Multilayers of (Co3 Å, Pt15 Å)x, x=15 or 30 repeats, with or without a 200 Å silver buffer layer, were grown on GaAs (111) substrates by molecular-beam epitaxy. Vibrating sample magnetometry measurements confirmed that the samples with the Ag buffer layer show strong uniaxial magnetic anisotropy perpendicular to the surface. The perpendicular anisotropy exhibited by these metallic superlattices is discussed in terms of the microstructure of the overall multilayer stack, as well as the structural characteristics of the Co interface layer. Samples grown on the Ag buffer layer show strong (111) texture with 30–40-nm-size twin-related grains. These grains, correspond to the two possible (111) stacking sequence for an fcc lattice, i.e., double positioning. However, direct growth on GaAs (111) results in randomly oriented 10–20 nm grains. All samples exhibit a repeat period of 1.83 nm in both low-angle reflectivity and high-angle Θ–2Θ x-ray scattering measurements. In addition, transverse scans through the low-angle multilayer Bragg peaks show the interfaces to be diffuse in nature indicative of considerable in-plane inhomogeneity and/or compound formation. High-resolution electron microscopy measurements of cross sections compared with image simulations confirm that the interface layer is diffuse and its stoichiometry is such that the Co occupation is less than 40%. Redistribution of Co should then extend over at least four monolayers. The nanostructure of the samples grown with the Ag buffer layer comprises an eight atomic layer repeat with the Co interface layer diffuse over four monolayers. The microstructure is strongly (111) textured with columns of twin related 30-nm-sized grains. It is suggested that the combination of interdiffusion, highly oriented but twin-related columnar growth, small grain size with a possible nanometer-scale second phase may be the key to the understanding of the perpendicular anisotropy observed in these (111) superlattices.

The structure of the electrochemical double layer at the interface between Ag(hkl) electrodes and 0.1 M NaOH electrolyte has been probed using in situ surface X-ray diffraction (SXRD) measurements. The three low-index Ag(hkl) surfaces were prepared and characterized under ultra-high vacuum (UHV) conditions before being transferred into the electrochemical environment. Crystal truncation rod (CTR) measurements were made at negative potentials (just negative of the pzc), where there is no specific adsorption onto the electrode surfaces, and at more positive potentials, where hydroxide species is specifically adsorbed. The measurements quantify the relaxation at the metal electrode surfaces and, through the specular CTR, give information about layering on the electrolyte side of the interface. Strong layering effects are observed on the Ag(111) and Ag(110) surfaces, whereas on Ag(001) the layering is much weaker and we attribute this to the symmetry mismatch to the adsorbing water network. The effect of saturating the electrolyte with carbon monoxide (CO) was also examined, and structural changes were only observed on the Ag(110) surface. The changes observed suggest a stabilization of a coadsorbed CO-OH adlayer due to charge redistribution that delays the onset of CO oxidation.

The presence of specifically adsorbed anions can significantly affect the electrochemical reactivity of a metal electrode which is of major interest for galvanic deposition, etching, corrosion and electrocatalysis. In-situ surface x-ray diffraction has enabled an atomic/molecular-level understanding of the interface under reactive conditions, including its potential and time dependence, to be developed. While information about the atomic structure of the electrode surface in electrochemical in-situ cells has been widely investigated, insight into the charge distribution and the structure of the electrolyte at the interface is still lacking. Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships. A fundamental understanding of the nature of the charge transfer, especially the influence of the applied potential and the screening by the electrolyte, is a major goal in electrochemistry to better understand electrochemical processes and charge transfer during adsorption and deposition. [1] Thus, combining x-ray spectroscopy and x-ray diffraction to gain site specific information about the charge distribution at buried interfaces is a promising tool. [2,3] Examples of how the use of surface x-ray scattering techniques can help to characterize electrochemical interfaces in-situ in order to link, structure, reactivity and stability will be presented. [4-5] Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships and to understand growth kinetics and electrochemical phase formation. References: [1] Y. Gründer and C. A. Lucas, Nano Energy 29, 378 (2016). [2] Y. Gründer, P. Thompson, A. Brownrigg, M. Darlington, and C. A. Lucas, Journal of Physical Chemistry C 116, 6283 (2012). [3] Y. Joly et al., Journal of Chemical Theory and Computation 14, 973 (2018). [4] Y. Grunder et al., Charge Reorganization at the Adsorbate Covered Electrode Surface Probed through in Situ Resonant X-ray Diffraction Combined with ab Initio Modeling; Phys. Chem. C 2022, 126, 9, 4612–4619 [5] Yvonne Soldo-Olivier et al., Unraveling the Charge Distribution at the Metal-Electrolyte Interface Coupling in Situ Surface Resonant X-Ray Diffraction with Ab Initio Calculations, ACS Catal. 2022, 12, 4, 2375–2380

