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89 items found (0 items not approved), displaying 1 to 10.[First/Prev] 1, 2, 3, 4, 5, 6, 7, 8 [Next/Last]

Instruments / Technical Area	Publication Details	Published
I15-Extreme Conditions	Dome-like pressure-temperature phase diagram of the cooperative Jahn–Teller distortion in NaNiO2 Liam A. V. Nagle-Cocco , James M. A. Steele , Shiyu Deng , Xiaotian Zhang , Dominik Daisenberger , Annalena R. Genreith-Schriever , Siddharth S. Saxena , Clare P. Grey , Sian E. Dutton Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/adcb0e Diamond Proposal Number(s): [31718] Open Access Show Abstract ▼ Abstract: NaNiO$_2$ is a Ni$^{3+}$-containing layered material consisting of alternating triangular networks of Ni and Na cations, separated by octahedrally-coordinated O anions. At ambient pressure, it features a collinear Jahn--Teller distortion below $T^\mathrm{JT}_\mathrm{onset}\approx480$\,K, which disappears in a broad first-order transition on heating to $T^\mathrm{JT}_\mathrm{end}\approx500$\,K, corresponding to the increase in symmetry from monoclinic to rhombohedral. It was previously studied by variable-pressure neutron diffraction [ACS Inorganic Chemistry 61.10 (2022): 4312-4321] and found to exhibit an increasing $T^\mathrm{JT}_\mathrm{onset}$ with pressure up to $\sim$5\,GPa. In this work, powdered NaNiO$_2$ was studied \textit{via} variable-pressure synchrotron x-ray diffraction up to pressures of $\sim$67\,GPa at 294\,K and 403\,K. Suppression of the collinear Jahn--Teller ordering is observed \textit{via} the emergence of a high-symmetry rhombohedral phase, with the onset pressure occurring at $\sim$18\,GPa at both studied temperatures. Further, a discontinuous decrease in unit cell volume is observed on transitioning from the monoclinic to the rhombohedral phase. These results taken together suggest that in the vicinity of the transition, application of pressure causes the Jahn--Teller transition temperature, $T^\mathrm{JT}_\mathrm{onset}$, to decrease rapidly. We conclude that the pressure-temperature phase diagram of the cooperative Jahn--Teller distortion in NaNiO$_2$ is dome-like.	Apr 2025
I11-High Resolution Powder Diffraction I15-1-X-ray Pair Distribution Function (XPDF)	Large dynamic scissoring mode displacements coupled to band gap opening in the cubic phase of the methylammonium lead halide perovskites Tobias A. Bird , Junsheng Chen , Manila Songvilay , Christopher Stock , Michael T. Wharmby , Nicholas C. Bristowe , Mark S. Senn Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ad5b44 Diamond Proposal Number(s): [21611, 18786] Open Access Show Abstract ▼ Abstract: Hybrid perovskites are a rapidly growing research area, having reached photovoltaic power conversion efficiencies of over 25 %. There is a increasing consensus that the structures of these materials, and hence their electronic structures, can not be understood purely from the time and space averaged crystal structures observable by conventional methods. We apply a symmetry-motivated analysis method to analyse X-ray pair distribution function data of the cubic phases of the hybrid perovskites MAPb$X_3$ ($X$ = I, Br, Cl). We demonstrate that, even in the cubic phase, the local structure of the inorganic components of MAPb$X_3$ ($X$ = I, Br, Cl), are dominated by scissoring type deformations of the Pb$X_6$ octahedra. We find these modes to have a larger amplitude than equivalent distortions in the $A$-site deficient perovskite ScF$_3$ and demonstrate that they show a significant departure from the harmonic approximation. Calculations performed on an inorganic perovskite analogue, FrPbBr3 show that the large amplitudes of the scissoring modes are coupled to a dynamic opening of the electronic band gap. Finally, we use density functional theory calculations to show that the organic MA cations reorientate to accommodate the large amplitude scissoring modes.	Jun 2024
I11-High Resolution Powder Diffraction	Structural and magnetic studies of the frustrated S = 1 kagome magnet NH4Ni2Mo2O10H3 Eamonn T. Connolly , Jack Wardell , David Boldrin , Chiu Tang , Andrew S. Wills Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ad2aab Diamond Proposal Number(s): [27235] Open Access Show Abstract ▼ Abstract: The strong geometric frustration of the kagome antiferromagnets (KAFMs) can destabilise conventional magnetic order and lead to exotic electronic states, such as the quantum spin-liquid state observed in some S = 1/2 KAFM materials. However, the ground state of S = 1 KAFM systems are less well understood. Spin nematic phases and valence bond solid ground states have been predicted to form but a paucity of experimental realisations restricts understanding. Here, the S = 1 KAFM NH4Ni2Mo2O10H3 is presented, which has the 3-fold symmetry of the kagome lattice but signiﬁcant site depletion, with ∼ 64 % site occupancy. Frustration and a competition between exchange interactions are evidenced through the suppression of order below the Weiss temperature \|θW\| and observation of ferromagnetic and antiferromagnetic characteristics in the magnetisation data. A semi spin glass ground state is predicted based on the ac-field frequency dependence of the magnetic transition and ferromagnetic signal.	Feb 2024
