Vincent Legrand

Crystallography Under Extreme Conditions: State of the Art and Perspectives

where S is a scale factor, Chkl is an experimental corrections term (including absorption, extinction, Lorentz-polarization correction…), Fhkl is the structure factor of a hkl reflection which depends of fj the form factor of the atom j with (xj, yj, zj) coordinates in the cell and of Tj the Debye-Waller factor given in the case of isotropic harmonic vibrations by:

where

U is the mean square atomic displacement of the atom j.

:

Introduction

Sir William Henry Bragg and his son William Lawrence Bragg were the pioneers of crystallography [START_REF] Bragg | X-rays and crystals[END_REF](Bragg, W. H., , 1913a(Bragg, W. H., , 1915a[START_REF] Bragg | The structure of magnetite and the spinels[END_REF][START_REF] Bragg | Crystals structure[END_REF]. In 1913, they published several articles, notably The reflection of X-rays by crystals [START_REF] Bragg | The reflection of X-rays by crystals[END_REF] and The structure of diamond [START_REF] Bragg | The structure of diamond[END_REF] were they wrote: "We have applied the new methods of investigation involving the use of X-rays to the case of the diamond, and have arrived at a result which seems of considerable interest. The structure is extremely simple". Two years later, they were jointly awarded the Nobel Prize in Physics for their works in the analysis of crystal structure by means of X-rays.

A century after the first crystallographic experiment, new computing facilities, modern technologies and new diffraction sources (synchrotron, neutron sources…) offer a large range of possibilities and opportunities for crystallographers to probe matter. Crystallography appears nowadays as a new science.

Performing structural analyses at ambient conditions or at low temperature (i.e. above 100 K using nitrogen jet-stream) is very common and popular in laboratories to obtain the structure of powder and single-crystal materials. To understand the mechanisms governing the behaviour of materials it is essential to well know the close relations between the structure and the physical and chemical properties. First, X-ray diffraction is a unique tool to obtain routinely a detailed description of atomic structure and thermal vibrations by analysing the diffracted intensities Ihkl of the crystallographic hkl reflections:

                (4)
where ρcore(r) and ρval(r) are spherically averaged core and valence electron densities calculated from Clementi Hartree-Fock wave functions for ground state isolated atoms [START_REF] Clementi | Roothaan-Hartree-Fock atomic wavefunctions[END_REF]. κ and κ' are contraction-expansion parameters and Pval is the atomic valence shell electron population. The deformation of the valence electron shell is projected on real spherical harmonics ylm±(θ,φ) times Slater-type radial functions Rnl(r). Plm± are the multipole population parameters.

In the later case, the accuracy of the results is highly dependant of the experimental conditions [START_REF] Destro | On the role of data quality in experimental charge density studies[END_REF][START_REF] Zhurov | Optimization and evaluation of the data quality for charge density studies[END_REF] and particularly of the crystal quality and the measurement temperature which must be chosen to reduce at the maximum the thermal vibrations of the atoms. These usual experiments at static low temperature give basic structural properties, and their variations as a function of temperature can reveal particular behaviour of matter as phase transition for example with changes in structural, electronic, optical and/or magnetic properties. But, in a general consideration, performing crystallographic measurements under external perturbations is of prime importance. Nowadays, experiments at different temperatures or hydrostatic pressures can be done almost routinely and exotic sample environments are more and more used to explore materials properties. If we just consider for the moment experiments involving temperature or pressure variations, a Web of Science search for "high pressure", "low temperature" and "high temperature" in the field of X-ray and neutron diffraction between the years 2005 and 2012 gave about 3500 experiments performed in 2005 but nearly 5000 in 2011 (figure 1). Two comments can be done observing figure 1. First, a large amount of the extreme conditions (temperature and pressure) experiments are carried out using X-ray diffraction compared to neutron diffraction. This difference is of course due to the large number of X-ray diffractometers available in laboratories, which are more and more outstanding in terms of source power and detector efficacy, combined with numerous sample environments especially designed for laboratory equipments. But this is also due to the building of several 3 rd generation of synchrotron sources which offer the possibility to conduct very quick measurements on a very small quantity of matter, allowing the measurement of materials under non ambient conditions, particularly in the domains of chemistry and biology sciences (neutron diffraction experiment demands more matter quantity, in general at least some mm 3 ).

The second observed aspect on figure 1 As said before, crystallography under extreme conditions is now more and more used and plays a key role as it offers helpful understanding of the physical, chemical and mechanical properties of the solid state. The term extreme conditions was first used to define non ambient thermodynamical conditions of pressure or temperature. It is also now employed when outof-equilibrium conditions are applied such as light irradiation, external magnetic or electric fields, specific chemical environments (e.g. under liquid or gas flux…) or applied strain. In situ measurements can also be considered as extreme conditions, for examples in the cases of time-resolved experiments (picosecond diffraction…) or chemical kinetic reactions (dehydrogenation reaction, diffusion process, decomposition pathway…). The present challenge is to combine two or more extreme conditions to explore new states of matter and new material properties, taking advantage of last generations of high brilliance sources (synchrotron and neutron sources) (figure 2). When extreme conditions are applied to a material, various changes occur. They may involve sample state variations (gaseous, liquid and solid phases), phase transitions (magnetic and structural), electronic structure modifications (the chemical bonds can change from covalent to ionic or metallic) and atomic bond lengths variations (which induce variations of the atomic vibrations, of the coordination numbers, of the diffusion processes…). A large number of the material properties are modified under extreme conditions, leading sometimes to metastable states, which open new investigation opportunities for the crystallographer, but also for all scientist who wants to go deeper in the understanding of the structure-properties relationships in order to design future materials. This chapter is reviewing different aspects of the use of crystallography under extreme conditions to investigate the nature, the mechanisms and the dynamics of out-ofequilibrium phase transitions as well as transformations driven by external applied conditions. The part 2. is focus on the advantages of using Large scale facilities around the world and what are now the extreme sample environments available to explore unknown properties of materials. Finally, an overview of perspectives and future developments in diffraction instruments and sample environments are presented in part 3. showing that modern crystallography is in perpetual evolution and that the present century is certainly the one of crystallography under extreme conditions.

