B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166, 32893, 14239]
Open Access
Abstract: Recent advances in anion-redox topochemistry have enabled the synthesis of metastable mixed-anion solids. Synthesis of the new transition metal oxychalcogenide Sr2MnO2Na1.6Se2 by topochemical Na intercalation into Sr2MnO2Se2 is reported here. Na intercalation is enabled by the redox activity of [Se2]2– perselenide dimers, where the Se–Se bonds are cleaved and a [Na2–xSe2](2+x)– antifluorite layer is formed. Freshly prepared samples have 16(1) % Na-site vacancies corresponding to a formal oxidation state of Mn of +2.32, a mixed-valence between Mn2+ (d5) and Mn3+ (d4). Samples are highly prone to deintercalation of Na, and over two years, even in an argon glovebox environment, the Na content decreased by 4(1) %, leading to slight oxidation of Mn and a significantly increased long-range ordered moment on the Mn site as measured using neutron powder diffraction. The magnetic structure derived from neutron powder diffraction at 5 K reveals that the compound orders magnetically with ferromagnetic MnO2 sheets coupled antiferromagnetically. The aged sample shows a metamagnetic transition from bulk antiferromagnetic to ferromagnetic behavior in an applied magnetic field of 2 T, in contrast to the Cu analogue, Sr2MnO2Cu1.55Se2, where there is only a hint that such a transition may occur at fields exceeding 7 T. This is presumably due to the higher ionic character of [Na2–xSe2](2+x)– layers compared to [Cu2–xSe2](2+x)– layers, reducing the strength of the antiferromagnetic interactions between MnO2 sheets. Electrochemical Na intercalation into Sr2MnO2Se2 leads to the formation of multiphase sodiated products. The work shows the potential of anion redox to yield novel compounds with intriguing physical properties.
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May 2024
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786, 25166, 32893]
Open Access
Abstract: The Nb2PdxS5 (x ≈ 0.74) superconductor with a Tc of 6.5 K is reduced by the intercalation of lithium in ammonia solution or electrochemically to produce an intercalated phase with expanded lattice parameters. The structure expands by 2% in volume and maintains the C2/m symmetry and rigidity due to the PdS4 units linking the layers. Experimental and computational analysis of the chemically synthesized bulk sample shows that Li occupies triangular prismatic sites between the layers with an occupancy of 0.33(4). This level of intercalation suppresses the superconductivity, with the injection of electrons into the metallic system observed to also reduce the Pauli paramagnetism by ∼40% as the bands are filled to a Fermi level with a lower density of states than in the host material. Deintercalation using iodine partially restores the superconductivity, albeit at a lower Tc of ∼5.5 K and with a smaller volume fraction than in fresh Nb2PdxS5. Electrochemical intercalation reproduces the chemical intercalation product at low Li content (<0.4) and also enables greater reduction, but at higher Li contents (≥0.4) accessed by this route, phase separation occurs with the indication that Li occupies another site.
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Jan 2024
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166, 32893]
Open Access
Abstract: The full characterisation of the high-pressure-synthesised, metastable layered oxyhalide Sr2NiO2Cl2 is reported. It is comprised of infinite NiO2 layers along with double rock salt Sr2Cl2 layers and is closely related to the n = 1 Ruddlesden-Popper phases. At ambient temperature, it crystallises in the tetragonal space group I4/mmm with 2 formula units per unit cell with the basal lattice parameters a = b = 4.03417(2) Å and the stacking direction lattice parameter, c = 15.1058(1) Å. A tiny cusp in the temperature dependence of the magnetic susceptibility in a previous report suggested that this oxyhalide underwent long range magnetic order and therefore contained high-spin Ni2+ ions. Powder neutron diffraction has confirmed that Sr2NiO2Cl2 is a localised-moment oxyhalide, adopting the high-spin S = 1 configuration with fully occupied dxz, dyz and dxy orbitals and partially occupied dz2 and dx2-y2 orbitals. The Ni2+ magnetic moments order antiferromagnetically below ≈180 K in a G-type arrangement on a √2a × √2a × c expansion of the nuclear cell with the propagation vector (½, ½, 0) and with a saturated long range ordered magnetic moment of 1.57(7) μB per Ni2+ cation which is in line with previously calculated computational results and showing that the ligand field around the Ni2+ cation is not sufficiently anisotropic to drive it into the low-spin, diamagnetic configuration found for this d8 cation in square planar or highly elongated octahedral coordination.
