Session M64: Local Structure and Orbital Physics in Dielectrics and Ferroics

Neutron diffraction and inelastic neutron scattering in the magnetodielectric regime of Ce2O3

The metastable material Ce₂O₃ has drawn recent attention due to reports of a giant magnetodielectric response of unknown origin, below a putative antiferromagnetic transition at T_N=6.2K. In the same temperature region, Raman spectroscopy has revealed the emergence of vibronic modes, which represent a relatively uncommon mixing of phonon and crystal field excitations.

Following up on these observations, we have performed a series of both neutron diffraction and inelastic scattering measurements on powders of Ce₂O₃. Our diffraction data allow us to comment on both the nature of the magnetically ordered state at low temperatures, and the variation in atomic structure as the material is varied through T_N. Our inelastic measurements focus on the temperature dependence of crystal field and phonon excitations at the energies of reported vibronic modes. I will present these data in the context of existing literature, and give our conclusions about the nature of the AF ground state and spin-lattice coupling in this material.
  

Local polarization in oxygen-deficient LaMnO3 induced by charge localization in the Jahn-Teller distorted structure

The functional properties of transition metal oxides result from a complex interplay between magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO₃ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO is an A-type antiferromagnet with a distorted orthorhombic perovskite structure. It is insulating due to the Jahn-Teller distortion that splits the e_g orbitals of the high-spin Mn³⁺ ions, leading to alternating long, short, and intermediate Mn-O bond lengths. Our DFT+U calculations show that, as a result of these peculiarities, the charge localization in LMO upon oxygen vacancy (V_O) formation is different compared to other manganites, like SrMnO₃, where the two extra electrons reduce the Mn sites adjacent to the vacancy. In LMO, relaxations around the V_O depend on which type of Mn-O bond is broken, affecting the d-orbital energies and leading to an asymmetric and hence polar localization of the excess electrons with respect to the vacancy. Furthermore, we show how isostatic and epitaxial strain can be used to affect the Mn-O bond lengths and orbital order and consequently the charge localization and polarity.   

Geometrical frustration and piezoelectric response in oxide ferroics

Despite years of investigation, the exact structural origin of the increased piezoelectric response of oxide ferroics is still unclear. We will present results from resonant high-energy x-ray diffraction experiments on exemplary sodium-potassium niobate ferroics. In particular, we will show that their increased piezoelectric response is due to a geometrical frustration in the underlying perovskite lattice induced by local fluctuations in the tilt pattern of the constituent niobium-oxygen octahedra, and not to a crystal-crystal phase transition or distinct nanodomains. The fluctuations peak when the sodium to potassium ratio approaches one, leading to softening of the perovskite lattice and easing of polarization rotation under electric field. Based on the experimental findings and model calculations, we will also show that the fluctuations are driven by the mismatch between the radii of sodium and potassium atoms, and the increased piezoelectric response of sodium-potassium niobates indeed scales with the variance in the distribution of these radii about their average value. The relevance of our work to the ongoing search for oxide ferroics with improved functionality will be briefly discussed.   

Magnetic Field Dependent Local Structure of Charge-Ordered AMnO3 Perovskites

Complex oxide perovskites exhibit closely lying structural phases at low temperatures. External parameters including pressure and magnetic field can be used to stabilize distinctly different structural phases. In this project, we utilize pair distribution function measurements to determine the changes in the local structure in AMnO3 perovskites when going from the charge-ordered insulating phased to the metallic phase at fixed temperature when applying an external magnetic field. The structure on a range of length scales will be explored. The results will be compared to the corresponding temperature dependence of the structure.
  

Infrared nano-spectroscopy of ferroelastic domain walls in hybrid improper ferroelectric Ca3Ti2O7

Ferroic materials are well known to exhibit heterogeneity in the form of domain walls. Understanding the properties of these boundaries is crucial for controlling functionality with external stimuli and for realizing their potential for ultra-low power memory and logic devices as well as novel computing architectures. In this work, we employ synchrotron-based near-field infrared nano-spectroscopy to reveal the vibrational properties of ferroelastic (90° ferroelectric) domain walls in the hybrid improper ferroelectric Ca₃Ti₂O₇. By locally mapping the Ti-O stretching and Ti-O-Ti bending modes, we reveal how structural order parameters rotate across a wall. Thus, we link observed near-field amplitude changes to underlying structural modulations and test ferroelectric switching models against real space measurements of local structure. This initiative opens the door to broadband infrared nano-imaging of heterogeneity in ferroics.   

