Victor Krayzman

Bio: Victor Krayzman is an academic researcher from National Institute of Standards and Technology. The author has contributed to research in topics: Neutron diffraction & Reverse Monte Carlo. The author has an hindex of 13, co-authored 23 publications receiving 557 citations.

Structural changes underlying the diffuse dielectric response in AgNbO 3

Abstract: Structural differences in the so-called M polymorphs of ${\text{AgNbO}}_{3}$ were analyzed using combined high-resolution x-ray diffraction, neutron total scattering, electron diffraction, and x-ray absorption fine-structure measurements. These polymorphs all crystallize with $Pbcm$ symmetry and lattice parameters $\ensuremath{\surd}2{a}_{c}\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}2{a}_{c}\ifmmode\times\else\texttimes\fi{}4{a}_{c}$ (where ${a}_{c}\ensuremath{\approx}4\text{ }\text{\AA{}}$ corresponds to the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system $({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}})/({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{+})$ involving a sequence of two in-phase and two antiphase rotations around the $c$ axis. Our results revealed that, similar to ${\text{KNbO}}_{3}$, the Nb cations in ${\text{AgNbO}}_{3}$ exhibit local off-center displacements correlated along Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature ${\text{AgNbO}}_{3}$ polymorphs where the Nb cations, on average, reside on the ideal fixed-coordinate sites. The onset of the $({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}})/({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{+})$ tilting in the M polymorphs lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolarlike array. This ordering preserves the average $Pbcm$ symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Structural states previously identified as the ${\text{M}}_{3}$ and ${\text{M}}_{2}$ phases represent different stages of displacive ordering rather than distinct thermodynamic phases. Rietveld refinements indicated intimate coupling between the displacive behavior on the oxygen, Nb, and Ag sublattices. The $Pbcm$ symmetry of the octahedral framework precludes a complete ordering of Nb displacements so that some positional disorder is retained. This partial disorder likely gives a source to the dielectric relaxation which, according to previous spectroscopic studies, is the origin of the diffuse dielectric response exhibited by M-type ${\text{AgNbO}}_{3}$ at $\ensuremath{\approx}250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$.

Local atomic order and hierarchical polar nanoregions in a classical relaxor ferroelectric

Abstract: The development of useful structure-function relationships for materials that exhibit correlated nanoscale disorder requires adequately large atomistic models which today are obtained mainly via theoretical simulations. Here, we exploit our recent advances in structure-refinement methodology to demonstrate how such models can be derived directly from simultaneous fitting of 3D diffuse- and total-scattering data, and we use this approach to elucidate the complex nanoscale atomic correlations in the classical relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN). Our results uncover details of ordering of Mg and Nb and reveal a hierarchical structure of polar nanoregions associated with the Pb and Nb displacements. The magnitudes of these displacements and their alignment vary smoothly across the nanoregion boundaries. No spatial correlations were found between the chemical ordering and the polar nanoregions. This work highlights a broadly applicable nanoscale structure-refinement method and provides insights into the structure of PMN that require rethinking its existing contentious models.

Local-structure origins of the sustained Curie temperature in (Ba,Ca)TiO3 ferroelectrics

Abstract: While the lattice volume in the solid-solution Ba1−xCaxTiO3 decreases with increasing x, the Curie temperature remains unaffected, in contrast to Ba1−xSrxTiO3. We have determined the origin of this phenomenon by comparing the local structures in (Ba,Ca)TiO3 and (Ba,Sr)TiO3. Reverse Monte Carlo refinements of instantaneous atomic positions using simultaneous fitting of multiple types of experimental data (neutron total scattering, X-ray absorption fine structure, patterns of diffuse scattering in electron diffraction) reveal both ferroelectric Ca displacements and their amplification of the Ti off-centering, which mitigate the lattice-volume effects. The activity of Ca is triggered by the anomalously strained Ca-O bonds.

Local structure of Ba(Ti,Zr)O3 perovskite-like solid solutions and its relation to the band-gap behavior

Abstract: Local structures in BaTi{sub 1-x}Zr{sub x}O{sub 3} solid solutions were analyzed using x-ray absorption fine structure (XAFS) measurements and density-functional theory (DFT) calculations. We demonstrate that for low concentrations of Ti, isolated Ti atoms in the relatively large octahedral sites of the BaZrO{sub 3} lattice acquire centrosymmetric coordination with average Ti-O distances shorter than those in BaTiO{sub 3}. In contrast for higher concentrations of Ti, Ti atoms having one or more Ti as their B-site nearest neighbors undergo polar off-center displacements. Our DFT calculations confirm both effects. These results combined with the previously published data suggest that isolated polarizable ions on the B sites of a relatively expanded host perovskite lattice remain nonpolar by symmetric relaxation of the nearest-neighbor oxygen atoms to yield nearly ideal bond lengths around the dopant species. For neighboring Ti atoms, such symmetric relaxation is impossible, and these atoms are displaced off center. Our XAFS measurements did not detect any significant deviations from a random distribution of Ti and Zr in the present samples except for compositions close to BaTiO{sub 3}. The DFT calculations suggest that the dominant effect of the local displacements on band-gap values for this system is determined by the shortest Ti-O bondsmore » due to strong Ti 3d-O 2p hybridization; however, local displacements have only a secondary effect on the band-gap behavior.« less

