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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I05-ARPES
I11-High Resolution Powder Diffraction
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J.-R.
Soh
,
F.
De Juan
,
M. G.
Vergniory
,
N. B. M.
Schröter
,
M. C.
Rahn
,
D. Y.
Yan
,
J.
Jiang
,
M.
Bristow
,
P. A.
Reiss
,
J. N.
Blandy
,
Y. F.
Guo
,
Y. G.
Shi
,
T. K.
Kim
,
A.
Mccollam
,
S. H.
Simon
,
Y.
Chen
,
A. I.
Coldea
,
A. T.
Boothroyd
Diamond Proposal Number(s):
[19234, 18786]
Abstract: We report theoretical and experimental evidence that
EuCd
2
As
2
in magnetic fields greater than 1.6 T applied along the
c
axis is a Weyl semimetal with a single pair of Weyl nodes. Ab initio electronic structure calculations, verified at zero field by angle-resolved photoemission spectra, predict Weyl nodes with wave vectors
k
=
(
0
,
0
,
±
0.03
)
×
2
π
/
c
at the Fermi level when the Eu spins are fully aligned along the
c
axis. Shubnikov–de Haas oscillations measured in fields parallel to
c
reveal a cyclotron effective mass of
m
∗
c
=
0.08
m
e
and a Fermi surface of extremal area
A
ext
=
0.24
nm
−
2
, corresponding to 0.1% of the area of the Brillouin zone. The small values of
m
∗
c
and
A
ext
are consistent with quasiparticles near a Weyl node. The identification of
EuCd
2
As
2
as a model Weyl semimetal opens the door to fundamental tests of Weyl physics.
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Nov 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284, 18786]
Open Access
Abstract: The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically.
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Jun 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Open Access
Abstract: The structures of two new oxide chalcogenide phases, Sr2CuO2Cu2S2 and Sr2CuO2Cu2Se2, are reported, both of which contain infinite CuO2 planes containing Cu2+ and which have Cu+ ions in the sulfide or selenide layers. Powder neutron diffraction measurements show that Sr2CuO2Cu2Se2 exhibits long-range magnetic ordering with a magnetic structure based on antiferromagnetic interactions between nearest-neighbor Cu2+ ions, leading to a √2a × √2a × 2c expansion of the nuclear cell. The ordered moment of 0.39(6) μB on the Cu2+ ions at 1.7 K is consistent with the value predicted by density functional theory calculations. The compounds are structurally related to the cuprate superconductors and may also be considered as analogues of the parent phases of this class of superconductor such as Sr2CuO2Cl2 or La2CuO4. In the present case, however, the top of the chalcogenide-based valence band is very close to the vacant Cu2+ 3d states of the conduction band, leading to relatively high measured conductivity.
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Nov 2018
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I11-High Resolution Powder Diffraction
I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[18324, 13639]
Abstract: For the collidine analogues of Barluenga's Reagent (IPy2BF4) reported, a flat cation is necessary for the generation of a modulated phase, in keeping with the ``Ratchet Model'' theory [Kim et al., Crystal Growth & Design, 2014, 14, 6294]. Attempts to study "diffuse modulation" in Br(Coll)2ClO4 have shown that these non-Bragg features disappear very rapidly on exposure to synchrotron radiation, an effect thought to be caused by the radiation damage disrupting the lattice vibrations that cause the modulation.
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Aug 2018
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Abstract: The antiferromagnetic structures of Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2 are solved using powder neutron diffraction. Both compounds adopt the same magnetic structure, based on a √2a × √2a × c expansion of the nuclear cell with magnetic space group PC42/n (86.72 in the Belov-Neronova-Smirnova notation). This structure is adopted as a result of nearest-neighbour antiferromagnetic interactions within the CoO2 planes. The refined long-range-ordered magnetic moments of Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2 are 3.7(1) and 3.97(3) μB per Co ion respectively. The refined moments are significantly greater than the value predicted from just considering the spin (3 μB); this is attributed to a significant orbital contribution to the magnetic moment in an analogous manner to that previously observed for Sr2CoO2Cu2S2 and the values conform to a relationship between the shape of the distended CoO4Ch2 (Ch = S, Se) octahedron and the size of the ordered moment established for a series of related compounds.
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May 2018
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I11-High Resolution Powder Diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
I19-Small Molecule Single Crystal Diffraction
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Open Access
Abstract: The structure, magnetic behaviour and chemistry of layered oxychalcogenides of composition A2MO2X2Ch2 (where A = Sr, Ba; M = Mn, Co, Ni, Cu, Zn; X = Cu, Ag and Ch = S, Se, Te) has been investigated by the synthesis of new compounds of this type and control of the properties of these compounds by oxidative deintercalation of Cu/Ag.