Large scale facilities and extreme sample environments

State-of-the-art sample environments at Laboratories

As noticed before, there are more than 5000 publications per year dealing with extreme conditions of temperature and pressure. Nevertheless, those conditions illustrate only 2 of the numberless non ambient thermodynamical and exotic conditions which can be used to probe the close links between structure and properties of materials. Changing the pressure or the temperature in a crystallographic experiment is performed quite routinely in laboratories. But if one wants for examples to apply electric or magnetic fields, to explore superconductivity properties below 1 K, to perform very fast measurements of transient species or quick chemical reactions, or to understand phase transitions under combined sample environments, the diffractometer performances become overriding and in particular the maximum power of the source, the quality of the beam collimators and the efficacy of the detector. This is very important to obtain exploitable data as the sample environment in those later examples is very bulky and could dramatically affect the beam intensity. Moreover, it is very rare for a laboratory to have sample equipments to perform sub-kelvin, time-resolved or high-magnetic field measurements as these sample environments are very expensive, maintenance demanding and hard to exploit on conventional diffractometers. For a long time, extreme conditions measurements were the privilege of expert laboratories in cryogenic, high fields or high pressures with ability and technical resources to build and to use such devices. Those techniques are nowadays distributed in a larger number of laboratories and, for some of them, take place to semi-industrial devices merchandised by high technology industries. Since ten or so years, apparatus able to be used on conventional diffractometers were punctually developed to respond to the requirement of probing new properties of materials. We can cite some of them. First, to probe the structural and the electronic properties of materials under light irradiation at low temperature, photocrystallographic experiments [START_REF] Coppens | Crystallographiy of molecular excited states. Transition metal nitrosyl complexes and the study of transient species[END_REF]Fomitchev et al., 2000) were developed first to study transient species of transition metal nitrosyl complexes. [START_REF] Coppens | Crystallographiy of molecular excited states. Transition metal nitrosyl complexes and the study of transient species[END_REF] wrote that "the study of photo-induced processes in crystals is a frontier area of crystallographic research, which requires development of novel experimental and computational methods". This is particularly true as the live-time of transient species is in general not related to the time of the experiment required to collect accurate data for structural refinement or experimental electron density modelling. The success of kind experiments consists in in situ laser irradiation of a cryogenically cooled crystal, with a N2 or He gas-flow system for less restricted access than closed cryostat [START_REF] White | Apparatus for the measurement of the electronic excited-state structure of single crystals using X-ray diffraction[END_REF], mounted on a diffractometer equipped with a CCD detector for fast data collection [START_REF] Graafsma | An X-ray charge density feasibility study at 56 keV of magnesium formate dihydrate using a CCD area detector[END_REF][START_REF] Muchmore | Experiences with CCD detectors on a home X-ray source[END_REF]. More recently, [START_REF] Legrand | Cristallographie et photo-cristallographie haute résolution de composés moléculaires à transition de spin : propriétés structurales, électroniques et mécanismes de conversion[END_REF] has developed a crystallographic experimental methodology approach [START_REF] Legrand | Photo-crystallography: from the structure towards the electron density of metastable states[END_REF]2007a;[START_REF] Legrand | Optical, magnetic and structural properties of the spin crossover complex [Fe(btr)2(NCS)2].H2O in the light-induced and thermally quenched metastable states[END_REF] and shown that it is possible to measure with accuracy photo-induced metastable states and to refine its experimental electron density below 35 K using conventional X-ray sources [START_REF] Legrand | Extension of the experimental electron density analysis to metastable states : a case example of the spin crossover complex Fe(btr)2(NCS)2.H2O[END_REF][START_REF] Pillet | Out-of-equilibrium charge density distribution of spin crossover complexes from steady-state photocrystallographic measurements : experimental methodology and results[END_REF].

