I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166]
Abstract: KBiNb2O7 was prepared from RbBiNb2O7 by a sequence of cation exchange reactions which first convert RbBiNb2O7 to LiBiNb2O7, before KBiNb2O7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb2O7 adopts a polar, layered, perovskite structure (space group A11m) in which the BiNb2O7 layers are stacked in a (0, ½, z) arrangement, with the K+ cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi3+ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb2O7 exhibits frequent stacking faults which convert the (0, ½, z) layer stacking to (½, 0, z) stacking and vice versa, essentially switching the x- and y-axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb2O7 it is estimated that each layer has approximately a ~9% chance of being defective - a high level which is attributed to the lack of cooperative NbO6 tilting in the material, which limits the lattice strain associated with each fault.
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Jan 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[24154]
Abstract: We present a comprehensive structural and magnetic characterization of the barlowite family of S = 1/2 kagomé magnets, Cu4(OH)6FX, where X = Cl, Br, or I. Through high-resolution synchrotron X-ray and neutron powder diffraction measurements, we reveal two sources of structural complexity within this family of materials, namely, compositional disorder of the halide species that occupy sites in between the kagomé layers and the positional disorder of the interlayer Cu2+ ions that persists well into the Pnma structural ground state. We demonstrate that understanding these inherent structural disorders is key as they correlate with the degree of partial order in the magnetic ground states of these quantum frustrated magnets.
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Dec 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Abstract: Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The ‘hybrid improper’ mechanism - in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure - offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb2O7, LiBiNb2O7 and NaBiNb2O7, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi3+ cations which are often observed to stabilize acentric crystal structures due to their 6s2 electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb2O7 and LiBiNb2O7 adopt polar crystal structures (space groups I2cm and B2cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi3+ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi3+ cations with Nd3+ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb2O7 (space group P212121) differs significantly from the centrosymmetric structure of NaNdNb2O7, which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi3+ cations.
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Oct 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Open Access
Abstract: Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li2SrTa2O7, which adopts a polar structure due to a cooperative tilting of its constituent TaO6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa2O7, can be prepared. On cooling below 43 K the Mn2+ centres in MnSrTa2O7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations.
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Aug 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17198]
Open Access
Abstract: The magnitude of ionic conductivity is known to depend upon both mobility and number of available carriers. For proton conductors, hydration is a key factor in determining the charge–carrier concentration in ABO3 perovskite oxides. Despite the high reported proton mobility of calcium titanate (CaTiO3), this titanate perovskite has thus far been regarded as a poor proton conductor due to the low hydration capability. Here, the enhanced proton conductivity of the defective calcium titanate Ca0.92TiO2.84(OH)0.16 prepared by replacing lattice oxygens with hydroxyl groups via a solvothermal route is shown. Conductivity measurements in a humidified Ar atmosphere reveal that, remarkably, this material exhibits one order of magnitude higher bulk conductivity (10−4 Scm−1 at 200 °C) than hydrated stoichiometric CaTiO3 prepared by traditional solid-state synthesis due to the higher concentration of protonic defects and variation in the crystal structure. The replacement of Ca2+ by Ni2+ in the Ca1−xTi1O3−2x(OH)2x, which mostly exsolve metallic Ni nanoparticles along orthorhombic (100) planes upon reduction, is also demonstrated. These results suggest a new strategy by tailoring the defect chemistry via hydration or cation doping followed by exsolution for targeted energy applications.
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Aug 2021
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Yili
Cao
,
Kun
Lin
,
Sergii
Khmelevskyi
,
Maxim
Avdeev
,
Keith M.
Taddei
,
Qiang
Zhang
,
Qingzhen
Huang
,
Qiang
Li
,
Kenichi
Kato
,
Chiu Chung
Tang
,
Alexandra
Gibbs
,
Chin-Wei
Wang
,
Jinxia
Deng
,
Jun
Chen
,
Hongjie
Zhang
,
Xianran
Xing
Abstract: Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of
R
2
(
Fe
,
Co
)
17
materials (
R
=
rare
Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3–461 K, almost twice the range of currently known SIV. In situ neutron diffraction, Mössbauer spectra and first-principles calculations reveal the
3
d
bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve “ultrawide” SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.
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Jul 2021
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I12-JEEP: Joint Engineering, Environmental and Processing
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Chi Ming
Yim
,
Soumendra Nath
Panja
,
Christopher
Trainer
,
Craig
Topping
,
Christoph
Heil
,
Alexandra S.
Gibbs
,
Oxana
Magdysyuk
,
Vladimir
Tsurkan
,
Alois
Loidl
,
Andreas W.