X-ray absorption fine structure studies of Mn coordination in doped perovskite SrTiO3

Abstract: The coordination of Mn in doped SrTiO3 ceramics having nominal compositions SrTi0.98Mn0.02O3 and Sr0.98Mn0.02TiO3 was analyzed using x-ray absorption fine structure (XAFS) measurements. As expected, Mn4+ substitution for Ti4+ leads to Mn occupancy of the octahedral B-sites of ABO3 perovskite lattice with a Mn–O bond distance of 1.902 A (compared to 1.953 A for Ti–O) and no significant local distortions around the Mn atoms. In contrast, for the composition Sr0.98Mn0.02TiO3, Mn segregates to both the A-sites (as Mn2+) and the B-sites (predominantly as Mn4+). Extended XAFS confirms strong (≈0.77 A) displacements of Mn2+ cations off the ideal A-site positions along ⟨001⟩ directions with a significant distortion of several coordination shells around the dopant atoms.

X-Ray Diffraction

Abstract: Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

The problem with determining atomic structure at the nanoscale.

Abstract: Emerging complex functional materials often have atomic order limited to the nanoscale. Examples include nanoparticles, species encapsulated in mesoporous hosts, and bulk crystals with intrinsic nanoscale order. The powerful methods that we have for solving the atomic structure of bulk crystals fail for such materials. Currently, no broadly applicable, quantitative, and robust methods exist to replace crystallography at the nanoscale. We provide an overview of various classes of nanostructured materials and review the methods that are currently used to study their structure. We suggest that successful solutions to these nanostructure problems will involve interactions among researchers from materials science, physics, chemistry, computer science, and applied mathematics, working within a "complex modeling" paradigm that combines theory and experiment in a self-consistent computational framework.

Lead-Free Antiferroelectric Silver Niobate Tantalate with High Energy Storage Performance.

Abstract: Antiferroelectric materials that display double ferroelectric hysteresis loops are receiving increasing attention for their superior energy storage density compared to their ferroelectric counterparts. Despite the good properties obtained in antiferroelectric La-doped Pb(Zr,Ti)O3 -based ceramics, lead-free alternatives are highly desired due to the environmental concerns, and AgNbO3 has been highlighted as a ferrielectric/antiferroelectric perovskite for energy storage applications. Enhanced energy storage performance, with recoverable energy density of 4.2 J cm-3 and high thermal stability of the energy storage density (with minimal variation of ≤±5%) over 20-120 °C, can be achieved in Ta-modified AgNbO3 ceramics. It is revealed that the incorporation of Ta to the Nb site can enhance the antiferroelectricity because of the reduced polarizability of B-site cations, which is confirmed by the polarization hysteresis, dielectric tunability, and selected-area electron diffraction measurements. Additionally, Ta addition in AgNbO3 leads to decreased grain size and increased bulk density, increasing the dielectric breakdown strength, up to 240 kV cm-1 versus 175 kV cm-1 for the pure counterpart, together with the enhanced antiferroelectricity, accounting for the high energy storage density.

The Antiferroelectric ↔ Ferroelectric Phase Transition in Lead-Containing and Lead-Free Perovskite Ceramics

Abstract: A comprehensive review on the latest development of the antiferroelectric ↔ ferroelectric phase transition is presented. The abrupt volume expansion and sudden development of polarization at the phase transition has been extensively investigated in PbZrO3-based perovskite ceramics. New research developments in these compositions, including the incommensurate domain structure, the auxetic behavior under electric fields in the induced ferroelectric phase, the ferroelastic behavior of the multicell cubic phase, the impact of radial compression, the unexpected electric field-induced ferroelectric-to-antiferroelectric transition, and the phase transition mechanical toughening effect have been summarized. Due to their significance to leadfree piezoelectric ceramics, compounds with antiferroelectric phases, including NaNbO3, AgNbO3, and (Bi1/2Na1/2)TiO3, are also critically reviewed. Focus has been placed on the (Bi1/2Na1/2)TiO3–BaTiO3 solid solution where the electric field-induced ferroelectric phase remains even after the applied field is removed at room temperature. Therefore, the electric field-induced antiferroelectric-to-ferroelectric phase transition is a key to the poling process to develop piezoelectricity in morphotropic phase boundary (MPB) compositions. The competing phase transition and domain switching processes in 0.93 (Bi1/2Na1/2)TiO3–0.07BaTiO3 are directly imaged with nanometer resolution using the unique in situ transmission electron microscopy (TEM) technique.

Antiferroelectrics for Energy Storage Applications: a Review

Abstract: Z.L., T.L., R.W., and Y.L. thank the Australian Research Council (ARCDP160104780) for financial support in the form of ARC Discovery Project. Z.L., J.Y., G.W., and X.D. thank the financial support of the National Natural Science Foundation of China (NSFC No. 11774366) and International Partnership Project of Chinese Academy of Science. Z.L. also acknowledges the support of Shanghai Sailing Program (No. 17YF1429700).