We can also mention an other development to do pressure measurements using particular pressure cell, up to 3.0 GPa and for T > 9 K [START_REF] Guionneau | Laboratory high pressure single-crystal X-ray diffraction -recent improvements and examples of studies[END_REF]) (figure 3). This last device was developed to probe phase transitions and structure-properties relationships in molecular conductor [START_REF] Chasseau | Crystal structures of (BEDT-TTF)3CuBr4 at 10 K and 10 kbar[END_REF] and spin transition complexes [START_REF] Guionneau | High pressure and very low temperature effects on the crystal structures of some iron(II) complexes[END_REF]. Doing in situ laboratory pressure investigations is still difficult as the pressure cell induce several perturbations affecting the quality of the data collection. The originality of this high pressure cell is that the X-ray beam goes through the gasket and not through the diamonds, offering a wide diffraction angle of 342° rotation. In this case, [START_REF] Guionneau | Laboratory high pressure single-crystal X-ray diffraction -recent improvements and examples of studies[END_REF] indicate that using a CCD area detector to collect Bragg peaks improves data and reduces the measurement time. To finish this non-exhaustive description, we can briefly mentioned two device developments in the field of very low temperature measurements. A mini-goniometer for X-ray diffraction studies down to 4 K on a four-circle diffractometer equipped with a CCD area detector [START_REF] Fertey | A mini-goniometer for X-ray diffraction studies down to 4 K on four-circle diffrctometers equipped with two-dimensional detectors[END_REF]. The authors used a helium-bath orange cryostat [START_REF] Brochier | Cryostat à température variable pour mesures neutroniques ou optiques[END_REF] and added a remarkable evolution: a magnetically coupled two-rotation-axis minigoniometer permanently installed in the sample chamber of the helium-bath cryostat. This original apparatus was tested with success on organic charge-transfer compounds [START_REF] Garcia | Crystallographic investigation of temperature-induced phase transition of the tetrathiafulvalene-pbromanil, TTF-BA charge transfer complex[END_REF][START_REF] Garcia | On the accuracy of intermolecular interactions and charge transfer: the case of TTF-CA[END_REF]. An other low temperature system was developed to analyse Jahn-Teller distortion of TmVO4 using conventional X-ray diffractometer between 0.22 and 3.5 K [START_REF] Suzuki | X-ray diffraction measurement below 1 K[END_REF]. The authors used a diffractometer equipped with a countermonochromator and a scintillation counter, and an X-ray generator with rotating Cu Anode.

To perform the ultra-low temperature measurements, a modified 3 He-4 He dilution refrigerator was mounted on the two-fold axis goniometer. In this later case, the study was also a success and the Jahn-Teller effect was fully investigated down to 0.22 K. In their paper, Suzuki et al. mentioned what was noticed above in this section concerning the use of bulky devices: "the x-ray beam generated by a Cu target, which passed through 4 walls of Be 2 mm thick and 4 aluminized mylar walls was reduced to approximately 1/100". This beam attenuation is generally inevitable when doing extreme conditions experiments and to guaranty good data collection, it must be used first class equipments on the diffractometer. Even if a lot of progresses concerning the performances of new diffractometers were obtained last years, extreme conditions measurements are sometimes impossible to perform at laboratories. Then, the best to do is to use Large scale facilities which benefits are now more and more highlighted. The term Large scale facilities stands for synchrotron research institutes and neutron facilities (spallation sources and reactors).

Why using large scale facilities in extreme crystallography?

The synchrotron working principle was shown during the Second World War by Oliphant (1943, University of Birmingham, UK). Three years later, it was shown for the first time [START_REF] Goward | Experimental 8 MeV synchrotron for electron acceleration[END_REF][START_REF] Elder | A 70 MeV Synchrotron[END_REF] that it is possible to built synchrotrons providing a satisfactory means of producing high energy electrons and X-rays. Nearly 70 years later, several synchrotrons of 3 rd generation are operational throughout the world and are of first necessity for all the scientists communities [START_REF] Bilderback | Review of third and next generation synchrotron light sources[END_REF]. The X-ray beam, produces by a synchrotron, is a light beam (electromagnetic wave) which interacts with electrons surrounding the nuclei of heavy elements [START_REF] Born | Modified field equations with a finite radius of the electron[END_REF][START_REF] Born | Quantised field theory and the mass of the proton[END_REF][START_REF] Born | The absolute field constant in the new field theory[END_REF]. The actual phenomenal success of the synchrotron for performing extreme conditions crystallographic measurements comes from numerous undeniable advantages and particularly it's very intense (brilliance) and focussing beam and it's time structure allowing the measurement of small or ultra-dilute samples at different time scales. Thus, it is possible to study transient species and fast chemical reactions or to perform biological and geological studies using bulky sample environment allowing temperature and pressure changes without affecting the data.

Neutron diffraction also plays a key role in actual crystallographic studies under extreme conditions. The first nuclear reactor was built in 1942 by Enrico Fermi and Leó Szilárd (University of Chicago, USA) [START_REF] Fermi | Elementary theory of the chain-reacting pile[END_REF]. In October 2011, the International Atomic Energy Agency (IAEA) database indexed 241 operational research reactors in the world.

Generally, experiments using neutrons to explore the properties of a material are based not only on the intensity of the beam (to exploit this criteria, synchrotron radiation would be preferable) but on the very nature of neutrons. It is a particle beam, allowing interactions with the nuclei and the magnetic moment of unpaired electrons in the sample, scattered by all elements also the light ones like hydrogen and its isotopes, with a deep penetration depth enables bulk studies of materials [START_REF] Fermi | Magnetic moment of atomic Nuclei[END_REF][START_REF] Fermi | Radioactivity induced by neutron bombardment[END_REF][START_REF] Fermi | Reactions produced by neutrons in heavy elements[END_REF][START_REF] Szilárd | Absorption of residual neutrons[END_REF]. As neutrons can penetrate deeper in matter (unless exceptions like cadmium, samarium, europium or gadolinium elements for examples), sample environments requiring to put the sample inside them (e. g. pressure cells, furnaces, gas handling inside a cryostat…) can be used even if further walls, generally in aluminium, must be crossed by neutrons before to hit the sample. This neutron property is very appreciable and neutron facilities now include very well equipped services and highly qualified staff to carry out invaluable experiments as the Services for Advanced Neutron Environment (SANE) at the Institut Laue-Langevin in France, where the Orange Cryostat [START_REF] Brochier | Cryostat à température variable pour mesures neutroniques ou optiques[END_REF] and the Cryopad [START_REF] Tasset | Zero-field neutron polarimetry[END_REF], among others equipments, were invented and built.