I16-Materials and Magnetism	Resonant x-ray diffraction measurements in charge ordered kagome superconductors KV3Sb5 and RbV3Sb5 Valerio Scagnoli , Lauren J. Riddiford , Shih Wen Huang , You-Guo Shi , Z. J. Tu , Hechang Lei , Alessandro Bombardi , Gareth Nisbet , Zurab Guguchia Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ad20a2 Diamond Proposal Number(s): [31963] Open Access Show Abstract ▼ Abstract: We report on (resonant) x-ray diffraction experiments on the normal state properties of kagome- lattice superconductors KV3Sb5 and RbV3Sb5. We have confirmed previous reports indicating that the charge density wave phase is characterized by a doubling of the unit cell in all three crystallographic directions. By monitoring the temperature dependence of Bragg peaks associated with the charge density wave phase, we ascertained that it develops gradually over several degrees, as opposed to CsV3Sb5, where the charge density wave peak intensity saturates promptly just below the charge density wave transition temperature. Diffraction measurements close to the V K edge and the Sb L1 edge show no sensitivity to inversion- or time-symmetry breaking, which are claimed to be associated with the onset of the charge density wave phase. Resonant diffraction intensity observed at the Sb L1 edge is associated with the anisotropy of the electron density of some of the Sb ions.	Jan 2024
	A theory of the spin-crossover phenomena in the rare earth perovskite LaCoO 3 induced by temperature or a magnetic field K. Katsumata , S. W. Lovesey Journal Of Physics: Condensed Matter 36 DOI: 10.1088/1361-648X/ad10cb Show Abstract ▼ Abstract: Published magnetic data for LaCoO3 are successfully analyzed with coexisting 5D and low-spin (LS) cobalt states. Energy levels of the two states are derived in analytic forms. To this end, fictitious orbital angular momentum l of magnitude one defines the Γ5 (5D) state. Our Hamiltonian includes the spin–orbit interaction, and a cubic crystal field embellished by a trigonal distortion $9B_{2}^{0} (l_{z}^2 - 2/3) - 80B_{4}^{0}(l_{z}^2 - 9/10)$. A singlet ground state with an energy gap to the first excited doublet is realized for certain values of the parameters. The temperature-independent paramagnetic susceptibility (TIPS) of the 5D state has a finite value, which accords with the observation. Whereas, TIPS is symmetry forbidden in the LS state. A rigorous calculation is made of the excitation spectrum in the LS state. The elementary excitation is modeled as a creation of an electron–hole pair that results in an energy level scheme in which the first excited quartet lies above the singlet ground state. The electron spin resonance data are successfully equated with transitions within the excited quartet. Available magnetization data delineate parameters in the 5D Hamiltonian. The temperature dependence of the susceptibility of our coexisting model is qualitatively reasonable. To improve on a quantitative outcome, we are led to introduce a temperature dependent concentration for the 5D and LS states. Calculated Bragg diffraction patterns gathered with x-rays tuned to the Co K-edge reveal potential to refine the current crystal structure and to shed light on the origin of the coexisting states.	Dec 2023
	Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet Roberto Sant , Alessandro De Vita , Vincent Polewczyk , Gian Marco Pierantozzi , Federico Mazzola , Giovanni Vinai , Gerrit Van Der Laan , Giancarlo Panaccione , Nicholas B. Brookes Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ace1bf Open Access Show Abstract ▼ Abstract: Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and X-ray magnetic circular dichroism spectra acquired at the V L2,3 edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the Vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the t22g ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn-Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.	Jun 2023