The dynamics of the structural changes in the electrochemical double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte have been probed using surface X-ray diffraction measurements. The X-ray measurements utilised a lock-in amplifier technique to obtain a time resolution down to the millisecond scale. Two potential step regions were explored in an attempt to separate the dynamics of the reversible adsorption/desorption of hydroxide species (OHad) and the subsequent cation (K+) ordering in the double layer. By probing different positions in reciprocal space, sensitive to different structural changes, the time-dependent response of the electrode surface was probed and time constants for the different associated processes were obtained.

Understanding the charge distribution and bonding mechanism at the polarized solid-liquid interface is a fundamental challenge in electrochemistry which impacts applications ranging from materials processing to renewable energy production. The applied potential, ions in solution, and the polarization of the interface all combine to alter the bond formation and the interfacial charge distribution; thus techniques must be developed to provide in situ characterization of the interface. Here we present a combination of in situ resonant surface X-ray diffraction studies and self-consistent DFT calculations to assess the charge distribution and bonding mechanism for the adsorption of bromide anions onto a single crystal Cu(001) electrode surface. A comparison between the experimental and modeled data gives detailed information about the charge distribution at the interface and the bonding of specific adsorbates, predicting a charge rearrangement rather than charging of the atoms involved and a surface dipole moment situated at the metal surface.

Electrical switching of ferroelectric domains and subsequent domain wall motion promotes strong piezoelectric activity, however, light scatters at refractive index discontinuities such as those found at domain wall boundaries. Thus, simultaneously achieving large piezoelectric effect and high optical transmissivity is generally deemed infeasible. Here, it is demonstrated that the ferroelectric domains in perovskite Pb(In1/2 Nb1/2 )O3 -Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 domain-engineered crystals can be manipulated by electrical field and mechanical stress to reversibly and repeatably, with small hysteresis, transform the opaque polydomain structure into a highly transparent monodomain state. This control of optical properties can be achieved at very low electric fields (less than 1.5 kV cm-1 ) and is accompanied by a large (>10 000 pm V-1 ) piezoelectric coefficient that is superior to linear state-of-the-art materials by a factor of three or more. The coexistence of tunable optical transmissivity and high piezoelectricity paves the way for a new class of photonic devices.

Combining and coupling both magnetic and electric properties in one single phase multiferroic material has attracted high interest recently to enable a broad range of novel devices and applications. To evaluate one potential route toward new multiferroics, we have studied 0.5% Fe-doped BaTiO3 single crystals and measured the ferroelectric, magnetic, and multiferroic properties. X-ray absorption spectroscopy shows the presence of Fe3+, and magnetic measurements confirmed that this has a significant impact on the magnetic properties. Doping of iron introduces paramagnetism from lone iron atoms as well as what appears to be a weak ferromagnetism. Multiferroicity and magnetoelectric (ME) coupling were observed in the polarization-electric field hysteresis loops with an applied magnetic field, yet there was no direct evidence that ME coupling persists when the sample was in the defect dipole-aligned state.

The atomic structure on the metal side of the electrochemical interface depends on the applied electric potential and the nature of the adsorbing species in the electrolyte solution. In this short article, we review some recent results probing surface stress and surface relaxation effects in single-crystal metal electrodes that are driven by potential changes. Both the potential and the structure in the electrolyte layers at the interface alter the metal electronic structure so that the surface in the electrochemical environment is strongly modified from the ultra-high vacuum counterpart. A methodology for linking experimental and theoretical approaches for a fundamental understanding of electrochemical reactions is proposed.