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Aug 2023
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166]
Open Access
Abstract: We report the solid-state synthesis of all eighteen layered oxide chalcogenides in the structural family O (A = K/Rb/Cs; M = Ti/V; Q = S/Se/Te), allowing the determination of trends in composition and reactivity within the series. All materials are isostructural, crystallising in the primitive tetragonal space group as reported previously for five compounds in the series. The titanium or vanadium ions have intermediate valency on a single crystallographic site, leading to temperature-independent paramagnetism and correlated electronic behaviour which is influenced by the compositional variation. Furthermore, the alkali metal ions in Image 2 and Image 3 can be removed by oxidative deintercalation using Image 4 at room temperature to produce a new metastable van der Waals layered phase, Image 1. During the deintercalation reaction the oxide chalcogenide layers undergo a relative shift by in the ab plane such that Image 1 is body-centered with space group .
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Aug 2023
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786, 25166]
Open Access
Abstract: A new reduced phase derived from the excitonic insulator candidate Ta2NiSe5 has been synthesized via the intercalation of lithium. LiTa2NiSe5 crystallizes in the orthorhombic space group Pmnb (no. 62) with lattice parameters a = 3.50247(3) Å, b = 13.4053(4) Å, c = 15.7396(2) Å, and Z = 4, with an increase of the unit cell volume by 5.44(1)% compared with Ta2NiSe5. Significant rearrangement of the Ta-Ni-Se layers is observed, in particular a very significant relative displacement of the layers compared to the parent phase, similar to that which occurs under hydrostatic pressure. Neutron powder diffraction experiments and computational analysis confirm that Li occupies a distorted triangular prismatic site formed by Se atoms of adjacent Ta2NiSe5 layers with an average Li–Se bond length of 2.724(2) Å. Li-NMR experiments show a single Li environment at ambient temperature. Intercalation suppresses the distortion to monoclinic symmetry that occurs in Ta2NiSe5 at 328 K and that is believed to be driven by the formation of an excitonic insulating state. Magnetometry data show that the reduced phase has a smaller net diamagnetic susceptibility than Ta2NiSe5 due to the enhancement of the temperature-independent Pauli paramagnetism caused by the increased density of states at the Fermi level evident also from the calculations, consistent with the injection of electrons during intercalation and formation of a metallic phase.
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Jul 2023
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B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Shunsuke
Sasaki
,
Souvik
Giri
,
Simon J.
Cassidy
,
Sunita
Dey
,
Maria
Batuk
,
Daphne
Vandemeulebroucke
,
Giannantonio
Cibin
,
Ronald I.
Smith
,
Philip
Holdship
,
Clare P.
Grey
,
Joke
Hadermann
,
Simon J.
Clarke
Diamond Proposal Number(s):
[25166, 14239]
Open Access
Abstract: Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr2MnO2Cu1.5Ch2 (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu1.5Ch2]2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr2MnO2Ch2 slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.
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May 2023
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I15-1-X-ray Pair Distribution Function (XPDF)
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Samuel W.
Coles
,
Viktoria
Falkowski
,
Harry S.
Geddes
,
Gabriel E.
Pérez
,
Samuel G.
Booth
,
Alexander G.
Squires
,
Conn
O'Rourke
,
Kit
Mccoll
,
Andrew L.
Goodwin
,
Serena A.
Cussen
,
Simon J.
Clarke
,
Saiful
Islam
,
Benjamin J.
