B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166]
Open Access
Abstract: Cation migration on electrochemical cycling can significantly influence the performance of li-ion cathode materials. Phases of composition LiFe2–xInxSbO6 (0 < x <1) adopt crystal structures described in space group Pnnm, consisting of a hexagonally close-packed array of oxide ions, with Fe/In and Sb cations ordered on octahedral sites, and lithium cations located within partially occupied tetrahedral sites. NPD, SXRD, and 57Fe Mössbauer data indicate that on reductive lithium insertion (either chemically or electrochemically), LiFe2SbO6 is converted to Li2Fe2SbO6 accompanied by large-scale cation migration, to form a partially Fe/Li cation-ordered and Fe2+/Fe3+ charge-ordered phase from which lithium cations cannot be easily removed, either chemically or electrochemically. Partial substitution of Fe with In suppresses the degree of cation migration that occurs on lithium insertion such that no structural change is observed when LiFeInSbO6 is converted into Li1.5FeInSbO6, allowing the system to be repeatedly electrochemically cycled between these two compositions. Phases with intermediate levels of In substitution exhibit low levels of Fe migration on Li insertion and electrochemical capacities which evolve on cycling. The mechanism by which the In3+ cations suppress the migration of Fe cations is discussed along with the cycling behavior of the LiFe1.5In0.5SbO6–Li1.75Fe1.5In0.5SbO6.
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Dec 2022
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I09-Surface and Interface Structural Analysis
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[29451]
Abstract: Topochemical reduction of the cation-disordered
perovskite oxides LaCo0.5Rh0.5O3 and LaNi0.5Rh0.5O3 with Zr yields
the partially anion-vacancy ordered phases LaCo0.5Rh0.5O2.25 and
LaNi0.5Rh0.5O2.25, respectively. Neutron diffraction and Hard X-ray
photoelectron spectroscopy (HAXPES) measurements reveal that
the anion-deficient phases contain Co1+/Ni1+ and a 1:1 mixture of
Rh1+ and Rh3+ cations within a disordered array of apex-linked
MO4 square-planar and MO5 square-based pyramidal coordination
sites. Neutron diffraction data indicate that LaCo0.5Rh0.5O2.25
adopts a complex antiferromagnetic ground state, which is the
sum of a C-type ordering (mM5
+) of the xy-components of the Co
spins and a G-type ordering (mΓ1
+) of the z-components of the Co
spins. On warming above 75 K, the magnitude of the mΓ1
+
component declines, attaining a zero value by 125 K, with the magnitude of the mM5
+ component remaining unchanged up to
175 K. This magnetic behavior is rationalized on the basis of the differing d-orbital fillings of the Co1+ cations in MO4 square-planar
and MO5 square-based pyramidal coordination sites. LaNi0.5Rh0.5O2.25 shows no sign of long-range magnetic order at 2 K − behavior
that can also be explained on the basis of the d-orbital occupation of the Ni1+ centers.
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Sep 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166]
Open Access
Abstract: LaSrNiIrO6 adopts a B-site cation-ordered double perovskite structure with a strong a–a–c + cooperative tilting distortion (space group P21/n a = 5.5931(1) Å, b = 5.5676(1) Å, c = 7.8850(1) Å, β = 90.01(1) o). Magnetization and neutron diffraction data indicate LaSrNiIrO6 adopts a ‘type II’ antiferromagnetic structure (TN = 70 K) in which the Ni spins are arranged in an antiferromagnetic manner with their next-nearest-neighbors, with no ordered moment observed for Ir. DFT calculations, including spin-orbit coupling effects, confirm S = 1 Ni2+ and Jeff = 0 Ir5+ local configurations and indicate strong Ni() (Kobayashi et al., 1999) [1]–O(2p)–Ir()0–O(2p)–Ni() (Kobayashi et al., 1999) [1] σ-type super-super-exchange is the dominant magnetic coupling interaction in the system.
