Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7-xSx

The arrangement of cations on the triangular pyrochlore lattice leads to a wealth of interesting physical phenomena influenced by geometric frustration. Although uncommon, several pyrochlore materials overcome this frustration and exhibit polar structures. Unraveling the origin of such behavior is key to understanding how broken inversion symmetry arises in complex crystal structures. Here, we investigate the effect of varying degrees of covalency in the pyrochlore lattice through a detailed structural and lattice dynamical analysis of the pyrochlore oxysulfide series Cd2Nb2O7-xSx above and below the ferroelectric transition temperatures (TC) using synchrotron X-ray diffraction and first principles calculations. All compositions exhibit the cubic Fd3 m pyrochlore aristotype above TC, whereas the amplitude and character of various structural distortions are found to be composition-dependent below TC. For x = 0, large Cd and Nb cation displacements occur to produce the polar Ima2 structure accompanied by a change in translational symmetry. Our symmetry and lattice dynamical calculations indicate that Cd2Nb2O7 undergoes a proper ferroelectric transition through TC. Analysis of the sulfur-substituted niobates indicates that although the polar space group Fdd2 is adopted by the nominal x = 0.25 sample, the transition into the polar phase is improper. For the nominally x = 0.7 composition, the lattice remains nearly cubic, but exhibits a high degree of structural disorder in the pyrochlore channel, with a deviation from the linear Cd-X′-Cd bond by nearly 15° to accommodate the large size of S while preventing extreme stretching of the Nb-O bond. This highly distorted Cd-X′ network is accompanied by a highly distorted NbO6 network, which is accommodated by the polarizable NbO6 coordination environment. This sheds light on the limited existence of oxysulfide pyrochlores; for example, the lack of reported S substitution in the case of the similar yet less-polarizable Cd2Ta2O7. Our work provides a new understanding of how inversion-symmetry lifting displacements arise and how anion substitution, which tunes covalent cation-anion interactions, is a useful strategy for manipulating polar behavior in a pyrochlore lattice

Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2

Magnetic materials with noncollinear spin textures are promising for spintronic applications. To realize practical devices, control over the length and energy scales of such spin textures is imperative. The chiral helimagnets Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic phase diagrams with different real-space periodicities and field dependence, positioning them as model systems for studying the relative strengths of the microscopic mechanisms giving rise to exotic spin textures. Here, we carry out a comparative study of the electronic structures of Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved photoemission spectroscopy and density functional theory. We show that bands in Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2 and connect this result to bonding and orbital overlap in these materials. We also unambiguously distinguish exchange splitting from surface termination effects by studying the dependence of their photoemission spectra on polarization, temperature, and beam size. We find strong evidence that hybridization between intercalant and host lattice electronic states mediates the magnetic exchange interactions in these materials, suggesting that band engineering is a route toward tuning their spin textures. Overall, these results underscore how the modular nature of intercalated transition metal dichalcogenides translates variation in composition and electronic structure to complex magnetism.Comment: 46 pages, 18 figures, 5 table

Structure-Function Relationships in Pyrochlore Oxides

Coherent off-centering of the A-site cation can be frustrated on the rigid pyrochlore lattice, preventing the ferroelectric-paraelectric phase transition in majority of pyrochlore oxide materials. One of the few ferroelectric pyrochlores is Cd2Nb2-xTaxO7. While it has been studied for some time, the low temperature structures are still not well understood, hindering the understanding of ferroelectric phase transitions and long–range ordering of distortions in pyrochlores. In this work synchrotron X-ray scattering data was used to characterize the low temperature ferroelectric structures of Cd2Nb2-xTaxO7 (x = 0, 1, 2) and better understand the structural changes accompanying the ferroelectric to paraelectric phase transitions. For the x= 0 material (Cd2Nb2O7), it was found that second order Jahn-Teller distortions, stemming from Nb(V), caused overall correlated displacements in both the NbO6 and Cd2O’ networks. Another pyrochlore of interest is Bi2Ti2O7, which exhibits local polar distortions, but lacks long-range ferroelectricity due to frustration of the A-site cations. Herein we attempted to relieve the frustration of Bi lone pairs through partial substitution of smaller cations with no lone pairs into the A-site. The solid solutions studied in this thesis are Bi2-xYxTi2O7 (x = 0.5, 1, 1.5, 2) and Bi2-xZnxTi2-xNbxO7 (x = 0.25, 0.75, 1). The average structure of all samples at 100K and 300K was characterized using synchrotron X-ray diffraction experiments. In addition, local structure at 300K of Bi2-xYxTi2O7 was investigated using pair distribution function (PDF) analysis. Analysis of Bi2-xYxTi2O7 solid solution revealed that increasing Bi content increases atomic motion in the A–site and local disorder in the 1 to 10 angstrom range. Even though yttrium substitution increased structural order, the frustration of Bi lone pairs was not relieved and a ferroelectric phase transition was not observed at 100K or 300K. Zinc substituted samples formed multiple pyrochlore phases, which were non-ferroelectric at the studied temperatures

Structure and Magnetism of Iron- and Chromium-Intercalated Niobium and Tantalum Disulfides

Transition metal dichalcogenides (TMDs) intercalated with spin-bearing transition metal centers are a diverse class of magnetic materials where the spin density and ordering behavior can be varied by the choice of host lattice, intercalant identity, level of intercalation, and intercalant disorder. Each of these degrees of freedom alters the interplay between several key magnetic interactions to produce disparate collective electronic and magnetic phases. The array of magnetic and electronic behavior typified by these systems renders them distinctive platforms for realizing tunable magnetism in solid-state materials and promising candidates for spin-based electronic devices. This Perspective provides an overview of the rich magnetism displayed by transition metal-intercalated TMDs by considering Fe- and Cr-intercalated NbS2 and TaS2. These four exemplars of this large family of materials exhibit a wide range of magnetic properties, including sharp switching of magnetic states, current-driven magnetic switching, and chiral spin textures. An understanding of the fundamental origins of the resultant magnetic/electronic phases in these materials is discussed in the context of composition, bonding, electronic structure, and magnetic anisotropy in each case study

Mapping the structural, magnetic and electronic behavior of (Eu\u3csub\u3e1- x\u3c/sub\u3eCa\u3csub\u3ex\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eIr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e7\u3c/sub\u3eacross a metal-insulator transition

© 2020 IOP Publishing Ltd. In this study, we employ bulk electronic properties characterization and x-ray scattering/spectroscopy techniques to map the structural, magnetic and electronic properties of (Eu1-xCax)2Ir2O7 as a function of Ca-doping. As expected, the metal-insulator transition temperature, T MIT, decreases with Ca-doping until a metallic state is realized down to 2 K. In contrast, T AFM becomes decoupled from the MIT and (likely short-range) AFM order persists into the metallic regime. This decoupling is understood as a result of the onset of an electronically phase separated state, the occurrence of which seemingly depends on both synthesis method and rare earth site magnetism. PDF analysis suggests that electronic phase separation occurs without accompanying chemical phase segregation or changes in the short-range crystallographic symmetry while synchrotron x-ray diffraction confirms that there is no change in the long-range crystallographic symmetry. X-ray absorption measurements confirm the J eff = 1/2 character of (Eu1-xCax)2Ir2O7. Surprisingly these measurements also indicate a net electron doping, rather than the expected hole doping, indicating a compensatory mechanism. Lastly, XMCD measurements show a weak Ir magnetic polarization that is largely unaffected by Ca-doping. Keywords: term, term, term