I2 can be used to oxidatively deintercalate Cu from Sr2MnO2Cu1.5S2, forming Sr2MnO2Cu1.33S2, an incommensurately modulated compound, with a completely different Cu/vacancy ordering and antiferromagnetic ordering structure to the parent. This reaction is also probed in real-time, using in situ powder X-ray diffraction.
Sr2MnO2Ag1.5Se2 was found to have an A-type magnetic ordering structure, similar to Sr2MnO2Cu1.5Se2. Sr2MnO2Cu1.8Te2 on the other hand with a lower Mn oxidation state shows only two-dimensional magnetic correlations, rather than long-range order.
Extending the reaction with I2 to several Co-containing analogues revealed that ∼ 25% Ag could be removed from Sr2CoO2Ag2Se2, sufficient to observe a change in magnetic behaviour, from antiferromagnetic to ferromagnetic. By contrast only ∼11% Cu can be deintercalated from Sr2CoO2Cu2S2 and even less (∼5%) from Sr2CoO2Cu2Se2. Neutron diffraction was used to examine the resultant changes in magnetic ordering.
The novel compounds Sr2CuO2Cu2Se2 and Ba2CuO2-xCu2Se2 are related by substitution of the alkali-earth metal, but while Sr2CuO2Cu2Se2 is a stoichiometric compound with metal-like character, Ba2CuO2-xCu2Se2 is an oxygen-deficient semiconductor, with tuneable oxygen content.
Unusual features are observed in the magnetic susceptibility measurements of Sr2NiO2Cu2Se2 that appear unrelated to this compound's long-range magnetic ordering, as probed by neutron diffraction. Furthermore, unusual peak splitting is observed in low-temperature powder X-ray diffraction patterns of this compound; this may plausibly be due to a photon-induced effect arising from the use of a high-energy beamline; although further measurements are required to examine this.
Overall the work shows the flexibility and range of behaviour exhibited by a series of the transition metal oxide chalcogenides.
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Sep 2017
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Open Access
Abstract: The synthesis of a high-purity sample of the layered oxide selenide Sr2MnO2Ag1.5Se2 is reported. At ambient temperature it crystallises in the space group I4/mmm with two formula units in the unit cell and lattice parameters a=4.08771(1) Å, c=19.13087(8) Å. The compound displays mixed-valent manganese in a formal oxidation state close to +2.5 and powder neutron diffraction measurements reveal that below the Néel temperature of 63(1) K this results in an antiferromagnetic structure which may be described as A-type, modelled in the magnetic space group PI4/mnc (128.410 in the Belov, Neronova and Smirnova (BNS) scheme) in which localised Mn moments of 3.99(2) μB are arranged in ferromagnetic layers which are coupled antiferromagnetically. In contrast to the isostructural compound Sr2MnO2Cu1.5S2, Sr2MnO2Ag1.5Se2 does not display long range ordering of coinage metal ions and vacancies, nor may significant amounts of the coinage metal readily be deintercalated using soft chemical methods.
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Nov 2016
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I11-High Resolution Powder Diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[12763, 13284, 13639]
Open Access
Abstract: Lithiation of hydrothermally synthesized Li1–xFex(OH)Fe1–ySe turns on high-temperature superconductivity when iron ions are displaced from the hydroxide layers by reductive lithiation to fill the vacancies in the iron selenide layers. Further lithiation results in reductive iron extrusion from the hydroxide layers, which turns off superconductivity again as the stoichiometric composition Li(OH)FeSe is approached. The results demonstrate the twin requirements of stoichiometric FeSe layers and reduction of Fe below the +2 oxidation state as found in several iron selenide superconductors.
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Sep 2016
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Abstract: CaCoSO, synthesized from CaO, Co, and S at 900 °C, is isostructural with CaZnSO and CaFeSO. The structure is non-centrosymmetric by virtue of the arrangement of the vertex-sharing CoS3O tetrahedra which are linked by their sulfide vertices to form layers. The crystal structure adopts space group P63mc (No. 186), and the lattice parameters are a = 3.7524(9) Å and c = 11.138(3) Å at room temperature with two formula units in the unit cell. The compound is highly insulating, and powder neutron diffraction measurements reveal long-range antiferromagnetic order with a propagation vector k = (1/3, 1/3, 1/2). The magnetic scattering from a powder sample can be modeled starting from a 120° arrangement of Co2+ spin vectors in the triangular planes and then applying a canting out of the planes which can be modeled in the magnetic space group Ccc (space group 9.40 in the Belov, Neronova, and Smirnova (BNS) scheme) with Co2+ moments of 2.72(5) μB. The antiferromagnetic structure of the recently reported compound BaCoSO, which has a very different crystal structure from CaCoSO, is also described, and this magnetic structure and the magnitude of the ordered moment (2.75(2) μB) are found by experiment to be similar to those predicted computationally.
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Feb 2016
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