Notice of CRISTAL beamline

This document describes the "Crystallography and Condensed Matter" beamline project, initiated by the "Association Française de Cristallographie" (AFC) and gathering a large French diffraction community at the Physic & Chemistry interface.

X-ray diffraction is one of the most efficient methods to obtain structural information on condensed matter. Thanks to the highly brilliant third-generation X-ray sources difficult issues can now be addressed: structure determination of incommensurate phases and quasicrystals, of materials under extreme conditions or out-of-equilibrium, measurement of electron density in the ground or excited states with very high accuracy. Moreover, the structure determination of large unit cell crystals from powders can be performed thus giving large opportunities to industrial applications as for instance in drug industry.
The high flux of the beam allows one to measure very weak but information-rich intensities, like diffuse scattering.
By coupling spectrometric measurements to crystallographic ones a valuable insight on the local environment of specific atoms can be derived.

Finally, the high performance SOLEIL source will provide an x-ray beam laterally and longitudinally coherent on a micrometer size, which opens up the possibility of measuring scattering without configurational average. This new method gives information on nano-objects and slow (>1ms) internal fluctuations at the microscopic and mesoscopic scales.

The philosophy of this project is thus to provide the French community with a consistent state-of-the-art X-ray diffraction techniques platform: standard single-crystals and powders methods with state-of-the art instruments and analysis tools on one hand, research activities using the specific features of third generation x-ray sources (brilliance and coherence) on the other hand. Because of the versatility of the beamline with three different dedicated instruments, the "Crystallography and Condensed Matter" project was conceived from the very beginning as a rather large partnership between about 10 French laboratories:

Specific technical parts of this beamline Project (optics, instruments, sample environments, …) will be conducted by the following laboratories in view of their particular expertise

• LCM3B will be involved in the 4-circles diffractometer development (S. Dahaoui, P. Fertey, S. Pillet, C. Lecomte)
• LPS will be involved in the N-circles conception, (D. Dallé, P. Launois, D. Le Bolloc’h, R. Moret, S. Ravy)
• CRISMAT and SPMS in the powder diffractometer conception (D. Grébille, G. Calvarin, G. Baldinozzi)
• LdC in the detector development: 2D pixel detector and multi-analyzer (J.-F. Bérar, J.-L. Hodeau)
• Le LURE in the powder instrument and the optics (E. Elkaïm, S. Rouzière)
• Le GMCM will be involved in the ultra-fast time-resolved project (E. Collet, M. Buron, M.-H. Lemée-Cailleau).

It must be stressed that the sine qua non condition for high precision diffraction data collection is the stability and the homogeneity of the beam. This stability is also mandatory for studies exploiting the time-resolved structure and the coherence of the beam. This can only be achieved with an undulator source allowing a "compact" optics and high enough flux even without ultimate focusing.

This project has been thought to be complementary to the beamlines already accessible to the French community at ESRF : CRG D2AM and IF, or planned at SOLEIL: Diff-Abs (H10 from LURE) and more specialized beamline projects in preparation.

The diffraction data should allow to reach the electron density of the crystal giving access to the chemical bonding.

In that context, it is worth noting that ultra high resolution data (q = 4πsin(θ)/λ up to 25 Å^–1) will quickly become prerequisite to allow proper modeling of valence electron shells of heavy atoms (2^nd series of transition elements, 4f rare earth electron) with many applications to catalysis (Pd, Pt) and molecular science (Gd). Access to 25 keV X-ray energies is needed for this purpose.

The use of synchrotron radiation can shed considerable light on the mechanisms of materials formation and ultimately control the processes.
Many materials (often those of major industrial or pharmaceutical importance) cannot be crystallized and exist only as powders, whose structure has tremendous importance on their expected properties.
Indeed, powder diffraction is an alternative x-ray method to unravel crystal structures and Ab initio structure determination is now an important application of powder diffraction: structure of crystals containing a few tens of atoms per unit cell can now be determined.

Very narrow vertical resolution is well suited for high angular resolution studies, (e.g. profile analysis, microstrains studies)

The increasing demand for beam time ( coming partly from industrial needs) at the dedicated beamline BM16 (now ID31) at the ESRF pleads for the construction of a dedicated powder diffraction instrument.