Morgan
Diamond Proposal Number(s):
[27702]
Open Access
Abstract: Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material Li2FeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLi4Fe2 oxygen coordination with a preference for polar cis-OLi4Fe2 over non-polar trans-OLi4Fe2 configurations. This preference for polar cation configurations produces long-range disorder, in agreement with experimental data. The predicted short-range-order preference contrasts with that for a simple point-charge model, which instead predicts preferential trans-OLi4Fe2 oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li2FeSO can therefore be attributed to the relative stability of cis-OLi4Fe2 and other non-OLi4Fe2 oxygen-coordination motifs. We show that this effect is associated with the polarisation of oxide and sulfide anions in polar coordination environments, which stabilises these polar short-range cation orderings. We propose similar anion-polarisation–directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis also illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, where other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.
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Apr 2023
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786, 25166]
Open Access
Abstract: Two new phases derived from the layered ternary chalcogenide Ta2PdS6 have been successfully synthesised via the intercalation of sodium or potassium. MTa2PdS6 (M = Na, K) crystallise in monoclinic space group I2/m (No. 12), Z = 2, with lattice parameters a = 8.1366(7) Å, b = 3.2876(1) Å, c = 15.3279(3) Å, β = 97.407(2)° for NaTa2PdS6, and a = 8.3845(3) Å, b = 3.2955(1) Å, c = 16.7416(8) Å, β = 86.339(3)° in the case of KTa2PdS6. The general structure of the parent phase is retained upon intercalation, with increased unit cell volumes of 17.3% and 33.2% for the Na and K intercalates respectively. SQUID magnetometry shows that both intercalates are net diamagnets, but shows a significant reduction of magnetic susceptibility from Ta2PdS6 due to the increase in a Pauli paramagnetic component due to partial band filling.
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Mar 2023
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I11-High Resolution Powder Diffraction
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Robert D.
Smyth
,
Jack N.
Blandy
,
Ziyu
Yu
,
Shuai
Liu
,
Craig V.
Topping
,
Simon J.
Cassidy
,
Catherine F.
Smura
,
Daniel N.
Woodruff
,
Pascal
Manuel
,
Craig L.
Bull
,
Nicholas P.
Funnell
,
Christopher J.
Ridley
,
John E.
Mcgrady
,
Simon J.
Clarke
Diamond Proposal Number(s):
[13284, 18786, 25166]
Open Access
Abstract: Sr2NiO2Cu2Se2, comprising alternating [Sr2NiO2]2+ and [Cu2Se2]2– layers, is reported. Powder neutron diffraction shows that the Ni2+ ions, which are in a highly elongated NiO4Se2 environment with D4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × √2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μB per Ni2+ ion. The adoption of the high-spin configuration for this d8 cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr2NiO2Cu2S2, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni2+, and accordingly, there is no evidence for magnetic moment on the Ni2+ ions. Examination of the solid solution Sr2NiO2Cu2(Se1–xSx)2 shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr2NiO2Cu2Se2 up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni2+ coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr2NiO2Cu2Se2 and Sr2NiO2Cu2S2, suggest that simple high-spin and low-spin models for Ni2+ may not be entirely appropriate and point to further complexities in these compounds.
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Oct 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786, 25166]
Open Access
Abstract: Two novel chromium oxide arsenide materials have been synthesized, Sr2CrO2Cr2OAs2 (i.e., Sr2Cr3As2O3) and Sr2CrO3CrAs (i.e., Sr2Cr2AsO3), both of which contain chromium ions in two distinct layers. Sr2CrO2Cr2OAs2 was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO4As2 octahedra containing Cr2+ and distorted CrO2As4 octahedra containing Cr3+. In contrast, Sr2CrO3CrAs incorporates Cr3+ in CrO5 square-pyramidal coordination in [Sr2CrO3]+ layers and Cr2+ ions in CrAs4 tetrahedra in [CrAs]− layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr2CrO3CrAs the Cr2+ moments in the [CrAs]− layers exhibit long-range ordering, while the Cr3+ moments in the [Sr2CrO3]+ layers only exhibit short-range ordering. However, in Sr2CrO2Cr2OAs2, both the Cr2+ moments in the CrO4As2 environments and the Cr3+ moments in the CrO2As4 polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr3+ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr2CrO2Cr2OAs2 below the magnetic ordering transitions.
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Jul 2022
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