Coupled structural distortions, domains, and control of phase competition in polar SmBaMn2O6

Complex oxides display highly tunable ground states, where small perturbations to the crystal structure can stabilize distinct electronic and magnetic phases. We use group theoretic methods and density functional theory calculations to analyze the polar crystal structure of A-site ordered SmBaMn₂O₆, which hosts a charge- and orbitally-ordered antiferromagnetic ground state. We show that a set of couplings between structural distortions are key for establishing the structural, electronic, and magnetic ground states and also establishes a hybrid improper ferroelectric mechanism in this material. We analyze the domain structure and show how the coupled degrees of freedom in SmBaMn₂O₆ leads to a network of coupled domains and domain wall vortices. Finally, we explore how competing electronic and magnetic phases may stabilize, for example at structural domain walls and in epitaxially strained thin films. This work provides new understanding of the complex physics realized across multiple length scales in SmBaMn₂O₆ and demonstrates a framework for the systematic exploration of correlated and structurally complex materials.   

Direct Imaging of Orbitals in Quantum Materials

The search for new quantum materials with novel properties is often focused on materials containing transition-metal or rare-earth elements. The presence of the atomic-like d or f orbitals provides a fruitful playground to generate novel phenomena and understanding the behavior of those d and f electrons is essential for designing and tuning new materials. Therefore, identifying the d or f orbitals that actively participate in the formation of the ground state is crucial. So far, these orbitals have mostly been deduced from optical, X-ray and neutron spectroscopies in which spectra must be analyzed using theory or modelling. This, however, is also a challenge in itself, since ab-initio calculations hit their limits due to the many-body nature of the problem.

Here we developed a new experimental method that circumvents the need for involved analysis and instead provides the information as measured. With this technique, we can make a direct image of the active orbital and determine what the atomic-like object looks like in a real solid. The method, s-core-level non-resonant inelastic X-ray scattering (s-NIXS), relies on high momentum transfer in the inelastic scattering process, which is necessary for dipole-forbidden terms to gain spectral weight. To demonstrate the strength of the technique, we imaged the text-book example, x²-y²/3x²-r² hole orbital of the Ni²⁺ ion in NiO single crystal [1]. We will present the basic principles of s-NIXS and its experimental implementation. We will also show how we can apply this technique to unveil the active orbitals in complex oxides as well as to determine the orbital character in highly metallic systems such as elemental Cr, Fe, and Ni.

[1] H. Yavas, M. Sundermann, K. Chen, A. Amorese, A. Severing, H. Gretarsson, M.W. Haverkort, L.H. Tjeng,
Nature Physics (2019) ; https://doi.org/10.1038/s41567-019-0471-2.
  

Interplay of orbital selectivity and local environment in correlated materials: the case of metal-insulator transition in CaFeO3

Density Functional Theory (DFT) with Embedded Dynamical Mean Field Theory (eDMFT) is a very successful method in describing novel electronic states of matter where (I) Mott and metallic orbitals coexist (orbital-selective Mott state) (II) Mott, metallic and semi-metallic orbitals coexist (site-orbital-selective Mott state) and (III) Mott and band orbitals coexist (band-Mott state). Recently, forces for structural relaxations within the DFT+eDMFT have been developed, and the predictive power of the method for electronic-structural interplay at finite temperatures was demonstrated on correlated materials that have novel states with orbital and site selectivity, such as NdNiO₃, LaMnO₃, BiMnO₃, TM₂Mo₃O₈ (TM=Mn, Fe). Through the use of this method, we apply DFT+eDMFT to study the electronic-structural interplay at finite temperature, in order to describe the interplay between Mott, band and metallic-like orbitals and the local environment (characterized by the bond length and bond angle) in CaFeO₃, as it goes through the metal to insulator transition.   

Atomic-scale insight into lattice and electronic modulation induced by the substitution of bismuth in iron garnet with enhanced magneto-optical effect

In this study, Bismuth substituted lutetium iron garnet prepared by liquid epitaxial method shows large faraday rotation up to 1.64 degree/µm at 633nm at room temperature, whose structure variation and element distribution in atomic scale have been directly interpreted by using analytical scanning transmission microscopy (STEM). The substituted Bismuth located in dodecahedral site of lutetium iron garnet can induced the change of lattice and furtherly will lead to the distortion of octahedral Fe sites, but magnetic order is not modified, which is all verified by using energy loss spectrum (EELS). Combined with density function theory (DFT) calculation, distorted octahedral magnetic sublattice contributes to the reduced crystal field splitting energy and orbital degeneracy.
[1] S. Higuchi, K. Ueda, F. Yahiro, Y. Nakata, H. Uetsuhara, T. Okada, M. Maeda, IEEE Trans. Magn. 2001, 37, 2451.
[2] Y. Qing-Hui, Z. Huai-Wu, W. Qi-Ye, L. Ying-Li, I. M. Syvorotka, I. I. Syvorotka, Chinese Physics Letters 2009, 26, 047401.   