Studies of phase transitions concern the crystal structure of the different phases, the mechanism of the transition (critical behavior, dynamics of fluctuations, local order, nucleation), the role of disorder and defects on the transition and the structural response to different external fields: temperature, pressure, electric or magnetic fields, laser excitations, or a combination of several of them.
In addition to standard Bragg diffraction data,the measurement of weak scattering effects, like diffuse scattering or weak Bragg reflections (profile and intensity) under different sample environments give access to:

• the interaction potentials driving the phase transition
• the structural effects induced by point-like or extended defects (elastic deformations, pinning of ordered domains, phase nucleation).
• the spatial correlation function of the ordered structure under consideration through diffuse scattering profile analysis

The high flux makes possible the study of nano-objects (e.g. nanotubes, artificial structures) and more generally of newly discovered crystals, which are often available in very small amounts.

Anomalous scattering corresponds to the variation of the scattering factor f(E) as a function of energy near the absorption edge of an element and can be used for all the structural studies mentioned previously.
 
The anomalous terms f'(E) and f"(E), which are due to virtual excitations to unoccupied states, contain information on the charge and the anisotropy of these states. Site selective f’(E) and f"(E) can also be extracted from diffracted intensities: diffraction and spectroscopy information can thus be combined. Furthermore near absorption edges, anomalous diffraction spectra are dependent of the local chemical environment of the anomalous scatterer with respect to the beam polarisation direction. This tensor character of resonant diffraction makes it also sensitive to the empty orbitals symmetries and occupations.
In summary, anomalous scattering can be used:

• To select the role of a specific element in a structural phase transitions
  
  
• To solve the phase problem for complex structures (MAD method for the study of incommensurate phases)
  
  
• To perform a selective spectroscopy of the element under consideration.
  
  
• To determine distortion symmetries from the study of the anisotropy of the scattering.

New synchrotron radiation technique, taking advantage of the coherence and time structure of the beam, allow now to study the dynamics of condensed matter, which was previously the realm of neutron or light scattering. This is the case of X-ray Photo-Correlation Spectroscopy (XPCS), which applies the principle of dynamic light scattering in the x-ray wavelength range. Another approach consists in taking advantage of the pulsed structure of the synchrotron beam to perform stroboscopic measurements of an excited sample.

Recently, XPCS has been successfully applied to soft matter physics, however, other topics could be investigated with this technique, let us mention :

• Critical slowing-down of fluctuations close to a phase transition.
  
  
• Domain motion under external excitations like charge density waves under an electric field or ferroelectric domains.
  
  
• Effect of impurities on second order phase transition (random field phenomena, charge density wave pinning). Nucleation phenomena in first order phase transitions. Separation of static and dynamic effects.
  
  
• Scattering from point-like defects. Dynamics of defects-induced deformation upon diffusion of the defects.

Besides these applications, the diffraction by a coherent beam gives the non-averaged Fourier transform of samples of micrometer sizes (nano-crystals, small individual grains), impossible to study with standard X-ray sources. Application in studying deformation of such tiny objects under stress or external fields are envisaged.

ms to ns time-resolved x-ray crystallography
For a long time, conventional X-ray crystallography has been limited to the study of the ground state and steady state structure and electron density of solids. Very recently, the molecular structure of a short-lived (50 µs) transient excited state has been determined for the first time by pump-probe (stroboscopic) synchrotron X-ray diffraction at NSLS .This preliminary study opens up the field for more precise time resolved monochromatic single crystaldiffraction measurements of transient species and fast dynamic processes in the ms to ns time scale by a careful optimization of the experimental setup (laser peak power, pulses frequency and duration, synchronization).
 
Among others, dynamics of photo-induced spin-transition is a typical example of possible investigation using such a method. Time-resolved (pump-probe) high resolution diffraction measurements would give access to the molecular and electronic structure of the short-lived intermediate states. By varying the delay between the laser pump and the X-ray probe, it would also allow to follow the relaxation path to better understand the physical process of relaxation.