Synchrotron and neutron facilities are complementary techniques which allow to analyse structural details of materials at the atomic scale. Microscopic properties are then known and the challenge is to derive the macroscopic ones. To achieve that goal, investigations must concern both surface and bulk phenomena of materials in both dynamic and steady states. Large scale facilities can give fine details on the nuclear structure of materials, on the physical and chemical properties, on the electronic and magnetic structures, they can give crucial information about surface and interface states. All materials are concerned: metals, alloys, polymers, composites, colloids, glass, cement, biological macromolecules (viruses, enzymes, proteins…), minerals, food products, superconductors… The world-wide scientific community can observe since about ten years state of the art measurements in the above mentioned fields.

One field where X-ray and neutron measurements are really complementary is the domain of spin-transition molecular magnetic materials which is highly studied under exotic environments since 20 years [START_REF] Gütlich | Spin crossover in transition metal compounds[END_REF][START_REF] Bousseksou | Molecular spin crossover phenomenon:recent achievements and pospects[END_REF]. These materials are especially interesting owing to their bistability properties which give them promising applications as data storage elements, thermal switches or display devices. These materials consist of a d 4 to d 7 transition metal coordinated to specific containing aromatic ligands whose ligand field is intermediate between weak and strong. Correlatively, the electron configuration of the central metal ion can be driven by several kinds of external perturbations such as pressure, intense magnetic field or light excitation. In the case of iron(II) complexes, the spin conversion is related to the electronic configuration of the ion changing from t2g 6 eg 0 in the low spin (LS) state to t2g 4 eg 2 in the high spin (HS) state. In addition to this electron redistribution, spin transition complexes present drastic structural variations between both phases, principally observed in the iron coordination sphere: an increase of the iron octahedron distortion and of the Fe-ligand bond length, by typically 0.2 Å, are observed for the LS → HS transition, generating a decrease of the molecular volume by several Å 3 . Thus, owing to their high electronic and structural contrast, spin transition materials are good candidates for crystallographic studies under extreme conditions especially using Large scale facilities. Pressure dependence of the lattice parameters of several iron(II) complexes were derived from neutron powder diffraction [START_REF] Legrand | Neutron powder diffraction studies of two spin transition Fe(II)-complexes under pressure[END_REF] and single-crystal X-ray synchrotron (Legrand, private communication) measurements at ambient and low temperatures. These studies highlight very promising results in the field of spin crossover phenomena under constraint, where crystallographic investigations under pressure are still very rare, especially because experimental conditions are difficult and sometimes not favourable. Concerning investigations under high magnetic field, [START_REF] Goujon | Photoinduced molecular switching studied by polarized neutron diffraction[END_REF] reported the photo-induced magnetization density of the iron(II) spin crossover compound Fe(ptz)6(BF4)2 using an experimental setup allowing combined application of light irradiation (λ = 473 nm) at low temperature (2 K) and high magnetic field (5 T) (figure 4a). Thanks to this polarized neutron diffraction measurement, the temperature dependence of the magnetization was well described and the thermal relaxation of the photo-induced state well observed.

Moreover, electronic and structural properties of iron(II) spin transition complexes were studied under light irradiation and low temperature (< 15 K) using neutron Laue [START_REF] Goujon | Neutron Laue diffraction on the spin crossover crystal [Fe(1-npropyltetrazole)6](BF4)2 showing continuous photoinduced transformation[END_REF] and X-ray synchrotron [START_REF] Legrand | On the precision and accuracy of structural analysis of light-induced metastable states[END_REF][START_REF] Pillet | Out-of-equilibrium charge density distribution of spin crossover complexes from steady-state photocrystallographic measurements : experimental methodology and results[END_REF] diffraction (figure 4b). These latter authors notably refined for the first time the experimental electron density of a photo-induced metastable state in a spin transition compound. Finally, probing photoinduced phase transitions in molecular materials can also be performed using ultra-fast measurements at a picosecond time scale (Coppens et al., 2004;Collet et al. 2012a;2012b). Other examples using the advantages of synchrotron radiations are numerous in structural biology, geology and materials sciences. The atomic structure of the bluetongue virus core (700 Å in diameter), containing about 1000 protein components self-assembled, was determined at a resolution of 3.5 Å [START_REF] Grimes | The atomic structure of the bluetongue virus core[END_REF]. The atomic structure of large ribosomal subunits were refined [START_REF] Ban | The complete atomic structure of the large ribosome subunit at 2.4 Å resolution[END_REF][START_REF] Wimberly | Structure of the 30S ribosomal subunit[END_REF] providing a wealth of information about RNA and protein structure, protein-RNA interactions and ribosome assembly. The high brilliance of synchrotron sources in combination with fast detectors was used to perform time-resolved radiography of metal foam formation (Banhardt et al., 2001) (figure 5a). The possibility to have a micro-focus beam was applied to the study of the structure of spider silk [START_REF] Riekel | In situ X-ray diffraction during forced silking of spider silk[END_REF][START_REF] Riekel | Spider silk fibre extrusion: combined wide-and small-angle X-ray microdiffraction experiments[END_REF] (figure 5b). The beam coherence properties enable new imaging techniques to do quantitative phase imaging and holotomography to probe materials with a micrometer resolution [START_REF] Cloetens | Holotomography: quantitative phase tomography with mocrometer resolution using hard synchrotron radiation X-rays[END_REF].