Field and Pressure Tuning the Coupling of Crystal Electric Field and Phonon Modes in Praseodymium Sesquioxide

The energetic overlap of phonons and low-lying crystal electric field (CEF) excitations in rare-earth sesquioxides is conducive to strong electron-phonon coupling and dramatic magnetostructural phenomena. For example, our previous temperature and magnetic field dependent Raman scattering studies on a member of this series, A-type Pr2O3, revealed significant phonon softening and a level repulsion between electronic and phononic levels. To explore these results further, we present a pressure-dependent Raman scattering study of Pr2O3 aimed at tuning the coupling between electron and phonon levels to modify the low temperature phase behavior. We observe that the degeneracy of the lowest lying CEF doublet is lifted under the application of pressure below 60 K and that the energy splitting increases linearly with increasing pressure or magnetic field. These results are indicative of a pressure-induced Structural Distortion that appears to be instigated by pressure-dependent electron phonon coupling. We discuss the possible effects of pressure on the magnetostructural properties of Pr2O3.   

Deciphering Anion Order in Oxysulfide Perovskites: Origins and Implications

Perovskite compounds—with a generic formula of ABX3— represent a versatile class of materials exhibiting tremendous synthetic flexibility and technological applications. Going beyond “single” perovskites, two or more species can occupy each sublattice. While origins and design rules of cation ordering in perovskites are well known, anion ordering in heteroanionic perovskites remains a largely uncharted territory. This talk will share insights from a first principles analysis of anion-ordered AB(O0.5S0.5)3 oxysulfide chemistries, studied in all possible anion configurations, with A=Ca,Sr,Ba and B=Ti,Zr,Hf. We find that the stable local ordering is always an all-cis arrangement (i.e., a fac-type ordering) in each BO3S3 octahedron, stabilized by a combination of electronic, strain and electrostatic interactions. We further study the relative stability of this ordering as a function of A and B site chemistries and probe its effect on the electronic structure. Most remarkably, we show that the ground state anion ordering breaks inversion symmetry to create a family of polar oxysulfides with polarization >30 µC/cm2, exhibiting a significant promise for electronic applications.   

Symmetry analysis of anion order, cation-vacancy order, and octahedral tilting in oxyfluoride double perovskites

In simple homoanionic double perovskites, the possible phase space based on octahedral tilting and cation ordering has been established and is useful in resolving known structures as well as performing computational studies to determine viable phase transitions [1] [2]. However, a similar study incorporating octahedral tilting and ordering in heteroanionic materials has yet to be carried out. Initial work in oxynitrides has determined that by coupling octahedral tilting and anion ordering, it is possible to further lower the phase symmetries of materials [3]. This coupling can also be used to break inversion symmetry, leading to improvement in dielectric responses and ferroelectricity. Here we use the P4/nmm phase of [ ]KNaNbOF₅ as a prototype structure to explore how independently and in combination anion ordering, A-site vacancy ordering, and octahedral tilting lead to variations in symmetries [4]. We apply materials physics principles to understand commonalities among the structure space and perform electronic structure calculation to ascertain the microscopic origins.
[1] Acta.Cryst.(2003).B59,463-471
[2] Jou.Sol.St.Chem.(2006)179,4,1076-1085
[3] Cryst.GrowthDes.(2013)13,10,4623-4629
[4] Crys. (2011),1,1,3-14   

Understanding the Appearance of Local and Long-Range Anion Order in Heteroanionic Layered Perovskites

The prediction of the appearance, or lack thereof, or anion ordering in mixed-anion and heteroanionic materials is an important, yet unsolved problem. This issue has a significant impact on the prediction of properties in these materials, especially properties whose appearance is reliant on long-range noncentrosymmetry, such as ferroelectricity, piezoelectricity and nonlinear optical pheonmena such as second harmonic generation. We examine this issue in Ruddlesden-Popper oxyfluorides where both anion ordered and disordered structures exist, such as Sr₂FeO₃F and Sr₂ScO₃F, and fluoride preferentially occupies the apical anion site of the 2D perovskite layers. We use density functional theory calculations to understand and attempt to quantify the contributions of the B-site cation size, covalency, and magnetism to the configurational entropy or the appearance of local and long-range ordered anions.