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Aug 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166]
Abstract: Reaction between Na2FeSbO5 and LiNO3 at 300 °C yields the metastable phase Li2FeSbO5 which is isostructural with the sodium “parent” phase (space group Pbna, a = 15.138(1) Å, b = 5.1440(3) Å, c = 10.0936(6) Å) consisting of an alternating stack of Li2Fe2O5 and Li2Sb2O5 sheets containing tetrahedral coordinated Fe3+ and octahedrally coordinated Sb5+, respectively. Further reaction between Li2FeSbO5 with NO2BF4 in acetonitrile at room temperature yields LiFeSbO5, which adopts an orthorhombic structure (space group Pbn21, a = 14.2943(4) Å, b = 5.2771(1) Å, c = 9.5610(3) Å) in which the LiFeO5 layers have shifted on lithium extraction, resulting in an octahedral coordination for the iron cations. 57Fe Mössbauer data indicate that the nominal Fe4+ cations present in LiFeSbO5 have disproportionated into a 1:1 combination of Fe3+ and Fe5+ centers which are ordered within the LiFeSbO5 structural framework. It is widely observed that Fe4+ centers tend to be unstable in delithiated Li–Fe–X–O phases currently proposed as lithium-ion battery cathode materials, so the apparent stability of highly oxidized Fe5+ centers in LiFeSbO5 is notable, suggesting cathode materials based on oxidizing Fe3+ could be possible. However, in this instance, the structural change which occurs on delithiation of Li2FeSbO5 prevents electrochemical cycling of this material.
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Feb 2022
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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):
[25166]
Open Access
Abstract: The progress of the topochemical reduction reaction that converts LaSrNiRuO6 into LaSrNiRuO4 depends on the synthesis conditions used to prepare the oxidized phase. Samples of LaSrNiRuO6 that have been quenched from high temperature can be readily and rapidly converted into LaSrNiRuO4. In contrast, samples that have been slow-cooled cannot be completely reduced. This reactivity difference is attributed to the differing microstructures of the quenched and slow-cooled samples, with the former having much smaller average crystalline domain sizes and larger lattice strains than the latter. A mechanism to explain this effect is presented, in which the greater “plasticity” of small crystalline domains helps lower the activation energy of the reduction reaction. In addition, we propose that the enhanced lattice strain in quenched samples also acts to destabilize the host phase, further enhancing reactivity. These observations suggest that the microstructure of a material can be used to “activate” topochemical reactions in the solid state, expanding the scope of phases that can be prepared by this type of reaction.
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Nov 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]
Abstract: LaxSr2–xNiRuO6, LaxSr4–xNiRuO8, and LaxSr3–xNiRuO7 are, respectively, the n = ∞, 1, and 2 members of the (Lax/2Sr1–(x/2))nSr(Ni0.5Ru0.5)nO3n+1 compositional series. Reaction with CaH2, in the case of the LaxSr2–xNiRuO6 perovskite phases, or Zr oxygen getters in the case of the LaxSr4–xNiRuO8 and LaxSr3–xNiRuO7 Ruddlesden–Popper phases, yields the corresponding topochemically reduced (Lax/2Sr1–(x/2))nSr(Ni0.5Ru0.5)nO3n–1 compounds (LaxSr2–xNiRuO4, LaxSr4–xNiRuO6, and LaxSr3–xNiRuO5), which contain Ni and Ru cations in square-planar coordination sites. The x = 1 members of each series (LaSrNiRuO4, LaSr3NiRuO6, and LaSr2NiRuO5) exhibit insulating ferromagnetic behavior at low temperature, attributable to exchange couplings between the Ni1+ and Ru2+ centers they contain. Increasing the La3+ concentration (x > 1) leads to a reduction of some of the Ru2+ centers to Ru1+ centers and a suppression of the ferromagnetic state (lower Tc, reduced saturated ferromagnet moment). In contrast, increasing the Sr2+ concentration (x < 1) oxidizes some of the Ru2+ centers to Ru3+ centers and enhances the ferromagnetic coupling (increased Tc, increased saturated ferromagnet moment) for the n = ∞ and n = 2 samples but appears to have no influence on the magnetic ordering temperature of the n = 1 samples. The magnetic couplings and influence of doping are discussed on the basis of superexchange and direct exchange couplings between the square-planar Ni and Ru centers.
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Sep 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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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Abstract: Neutron powder diffraction data indicate that Li2La(TaTi)O7 adopts a polar, a–a–c+/a–a–c+ distorted, n = 2 Ruddlesden–Popper structure described in space group A21am (#36), consistent with a hybrid-improper stabilization of the polar structure via a trilinear coupling between the X3–, X2+, and Γ5– distortion modes. In contrast, Na2La(TaTi)O7 adopts a polar, a–a–c–/a–a–(-c–) distorted, n = 2 Ruddlesden–Popper structure described in space group Pna21 (#33) with the polar Γ3– distortion mode apparently stabilized by a second-order Jahn–Teller distortion of the d0 Ta5+/Ti4+ cations. The change in class of polar distortion of the A2La(TaTi)O7 framework, from hybrid improper (trilinearly coupled) to proper (second-order Jahn–Teller stabilized), on A-cation substitution suggests that the two stabilization mechanisms are in competition in these two materials and many other hybrid-improper ferroelectric phases.
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Mar 2021
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