen vacancies or interstitial zinc. Here, we investigate the material properties of ZnO polycrystalline thin films prepared by thermal Atomic Layer Deposition (ALD) with the presence of molecular oxygen pulsing during the growth. By means of structural, electrical and optical characterizations, we identify key growth parameters of this unusual ALD process. Unexpectedly, the influence of oxygen molecules on the crystallography, microstructure and morphology of ZnO films is significant from hundred-nanometers to micrometer thick film. The electrical resistivity of the films grown with oxygen gas shows a dramatic increase from 3 to 4 orders of magnitude. Additionally, photoluminescence measurements reveal that deep-level emissions caused by defects located deep in the band gap can be reduced by applying an adequate pulsing of oxygen gas during the process. Finally, we conclude with a discussion about the degree of consistency between the chemical composition, the inner strain and the optical and electrical properties of the films obtained with the different thermodynamic parameters of growth. Several hypotheses are discussed in order to understand the dominance of (002) orientation in the presence of oxygen during the ALD growth process. [less ▲]

Optical studies of ferroelectric and ferroelastic domain walls

in Journal of Physics: Condensed Matter (2020), 32(18), 183001

Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the ... [more ▼]

Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the domains, such as enhanced conductivity in insulators, or polar properties in non-polar materials. In this review we show that optical techniques, in spite of the diffraction limit, also provide key insights into the structure and physical properties of ferroelectric and ferroelastic domain walls. We give an overview of the uses, specificities and limits of these techniques, and emphasize the properties of the domain walls that they can probe. We then highlight some open questions of the physics of domain walls that could benefit from their use. [less ▲]

Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials

in Nature Reviews. Physics (2020), 2(11), 634-648

Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional ... [more ▼]

Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional properties, such as polarity in non-polar systems or electrical conductivity in otherwise insulating materials, it becomes clear that domain walls represent new and exciting objects in matter. In this Review, we discuss the exotic polarization profiles that can arise at domain walls with multiple order parameters and the different mechanisms that lead to domain-wall polarity in non-polar ferroelastic materials. The emergence of energetically degenerate variants of the domain walls themselves suggests the existence of interesting quasi-1D topological defects within such walls. We also provide an overview of the general notions that have been postulated as fundamental mechanisms responsible for domain-wall conduction in ferroelectrics. We then discuss the prospect of combining domain walls with transition regions observed at phase boundaries, homo- and heterointerfaces, and other quasi-2D objects, enabling emergent properties beyond those available in today's topological systems. Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level and emergent functional properties. This Review discusses the exotic polarization profiles that arise at domain walls and the fundamental mechanisms responsible for domain-wall conduction. [less ▲]

High-contrast imaging of 180 degrees ferroelectric domains by optical microscopy using ferroelectric liquid crystals

in APPLIED PHYSICS LETTERS (2020), 116(21),

Ferroelectric liquid crystals (FLCs) couple the direction of their spontaneous electric polarization to the direction of tilt of their optic axis. Consequently, reversal of the electric polarization by an ... [more ▼]

Ferroelectric liquid crystals (FLCs) couple the direction of their spontaneous electric polarization to the direction of tilt of their optic axis. Consequently, reversal of the electric polarization by an electric field gives rise to an immediate and lasting optical response when an appropriately aligned FLC is observed between crossed polarizers, with one field direction yielding a dark image and the opposite direction yielding a bright image. Here, this peculiar electro-optic response is used to image, with high optical contrast, 180 degrees ferroelectric domains in a crystalline substrate of magnesium-doped lithium niobate. The lithium niobate substrate contains a few domains with upward electric polarization surrounded by regions with downward electric polarization. In contrast to a reference non-chiral liquid crystal that is unable to show ferroelectric behavior due to its high symmetry, the FLC, which is used as a thin film confined between the lithium niobate substrate and an inert aligning substrate, reveals ferroelectric domains as well as their boundaries, with strong black and white contrast. The results show that FLCs can be used for non-destructive readout of domains in underlying ferroelectrics, with potential applications in, e.g., photonic devices and non-volatile ferroelectric memories. [less ▲]

Vibrational properties of LaNiO3 films in the ultrathin regime

in APL MATERIALS (2020), 8(6),

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films ... [more ▼]

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (similar to 1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit. [less ▲]