In this work, we present a grazing incidence X-ray diffraction study of the surface of a 0.24Pb(In1/2Nb1/2)O-3-Pb(Mg1/3Nb2/3)O-3-PbTiO3(PIN-PMN-PT) [011] poled rhombohedral single crystal. The near surface microstructure (the top several tens to hundreds of unit cells) was measured in situ under an applied electric field. The strains calculated from the change in lattice parameters have been compared to the macroscopic strain measured with a strain gauge affixed to the sample surface. The depth dependence of the electrostrain at the crystal surface was investigated as a function of temperature. The analysis revealed hidden sweet spots featuring unusually high strains that were observed as a function of depth, temperature and orientation of the lattice planes.

In this work, we demonstrate that xPb(In1/2Nb1/2)O3-(1-x-y)Pb(Mg1/3Nb2/3)O3-yPbTiO3 [110]-poled domain-engineered relaxor single crystals can be dynamically and reversibly driven through a ferroelectric–ferroelectric phase transition exhibiting a highly enhanced piezoelectric response in a wide range of frequencies. Realization of this phase switching requires an applied compressive stress close to the critical values for the inter-ferroelectric phase transition, which can then be induced by a relatively small electric field (≤0.2 kV/mm). The required critical stress was established by in situ stress and x-ray diffraction measurements. The effective d32 coefficient measured dynamically up to 70 Hz was shown to be consistently twice that of the linear piezoelectric mode measured below the phase transformation region. The crystal was installed into a prototype transducer based on a Tonpilz configuration. The performance of the transducer was tested in water and showed up to 15 dBSPL higher acoustic power radiated when the crystal was driven through the phase transition than when operating in the linear piezoelectric regime.

The presence of specifically adsorbed anions can significally affect the electrochemical reactivity of a metal electrode which is of major interest for galvanic deposition, etching, corrosion and electrocatalysis. Halogens on metal surfaces are prototypic adsorbate systems but the adsorption of halide ions especially on copper is also of major importance for on-chip metallization in ULSI microchip production. Halide ions on Cu surfaces form an inhibiting adsorbate layer with polyethylene glycol (PEG). Even though the influence of the additives combination on the shape evolution of the Cu deposit was subject of numerous studies, their precise role during the elementary steps of the deposition with regards to altering the charge distribution and dipole moment at the interface is largely not understood. To obtain insights into the influence of the applied potential and the screening by the electrolyte to better understand electrochemical processes and charge transfer during adsorption and deposition is a major goal .[1] Thus combining x-ray spectroscopy and x-ray diffraction to gain site-specific information about the charge distribution at buried interfaces is a promising tool. [2,3] We will present studies on the metal-halide interface and how the use of surface x-ray scattering techniques can help to characterise electrochemical interfaces in-situ in order to link, structure and stability. [3] Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships which will help to understand growth kinetics. References: [1] Y. Gründer, P. Thompson, A. Brownrigg, M. Darlington, and C. A. Lucas, Journal of Physical Chemistry C, 2012,116, 6283 [2] Y. Gründer and C. A. Lucas, Physical Chemistry Chemical Physics, 2017, 19, 8416 [3] Y. Joly et al., J. Chem. Theory Comput,. 2018, 14, 973−980

This paper describes the motivation for the design and construction of a beamline at the European Synchrotron Radiation Facility (ESRF) for the use of UK material scientists. Although originally focused on the study of magnetic materials, the beamline has been running for 20 years and currently supports a very broad range of science as evidenced by the research topics highlighted in this article. We describe how the beamline will adapt to align with the ESRF's upgrade to a diffraction limited storage ring. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

In situ electrochemical surface X-ray diffraction was employed to investigate the atomic scale structure of the electrochemical double layer and the relaxation at the Pt(111) electrode surface in non-aqueous and aqueous acetonitrile electrolytes under potential control. The X-ray measurements provide insight into the potential-dependence of the interface structure by combining potentiodynamic measurements (X-ray voltammetry) with potentiostatic measurements (crystal truncation rod data) to probe both the metal and electrolyte sides of the interface. The crystal truncation rod measurements are consistent with the potential dependent reorientation of acetonitrile in the absence of water and a parallel arrangement in the presence of water. As acetonitrile concentration increases, the electron density closest to the electrode surface also increases. Finally, Pt surface relaxation in a range of aqueous and non-aqueous solvents is discussed in general with regards to the structure of the electrochemical double layer.