I21-Resonant Inelastic X-ray Scattering (RIXS)	Resonant inelastic x-ray scattering from U3O8 and UN Eleanor Lawrence Bright , Lei Xu , Lottie Harding , Ross Springell , Andrew Walters , Martin Sundermann , Mirian Garcia-Fernandez , Stefano Agrestini , Roberto Caciuffo , Gerrit Van Der Laan , Gerry H. Lander Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/acb8f4 Diamond Proposal Number(s): [29274] Show Abstract ▼ Abstract: Resonant inelastic x-ray scattering (RIXS) using an incident energy tuned to the uranium N4,5 absorption edges is reported from epitaxial films of α-U3O8 and UN. Theory shows that for U3O8 the multiplets associated with a 5f1 configuration with a ground state of 2F5/2 and the excited state of 2F7/2 are observed. However, the strong transition predicted at a transfer energy of 1.67 eV is not observed. We assume this is a consequence of the intermediate state lifetime broadening due to interaction with continuum states when the transferred energy exceeds the onset of the continuum in the presence of the core hole. This hypothesis is supported by the results obtained for the 5f-itinerant system UN, where no sharp transitions have been observed, although the broad scattering response centered at ∼ 1 eV is considered a signature of a predominantly 5f3 configuration in this band-like semi-metallic system. These experiments and theory add important information on these materials, both of which have been investigated since the 1960s, as well as whether RIXS at the uranium N edge can become a valuable tool for actinide research.	Feb 2023
	Polar magnetism and chemical bond in α-RuCl 3 Stephen W. Lovesey Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/acae12 Open Access Show Abstract ▼ Abstract: The micaceous black allotrope of ruthenium trichloride is the subject of many recent experimental and theoretical studies. Johnson et al. infer a candidate for the magnetic structure (Cc2/m) from results gathered in an extensive set of experiments on an untwined sample of α-RuCl3 [Phys. Rev. B 92, 235119 (2015)]. The proposed zigzag antiferromagnetic ground state of Ru ions does not respond to bulk magnetic probes, with optical rotation and all forms of dichroism prohibited by symmetry. Experimental techniques exploited by Johnson et al. included x-ray and magnetic neutron diffraction. Properties of the candidate magnetic structure not previously explored include polar magnetism. In a general case, anapoles are capable of generating interactions between magnetic ions, as in an electrical Dzyaloshinskii-Moryia interaction [T. A. Kaplan and S. D. Mahanti, Phys. Rev. B 83, 174432 (2011) and H. J. Zhao et al., Nat. Mat. 20, 341 (2021)]. From symmetry-informed calculations we show that, the magnetic candidate permits Bragg spots that arise solely from Dirac multipoles. Stringent tests of Cc2/m can also be accomplished by performing resonant x-ray diffraction with signal enhancement from the chlorine K-edge. X-ray absorption spectra published for α-RuCl3 possess a significant low-energy feature [K. W. Plumb et al., Phys. Rev. B 90, 041112(R) (2014)]. Many experimental studies of other Cl-metal compounds concluded that identical features hallmark the chemical bond. Using a monoclinic Cc2/m structure, we predict the contribution to Bragg diffraction at the Cl K-edge absorption. Specifically, the variation of intensity of Bragg spots with rotation of the sample about the reflection vector. The two principal topics of our studies, polar magnetism and the chemical bond in α-RuCl3, are brought together in a minimal model of magnetic Ru ions in Cc2/m.	Dec 2022
	Ligand-metal bonding effects in resonance enhanced x-ray Bragg diffraction Stephen W. Lovesey , Gerrit Van Der Laan Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ac7e11 Open Access Show Abstract ▼ Abstract: Chlorine covalently bonded to an open shell metal is present in many materials with desirable or intriguing physical properties. Materials include highly luminescent nontoxic alternatives to lead halide perovskites for optoelectronic applications K2CuCl3 and Rb2CuCl3, enantiomorphic CsCuCl3 that presents magneto-chiral dichroism at a low temperature, and cubic K2RuCl6 that possesses a singlet ground state generated by antiparallel spin and orbital angular momenta. Structural chirality of CsCuCl3 has been confirmed by resonant x-ray Bragg diffraction. We explore likely benefits of the technique at the chlorine K-edge using a symmetry inspired method of calculation applied to chlorine multipoles. Already, a low energy feature in corresponding x-ray absorption spectra of many compounds has been related to the chlorine-metal bond. Bragg diffraction from chlorine in cubic K2RuCl6 is treated in detail. Diffraction patterns for rhombohedral compounds that present space-group forbidden Bragg spots are found to be relatively simple.	Jul 2022
	Structure and reactivity of model CeO 2 surfaces David Grinter , Geoff Thornton Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ac5d89 Open Access Show Abstract ▼ Abstract: As a key component in many industrial heterogeneous catalysts, the surface structure and reactivity of ceria, CeO2, has attracted a lot of attention. In this topical review we discuss some of the approaches taken to form a deeper understanding of the surface physics and chemistry of this important and interesting material. In particular, we focus on the preparation of ultrathin ceria films, nanostructures and supported metal nanoparticles. Cutting-edge microscopic and spectroscopic experimental techniques are highlighted which can probe the behaviour of oxygen species and atomic defects on these model surfaces.	Mar 2022

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