Nowadays, highlight articles are still numerous and new instrumental and device developments allow unthinkable experiments even ten years ago. Using highly brilliant X-ray beams at the APS (Advanced Photon Source, USA), ESRF (European Synchrotron these extreme conditions were generated with tiny samples heated thanks to high electric current while squashing them in a press (figure 6); knowing the attenuation of the synchrotron X-ray beam through both the solid and molten parts of the sample, the density at high pressure and high temperature could be measured. High pressure in situ X-ray diffraction and specific volume measurements on isotactic poly(4-methyl-1-pentene) melt have uncovered abrupt changes in the pressure dependence of microscopic structure as well as that of macroscopic density [START_REF] Chiba | Pressure-induced structural change of intermediaterange order in poly(4-methyl-1-pentene) melt[END_REF]. It was proved another time that pressure has an essential role in the production and control of superconductivity [START_REF] Sun | Re-emerging superconductivity at 48 kelvin in iron chalcogenides[END_REF]: it is reported that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges at a critical temperature Tc reaching 48.0 K and above 11.5 GPa, after the Tc drops from the first maximum of 32 K at 1 GPa. Metallic liquid silicon at 1787 K was investigated using X-ray scattering [START_REF] Okada | Persistence of covalent bonding in liquid silicon probed by inelastic x-ray scattering[END_REF]: the results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled states. To finish, it can be pointed out the crystallographic structure refinement of a protein, PthXo1 (which is a transcription activator-like, TAL, effector), encoded by an important group of harmful plant pathogens [START_REF] Mak | The crystal structure of TAL effector PthXo1 bound to its DNA target[END_REF]: understanding DNA recognition by TAL effectors may facilitate rational design of DNA-binding proteins with biotechnological applications.

Using neutron diffraction on reactors and spallation sources, researchers also push back the limits of measurements as for examples in the observation of a roton-like excitation in a monolayer of liquid 3 He, a Fermi liquid [START_REF] Godfrin | Observation of a roton collective mode in a two-dimensional fermi liquid[END_REF]. A new class of magnetic ionic liquid surfactants showing remarkable effects on surface and interfacial tension and allowing access to magneto-responsive emulsions and new methods of separation, recovery, catalysis, and potential magnetophoretic applications were analyses by small-angle neutron scattering (SANS) [START_REF] Brown | Magnetic control over liquid surface properties with responsive surfactants[END_REF]. Using neutron diffraction studies, along with computational chemistry, the mystery of lead oxide, the battery anode material of lead-acid batteries, was solved [START_REF] Scanlon | Nature of the band gap and origin of the conductivity of PbO2 revealed by theory and experiment[END_REF]: oxygen vacancies give metallic behaviour by freeing up electrons to carry electrical current, neutron experiment put in evidence that commercial lead oxide powder is oxygen deficient by showing that oxygen sites were 1.6% vacant. Using high resolution neutron diffraction under high temperature and high magnetic field, it was shown that the thermal evolution of the helimagnetic state in CoMnSi is accompanied by a change in interatomic distances of up to 2%, the largest ever found in a metallic magnet [START_REF] Barcza | Giant Magnetoelastic Coupling in a metallic helical metamagnet[END_REF]; this important result could lead to more efficient fridges and cooling systems.

The above overview has to remain incomplete with a number of fields left out. However, one can realize the wealth of extreme condition experiments which can be performed nowadays at laboratories and, particularly, using Large scale facilities which benefit of the last innovations in terms of sample environments.

Crystallographic experiments: advances in sample environments

Sample environment equipments like pressure cells, cryostats, etc, are essential adjuncts to perform with success the analysis of materials into a state or a phase with special behaviours and properties. This section is a brief overview of the most employed stately sample environments available mainly on Large scale facilities. The technical points of the discussed devices are not detailed here and the reader could find further information in the references given through section 2.3 and in specific reviews as written by [START_REF] Bailey | A review of sample environments in neutron scattering[END_REF] or [START_REF] Mignot | Diffusion neutronique sous conditions extrêmes[END_REF]. Specific Information is also related in reviews from the Workshop "sample environments in neutron and X-ray experiments" held at the Institut Laue-Langevin (France) in 1984 [START_REF] Vettier | Workshop "sample environments in neutron and X-ray experiments[END_REF]. There are various ways of tuning materials properties. In this section, the most employed sample environments are described, but the possibilities are miscellaneous and it is not possible to make an exhaustive list. Exotic sample environments like, among others, electric fields [START_REF] Guillot | Diffraction measurements from crystals under electric fields: instrumentation[END_REF][START_REF] Hansen | Studies of electric field induced structural and electron-density modifications by X-ray diffraction[END_REF] or in situ reactions using gas handling [START_REF] Walker | High capacity multicomponent hydrogen storage materials: investigation of the effect of stoichiometry and decomposition conditions on the cycling behaviour of LiBH4-MgH2[END_REF][START_REF] Price | Enhanced kinetics for the LiBH4:MgH2 multi-component hydrogen storage system -the effects of stoichiometry and decomposition environment on cycling behavior[END_REF], as well as experiments performed with time resolved methods (Gembicky & Coppens, 2006;[START_REF] Nozawa | Developing 100 ps-resolved Xray structural analysis capabilities on beamline NW14A at the Photon Factory Advanced Ring[END_REF] are not discussed. The investigations using these increasingly sophisticated experimental methods contribute enormously to the understanding of structural properties and progress in solid state physics and chemistry, biology and in geosciences. Advances in sample environments help to perform crystallographic experiments in extreme conditions and extend current areas of investigations.