The surfaces of gold exhibit a rich physical behaviour that is interesting not only from a structural perspective but also for applications in areas such as heterogeneous catalysis and electrocatalysis. In this paper we show that the hexagonal reconstructions of both the Au(111) and the cubic Au(001) surfaces in alkaline electrolyte exhibit a potential-dependent in-plane compression that is remarkably similar despite the substantial difference in the geometry of the underlying substrate. The compressibility is linked to the charge on the surface Au atoms within a simple free electron model. The interplay between surface charge and the adsorption of hydroxide species determines both the surface compression and the reversible lifting of the reconstructions. In the presence of adsorbed carbon monoxide both the potential-induced changes in the surface compression and the lifting of the reconstruction are suppressed leading to the promotion of electrocatalytic reactivity.

The XMaS bending magnet beamline at the ESRF has been in regular user operation since the autumn of 1998 and has employed a very simple optical system consisting of a Si <111> monochromator and a toroidal mirror. The ESRF extremely brilliant source (EBS) upgrade program presents the bending magnet beamlines with a series of challenges and exciting new opportunities to extend the range of science performed, with emphasis on in-operando and in-situ studies. Geometrically, the new EBS lattice will move the source for bending magnet beamlines some 3 meters upstream and XMaS will use a newly designed 0.86 Tesla short bend, instead of the present 0.4 Tesla bending magnet. The higher field of the new source increases the available flux at high energies (>25 keV) by an order of magnitude and will result in a smaller brighter beam. To exploit the extended energy range, a dual toroidal mirror system, coated with chromium and platinum, will provide the focusing optics and enable continuous operations from 2.035 keV to 33 keV which will be coupled to a fast scanning LN2 cooled, constant offset monochromator. We report here on the opportunities presented by the new machine lattice and the solutions chosen to deliver a state of the art beamline that utilizes a very wide range of x-ray techniques including scattering and spectroscopy from a broad spectrum of materials characterization.

Copper and copper oxide electrode surfaces are suitable for the electrochemical reduction of CO2 and produce a range of products, with the product selectivity being strongly influenced by the surface structure of the copper electrode. In this paper, we present in-situ surface X-ray diffraction studies on Cu(111) electrodes in neutral phosphate buffered electrolyte solution. The underlying mechanism of the phosphate adsorption and deprotonation of the (di)-hydrogen phosphate is accompanied by a roughening of the copper surface. A change in morphology of the copper surface induced by a roughening process caused by the formation of a mixed copper–oxygen layer could also be observed. The stability of the Cu(111) surface and the change of morphology upon potential cycling strongly depends on the preparation method and history of the electrode. The presence of copper islands on the surface of the Cu(111) electrode leads to irreversible changes in surface morphology via a 3D Cu growth mechanism.

The development of new functional materials is transforming many technological areas of great interest from electronics and catalysis to detection and bio-sensing. Surface alloying and self-organization phenomena, driven by the surface stress and elastic interactions in metals, are key to these developments. While much has been done theoretically and explored experimentally in ultra-high vacuum (UHV), very little has been exploited in practical applications. Electrochemistry is the ideal science to exploit the physics and chemistry of such functional materials. Alloying between two components in general is driven by negative enthalpy of mixing and the lowering of surface free energy. However, when confined to a few surface layers, it can be observed even in systems that do not form alloys in the bulk phase (so called immiscible systems). UHV studies at elevated temperatures have found that the formation of immiscible surface alloys is a rather common phenomenon in systems with large lattice mismatch and that it is driven by surface stress relaxation and elastic energy. In this talk in-situ surface x-ray scattering studies of alloy formation and alloy surfaces will be presented. This will include alloys prepared by UHV methods, in particular both bulk and surface Pt 3 Sn(111) alloys, and alloys prepared directly in the electrochemical environment, Pb-Au(111). For the Pt 3 Sn system, of interest in electrocatalysis due to its tolerance towards CO poisoning, the surface has been studied under UHV conditions and during the adsorption of water and the results are compared to measurements of bulk Pt 3 Sn alloys in 0.5 M H 2 SO 4 electrolyte under potential control. For the Pb-Au(111) system, resonant x-ray scattering methods are used to identify the onset of surface alloying and correlate the changes in surface stress with the out-of-plane atomic structure.