The pulsed structure of the synchrotron radiation makes it possible to reduce the time resolution to about 20-50 ps, which is the length of an electron bunch within the storage ring. Another possibility to improve the time resolution has recently been demonstrated to generate femtosecond (fs) X-ray pulses by means of interaction between an electron bunch and an external intense fs-laser pulse. We want to benefit from these two kinds of time-structure at SOLEIL for the study of ultra-fast photo-induced structural phenomena in condensed matter. The three diffractometers proposed to be installed on the beam-line should be able to perform time-resolved experiments. Therefore different types of instantaneous complete diffraction patterns (single-crystal or powder) could be recorded according to the scientific objectives.
 
This is particularly adapted to the study of photo-induced phase transformations, i.e. ultra-fast light-driven switching between two macroscopic states. These types of phenomena take place in highly-correlated materials such as charge-transfer complexes, mixed-valence chains, oxides, quantum ferroelectrics.
 
These modern time-resolved diffraction techniques give an exceptional opportunity to explore strongly non-linear, non-adiabatic and non-equilibrium processes in condensed matter which can dominate the functionality of advanced materials.
 
The development of a diffraction line at SOLEIL, using fs x-ray pulses, will give significant advances in structural and chemical dynamics, with studies of:

• photo-induced symmetry changes,
  
  
• photo-induced structural and molecular rearrangements,
  
  
• phase separation,
  
  
• precursor phenomena (co-operative diffuse scattering),
  
  
• coherent phonons,

structure and electron density of molecular excited state.

The concerned scientific community is very broad. It involves solid state physics and chemistry laboratories, Already 16 laboratories or groups, in addition the the 10 directly involved in the project have expressed their interest
 
Recommendations of the Scientific Advisory Committee (17/05/02)
The proposed crystallography beamline is designed to collect data of the highest quality to answer questions at the forefront of research in chemistry, material research and condensed matter physics. The scientific case is well established, including challenging time resolved experiments and measurement of electron density in the excited states. The project is supported by excellent laboratories, the promoters are highly experienced in crystallography, and there is no doubt that they will succeed in building a highly competitive beamline. The potential community of users is very large, and the proposed scientific programme huge, then SAC recommends to build this line with a very high priority. SAC also wants to draw attention to the following points :

• the proposal is supported by 26 users groups with great expertise and experience in sophisticated crystallographic investigations. This is a good point, but the drawback could be the difficulty to run all the programmes in the best conditions. The scientific team in charge of this beamline will have to be very attentive to the good coordination between the different activities, both during the construction and the operation periods;
  
  
• among the three proposed instruments, two are high throughput set-ups (A, for straightforward diffraction measurements and C, for powder diffraction) whereas the third one (B) is set for more demanding experiments dealing with interesting novel physics. This choice is relevant, and it seems that instrument A could be in use very rapidly;
  
  
• considering the present evolution of material science and the increase in industrial applications, SAC believes that powder diffraction will be an extremely important technique in the future. SAC suggests that much effort should be put on instrument C, which could require a dedicated beamline in the future;

In conclusion, the proposal includes a well developed scientific case and has properly considered the beamline requirements both to implement existing methods (extended to new extremes) and to develop novel techniques to conduct high impact research for a very broad scientific community. So SAC recommends to build this line with the highest priority.

La direction de SOLEIL demande l’aval du Conseil pour le lancement de la construction de la ligne « Cristallographie et structure de la matière condensée ». Les possibilités offertes par une telle ligne sur un onduleur ouvriront des domaines d’expériences d’une grande importance en recherche fondamentale et appliquée. La conduite du projet a été exemplaire par son ouverture sur la communauté scientifique. La direction de SOLEIL, consciente du budget élevé correspondant aux dispositifs expérimentaux prévus, souhaite mettre en place avec les laboratoires concernés un programme d’accompagnement du développement de l’instrumentation qui permette d’atteindre les objectifs fixés. L’importance de la communauté concernée par ce projet et le développement souhaité de la demande industrielle, en particulier dans le domaine des poudres, pourrait justifier à l’avenir la construction d’une ligne dédiée à cette technique.