Rost
,
Peter
Wahl
Diamond Proposal Number(s):
[22974]
Open Access
Abstract: A key property of many quantum materials is that their ground state depends sensitively on small changes of an external tuning parameter, e.g., doping, magnetic field, or pressure, creating opportunities for potential technological applications. Here, we explore tuning of the ground state of the nonsuperconducting parent compound, Fe1+xTe, of the iron chalcogenides by uniaxial strain. Iron telluride exhibits a peculiar (π, 0) antiferromagnetic order unlike the (π, π) order observed in the Fe-pnictide superconductors. The (π, 0) order is accompanied by a significant monoclinic distortion. We explore tuning of the ground state by uniaxial strain combined with low-temperature scanning tunneling microscopy. We demonstrate that, indeed under strain, the surface of Fe1.1Te undergoes a transition to a (π, π)-charge-ordered state. Comparison with transport experiments on uniaxially strained samples shows that this is a surface phase, demonstrating the opportunities afforded by 2D correlated phases stabilized near surfaces and interfaces.
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Apr 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Abstract: Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM2O7 (M = Nb, Ta) yields the corresponding, metasta-ble KNdM2O7 phases. Synchrotron X-ray and neutron powder diffraction data, combined with powder SHG data, reveal that the KNdM2O7 phases adopt a polar structure (space group Im2m) consisting of NdM2O7 double perovskite sheets stacked in a (0, ½, z) manner with K+ cations ordered within the 6-coordinate prismatic inter-layer sites. The perovskite double sheets adopt an (a0b+c0/a0-b+c0) tilting distortion, however unlike other A’AB2O7 phases this distortion is not the origin of the non-centrosymmetric structure, which is attributed to a second-order Jahn-Teller distortion of the MO6 units. First-principles DFT calculations confirm the polar Im2m phase is more stable than the corresponding centrosymmetric alternative. The role of the A’- and A- cations in di-recting the stacking patterns and tilting distortions of A’AB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.
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Aug 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Abstract: There has been much interest recently in hybrid improper ferroelectrics – materials which adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related struc-tures. Using a combination of synchrotron X-ray powder diffraction and high-resolution neutron powder diffraction data the temperature-dependent phase transitions of a series of n = 2 Dion-Jacobson oxides have been investigated. RbNdM2O7 undergoes a transition from a polar, aˉaˉc+/-(aˉaˉc+) distorted I2cm phase to an antipolar, aˉb0cˉ/-(aˉb0)cˉ distorted Cmca phase at T = 790 K and 500 K for M = Nb and Ta respectively. There is a subsequent transition to an a0a0cˉ/a0a0-cˉ distorted I4/mcm structure at 865 K and 950 K for M = Nb and Ta respectively, before a transition to the undistorted P4/mmm aristotype structure. In contrast CsNdM2O7 undergoes a transition from a polar, aˉaˉc+ distorted P21am structure to an antipolar, aˉb0cˉ distorted C2/m phase at T = 625 K and 330 K for M = Nb and Ta respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 K and 820 K for M = Nb and Ta respectively. A plot of Tc against the relative stability of the 4 polar Dion-Jacobson phases compared to the corre-sponding aristotype P4/mmm structures (calculated from first-principles DFT), yields a strong linear relation suggest-ing Tc is not proportional to the enthalpy change at the ferroelectric phase transition.
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Apr 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Abstract: Cation exchange reactions performed on the n = 2 Dion-Jacobson phases RbNdNb2O7 and RbNdTa2O7, using LiNO3 and NaNO3, yield the corresponding LiNdM2O7 and NaNdM2O7 (M = Nb, Ta) phases. Synchrotron X-ray and neutron powder diffraction data, in combination with second-harmonic generation data and supported by first-principles DFT calculations, reveal that the LiNdM2O7 phases adopt n = 2 Ruddlesden-Popper type structures with an aˉaˉc+/-(aˉaˉc+) distortion described in the polar space group B2cm. In contrast the NaNdM2O7 phases adopt n = 2 Ruddlesden-Popper type structures with an aˉb0c0/b0aˉc0 distortion, described in the centrosymmetric space group P42/mnm. The differing structures adopted by the LiNdM2O7 and NaNdM2O7 phases are rationalized on the basis of a completion between i) optimizing the size Li/Na coordination site via octahedral tilting and ii) ordering the Na/Li cations within the (Li/Na)O2 sheets to minimize cation-cation repulsion – the former appears to be the dominant factor for the Li phases and the latter factor dominates for the Na phases. The strong A’-cation dependence of the tilting distortions adopted by the A’NdM2O7 phases suggests that by careful selection of the substituting cation the tilting distortions of layered perovskite phases can be rationally tuned to adopt polar configurations, and thus new ferroelectric phases can be synthesized.
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Nov 2018
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