Low temperature

Cryogenic conditions are by far the most used sample environments principally to reduce vibrations and thermal motions of atoms, thus the structural refinement is improved. Moreover, phenomena like phase transitions are also generally induced at low temperature. Different devices could be used for low temperature experiments depending the desired temperature range: liquid helium bath cryostat (Orange Cryostat [START_REF] Brochier | Cryostat à température variable pour mesures neutroniques ou optiques[END_REF]) for 300 K to 1.5 K; 3 He sorption systems for 3 K to 300 mK; 3 He/ 4 He dilution refrigerator systems below 1 K. Of course, there exist many others systems allowing to reach low and very low temperatures as N2 or He gas-flow systems, helium closed cycle refrigerators (Displex), etc.

The Orange cryostat is now regarded as the international standard for low temperature experiments performed above 1.5 K. Some variations exist differing in the sample stick geometry (diameters and length). Cryofurnace and Cryomagnet versions were also developed. Nowadays, more and more experiments are based on combining sample environments. Thus, special versions of the Orange cryostat or Displex were built to meet the demand of users. Among the list of special purposes, it can be noted versions allowing the use of a high pressure cell (even Paris-Edinburgh cell) (figure 7), a gas handling for in situ chemical reactions analysis, an omega rotating for single crystals measurements, a sample changer. Because these cryostats are very common at Large scale facilities and are user friendly, recently improvements were made to have the possibility to do ultra-low temperature measurements using new sample stick inserts including high performance dilution refrigerator with a base temperature of less than 15 mK [START_REF] Neumaier | A dilution refrigerator insert for standard ILL cryostat[END_REF].

High temperature

Contrary to cryogenic devices, there is no standard high temperature apparatus. The designs depend of the working temperature range, of the type of experiment, of the diffraction beam (X-ray or neutron). However, a typical furnace configuration places a sample into the centre of a thin cylindrical metal foil element, of about 50 μm thick, surrounded by heat shields. According to the chosen metal, the achieved temperature could be typically up to 1400-2000 K. Closed-shell furnaces [START_REF] Kuhs | A closed-shell furnace for neutron single-crystal diffraction[END_REF] and mirror furnaces [START_REF] Lorenz | A mirror furnace for neutron diffraction up to 2300[END_REF] were developed for powder and single-crystal analysis at high temperature. This last furnace type has the advantage to allow air-operated experiments with no heat shield, thus removing apparatus diffraction. As for cryogenic, variations are developed to combine for example high temperature and high pressure [START_REF] Falconi | X-ray diffraction study of liquid Cs up to 9.8 GPa[END_REF]. Finally, the last developments concern the design and built of furnace using aerodynamic levitation for reaching ultra-high temperatures up to 3000 K. This technique is well described by [START_REF] Hennet | Structure and dynamics of levitated liquid aluminates[END_REF]: "A spherical sample (3mm in diameter) is placed on a levitator that contains a convergent-divergent nozzle enabling the diffusion of a regulated gas flow onto the sample from below. This enables the sphere to remain in a stable position without any contact with the nozzle. When the sample levitates, it is heated by two CO2 laser from the bottom through the hole in the nozzle. The temperature is measured with one or two optical pyrometers". Aerodynamic levitation apparatus were optimized for neutron [START_REF] Hennet | Levitation apparatus for neutron diffraction investigations on high temperature liquids[END_REF] and X-ray [START_REF] Hennet | Fast X-ray scattering measurements on molten alumina using a 120° curved position sensitive detector[END_REF][START_REF] Sakai | Aerodynamic levitation apparatus for structure study of high temperature materials coupled with Debye-Scherrer Camera at BL19B2 of Spring-8[END_REF] scattering measurements. They were applied with success for examples to study the liquid states and the solidification of CaAl2O4 [START_REF] Hennet | Fast X-ray scattering measurements on high temperature levitated liquids[END_REF] and Y2O3 [START_REF] Hennet | Melting behaviour of levitated Y2O3[END_REF]Cristiglio et al., 2007).

High magnetic field

Studying materials under high magnetic field could be done ether by X-ray diffraction [START_REF] Katsumata | Synchrotron X-ray diffraction studies of magnetic materials under extreme conditions[END_REF] or by neutron scattering [START_REF] Brown | Determination of the magnetization distribution in Cr2O3 using spherical neutron polarimetry[END_REF][START_REF] Givord | The cerium magnetic form facto rand diffuse polarization in CeRh3B2 as functions of temperature[END_REF].

However, one of the advantages of neutrons is their possible interaction with the unpaired electron magnetic field. Thus, magnetic studies deal with a wide proportion of the total neutron diffraction experiments. As magnetic properties variations are generally observed at low temperature, superconductor cryomagnets are used for kind studies, with applied vertical or horizontal fields. The available magnetic field range is wide and nowadays fields around 10 T are commonly obtained, some devices also produce fields between 14.5 T and 17 T [START_REF] Nietz | Prospects for the use of the pulsed fields in neutron research of condensed matter[END_REF]. It can finally be noticed that punctual apparatus are developed through the world allowing 1 s pulsed magnetic fields up to 25 T (Nojiri et al., 1998) or 40 T [START_REF] Grössinger | Austromag: pulsed magnetic fields beyond 40[END_REF], in order to achieve fields up to 70 T in the future. Thermal neutrons are thus largely used to refine magnetic structures in addition to the nuclear structure. Hot polarized neutron diffraction is also a very interesting technique used to obtain crucial information about the magnetic form factors (reflecting the state of the magnetic ions) and magnetisation distributions of para-, ferro-and some antiferro-magnetic single-crystal materials if measurements are performed under high magnetic field configuration. On the other hand, one can performed zero-field neutron polarimetry measurements using Cryopad (CRYOgenic Polarization Analysis Device) [START_REF] Tasset | Spherical neutron polarimetry with Cryopad-II[END_REF][START_REF] Lelièvre-Berna | Advances in spherical neutron polarimetry with Cryopad[END_REF][START_REF] Takeda | Cryopad on the triple-axis spectrometer TAS-1 at JAERI[END_REF] to refine complex antiferromagnetic structures or to perform accurate determination of antiferromagnetic distributions.