An unsolved and crucial question in electrochemistry is the issue of charge transfer between the metal electrode and the adsorbate as this defines the nature of the bonding. Although there have been several theoretical studies of the charge transfer mechanism, initially in ideal UHV conditions but recently with extension to the electrochemical interface, there have been few experimental electrochemical studies due to the complexity of the electrochemical environment which is inaccessible to most surface probes of charge transfer. A fundamental understanding of the nature of the charge transfer, especially the influence of the applied potential and the screening by the electrolyte, is a major goal in electrochemistry. However, in-situ studies of the chemical bonding are rather difficult due to the presence of the electrolyte as standard characterisation techniques which are mostly UHV based, cannot be applied. We performed in-situ resonant scattering experiments for the halide Cu(001) and halide Cuupd-Au(001) system to investigate the suitability of this technique to probe the charge transfer and nature of the chemical bonding in-situ for such systems and to give detailed information of the atomic subsurface charge at the interface. Halide adsorption onto Cu(001) electrodes is an ideal system [1] in which to attempt this measurement, as the halides, Br and Cl, both form simple ordered c(2x2) adlayers on the Cu(001) surface. For both Cl and Br adsorption onto Cu(001) the adsorbates form c(2x2) adlayers, which are simple square structures with a coverage of 0.5 halide atoms per surface Cu atom and adsorption uniquely into the 4-fold Cu hollow site. The c(2x2) cell gives rise to additional surface scattering that is independent from the Cu crystal truncations rods (CTR’s). In the experiment it is therefore possible to probe the surface Cu atoms, by performing resonant measurements at the ‘anti-Bragg’ positions of the Cu and at the c(2x2) superstructure positions. A shift in the adsorption edge can be seen indicative of a change in the charge state of the surface Cu atoms. More detailed information can be obtained by modelling the experimental data through single atomic charges. Similar experiments were carried out for the Br adsorption on Au(001) and Cuupd halide Au(001) system [2] for which a difference in bonding, compared to the halide Cu(001) system is expected. This is the first direct experimental proof of partial charge transfer and subsurface charge distribution at the electrochemical interface. These measurements show that resonant x-ray scattering is an adequate tool to investigate the charge transfer at an electrochemical interface and will shed light on the nature of the chemical bonding at the interface and the role of the double layer. References [1] Y. Gründer, D. Kaminski, F.Golks ,K. Krug, J. Stettner, O.M. Magnussen et al., PhysRevB. 81 174114 (2010) [2] Y. Gründer, P. Thompson, A. Brownrigg, M. Darlington, C.A. Lucas CA. , J. Phys. Chem. C. 116, 6283 (2012)

We have studied the adsorption of water on a Cl covered Cu(100) surface using both low energy electron diffraction (LEED) experiments and density functional theory (DFT) calculations. On the Cu{100}-c(2×2)-Cl surface water is shown to form a bilayer, which is weakly bound to the surface.

The electrode/electrolyte interface is central to many electrochemical systems; however, gaining insight into the electronic structure at the interface is challenging. Due to its buried nature it is difficult to employ traditional techniques that provide spectroscopic information of localised atoms. To gain new insight into the charge distribution at the interface, we used resonant surface X-ray diffraction to select atoms at the interface via the diffraction conditions and obtained spectroscopic information simultaneously. Coupling the polarisation of the incident X-ray beam with the electron density at the interface allows direct probing of the charge transfer between the metal electrode and the adsorbing species in the electrolyte solution. Results for the adsorption of halide anions onto Cu and Au single crystal electrode surfaces reveal that there is significant modification of the charge distribution of both the surface and sub-surface atomic metal adlayers in the case of ionic bond formation. This has potential impact both in developing a theoretical understanding of the interface structure and in designing new materials for electrochemical applications.