High pressure

High pressure condition is certainly the more used sample environment after low/high temperature. This field of research is also one of the most well-informed facing the number of published papers and reviews concerning experimental results or technical information (see for examples [START_REF] Paszkowicz | High pressure powder X-ray diffraction at the turn of the century[END_REF][START_REF] Boldyreva | High-pressure diffraction studies of molecular organic solids. A personal view[END_REF][START_REF] Katrusiak | High-pressure crystallography[END_REF]and references therein). Material studies under pressure are nowadays numerous due to the increase capabilities of modern X-ray diffractometer and the making of dedicated instruments on synchrotrons and neutron facilities. At the present time, about 30 synchrotron beamlines in the world (at 11 synchrotrons) allow high-pressure measurements to be performed, with possibility of high temperature too. It could be pointed out that 2012 marks the 35 th anniversary of high pressure diffraction at synchrotron (Buras et al., 1977). The application of pressure on a material may induce major structural variations, particularly a modification in the weak intermolecular contacts and reorganization of the electronic structure, but also phase transitions or chemical reactions. Anyway, the application of pressure on a system induces more effects than the application of cryogenic temperatures, which is of interest to physicists, chemists, biologists and geologists. Pressure can give substantial information about structure-properties relationships and new thermodynamic phenomena (phase diagrams, polymorphism, cohesion forces, transformations…). Moreover, temperature variations can now be applied in combination to high pressure due to the high brilliance of modern sources at facilities which can bypass the unavoidable absorption of the beam by the cryostat or furnace and the pressure cell containing the tiny studied sample. Now, it is possible to perform experiments at 100 GPa and further thousands of Celsius degrees [START_REF] Fiquet | Powder X-ray diffraction under extreme conditions of pressure and temperature[END_REF][START_REF] Akahama | A new distorded body-centred cubic phase of titanium (δ-Ti) at pressure up to 220 GPa[END_REF][START_REF] Guo | The phase transitions of CoO under static pressure to 104 Gpa[END_REF][START_REF] Hu | X-ray diffraction and laser heating: application of a moissanite anvil cell[END_REF][START_REF] Orosel | Structure of a new highpressure -high temperature modification of antimony(III)oxyde, γ-Sb2O3, from highresolution synchrotron powder diffraction data[END_REF] using synchrotron radiations, this is achieved with the laser heating technique [START_REF] Bassett | The birth and development of laser heating in diamond anvil cells[END_REF]. Due to the relatively large sample volume required for neutron diffraction, high pressure measurement are always a challenge and reached maximum pressures are not so high: it was recently reported a high-pressure single-crystal neutron diffraction to 10 GPa at ambient temperature taking advantage of the neutron diffractometer D9 at the Institut Laue-Langevin (France) [START_REF] Bull | High-pressure single-crystal neutron diffraction to 10 GPa by angle-dispersive techniques[END_REF]. To succeed those measurements, teams generally use DAC (Diamond Anvil Cell) [START_REF] Jamieson | New device for obtaining X-ray diffraction patterns from substances exposed to high pressure[END_REF][START_REF] Weir | Infrared studies in the 1-to 15-micron region to 30,000 atmospheres[END_REF], MAC (Moissanite Anvil Cell) [START_REF] Xu | The moissanite anvil cell: a new tool for high-pressure research[END_REF] or LAC (Large Anvil Cell, as Paris-Edinburgh cell) (figure 7c) [START_REF] Ohtani | Multi-anvil apparatus for high pressure X-ray diffraction[END_REF]Besson et al., 1992;[START_REF] Mezouar | Paris-Edinburgh large volume cell coupled with a fast imaging-plate system for structural investigation at high pressure and high temperature[END_REF][START_REF] Godec | Recent developments using the Paris-Edinburgh cell for neutron diffraction at high pressure and high temperature and some applications[END_REF] pressure cells. For low and moderate pressures up to about 3 GPa at ambient temperature, there are also piston-in cylinder clamped cells [START_REF] Mcwhan | Apparatus for neutron diffrction at high pressure[END_REF] and continuously loaded cells [START_REF] Paureau | Nouveau dispositif d'étanchéité pour hautes pressions[END_REF]. From the firsts diffraction measurements under pressure (Barnett et al., 1963;Bassett et al., 1966) to the last ones [START_REF] Basu | Reppearance of ferroelectric soft modes in the paraelectric phase of Pb1-xCaxTiO3 at high pressures : Raman and Xray diffraction studies[END_REF][START_REF] Orosel | Structure of a new highpressure -high temperature modification of antimony(III)oxyde, γ-Sb2O3, from highresolution synchrotron powder diffraction data[END_REF][START_REF] Tulk | Cage occupancies in the high pressure structure H methane hydrate: a neutron diffraction study[END_REF] efforts still continue to increase the available pressure range in both static and dynamic conditions, and also to improve instruments [START_REF] Utsumi | High-pressure science with a multi-anvil apparatus at Spring-8[END_REF] or allow new techniques [START_REF] Bromiley | A portable high-pressure stress cell based on the V7 Paris-Edinburgh apparatus[END_REF].

New instrumental developments at dawn of the third millennium

Using extreme conditions, researchers can consider not only to extend the accessible parameters scale to measure some physical constants or to verify existing theories, but also to point out new behaviours and states which increase our knowledge of materials properties. With progresses in the domains of available sources as 3 rd generation synchrotrons or powerful neutron sources and of new sample environment devices as explained in part 2., extreme conditions measurements have known since 10 years a meteoric rise in all scientific fields for fundamental studies, for examples, of complex fluids, high correlated electrons systems, geological and biological materials at the origin of earth or life. However, will this keen interest continue for a long time? Are technological innovations always possible in that scientific area to build more efficient instruments and devices? Could it be possible to go further the actual limits of high pressures, high and low temperatures, applied magnetic fields? Is it really necessary to cross these limits as numerous material properties are still unexplored? All these questions have one simple answer: yes. Going further in the field of crystallography under extreme conditions is also expanding our knowledge of the world and life by probing the matter at the border of the unknown. Scientist needs everyday new challenges and the one to push back those limits is a necessity for him, but mostly essential for mankind.

In this way, extreme conditions beamline projects have emerged throughout the world at Large scale facilities. It will be pointed out thereafter some examples of instrumental projects which were, or will be, realized to explore materials properties giving access to new nonambient conditions. During the last 10 years, numerous Large scale facilities begun an upgrade of their instruments and installations. This is partly due to the increase demand for high-brilliance X-ray and high-flux neutron beams by user communities requiring always an increase of the instrument performances and of the allocated beamtime. Users want to perform measurements on smaller samples mostly in exotic environments. One of the engineering goals for the upgrades is making available advanced sample environments with extreme conditions on the beamlines whilst fitting in the tight space around the sample also detectors and monitoring equipment. The challenge for the facilities will be to commonly supply extreme environments like pressures up to 100 GPa, low temperature below 1 K, high temperature above 3000 K, high magnetic field up to 50 T in pulsed mode and 30 T in continuous mode. To achieve these goals, facilities must also invest in the engineering of new dedicated instruments and particularly in the development of new detectors which have to be more sensitive, more efficient (especially for high-energy X-rays) and faster (notably in the sub-millisecond range to perform time resolved measurements). Upgraded a Large scale facility is difficult, long and very expensive (table 2). SPring-8 light source will be upgraded in order to advance promising science and to support industrial innovations that will improve our life and contribute to a more sustainable society (SPring-8, 2012). This upgrade programme is principally focused on the construction of a new ring in place of the actual one, retaining the existing insertion device beamlines. In 2019, the new ring (SPring-8 II) would produce 1000 times higher brilliance than the present SPing-8 (new stored current of 300 mA instead of 100 mA presently) with a short X-ray pulse option of around 1 ps. The frontiers of science will be definitely opened. Instruments like BL02B1 (designed for analyses of single crystal structures and for investigations of phase transitions under external fields), BL04B1 (dedicated to high pressures and high temperatures researches in geoscience), BL10XU (designed to perform X-ray diffraction structure analyses under high pressure -300 GPa -and low temperature -10 K -or high temperature -3000 K -) or BL40XU (high flux beamline for time-resolved x-ray diffraction) will benefit of new opportunities.

On April 2010, the US Department of Energy approved the Advanced Photon Source (APS) upgrade project (APS, 2011). The upgrade will provide high energy, high brilliance, short pulse, new and upgraded instruments. The objective is to push the stored current to 150 mA, to upgrade 6 beamlines and to build at least 6 new ones between 2010 and 2018 [START_REF] Mills | The Advanced Photon Source -where we are and where we are going[END_REF]. The new beamlines will especially deal with the overarching theme Real materials under real conditions in real time. It means an easier access for users to high pressures, low/high temperatures and in situ chemical reactions. One frontier for X-ray science in the 21 st century is to combine atomic-level spatial and temporal information, a frontier which would be crossed after the upgrade. For extreme conditions experiments, the APS had plane to upgrade 4 instruments and to build 2 new ones. For examples, the outboard branch of 16-ID will be dedicated to high pressure spectroscopy using sub-μm probes (beam size of 100-500 nm with a flux ≥ 10 12 photons.s -1 at sample position).

In this context, the ESRF (ESRF, 2007) has planned an upgrade programme for its instruments, like ID20 (beamline for magnetic and resonant X-ray scattering investigations under extreme conditions; [START_REF] Paolasini | ID20: a beamline for magnetic and resonant X-ray scattering investigations under extreme conditions[END_REF], ID22 (micro-fluorescence, imaging and diffraction beamline, [START_REF] Martinez-Criado | New cryogenic environment for beamline ID22 at the European Synchrotron Radiation Facility[END_REF], ID24 (Energy dispersive X-ray absorption spectroscopy beamline) or ID27 (high pressure beamline; [START_REF] Mezouar | development of a new state-of-the-art beamline optimized for monochromatic singlecrystal and powder X-ray diffraction under extreme conditions at the ESRF[END_REF]. The ID27 source was upgraded to the most recently developed cryogenic or superconducting undulator to obtain the maximum flux from the ESRF machine after its upgrade. This very high flux beamline will provide new approaches to very challenging problems such as the search for a new metallic superfluid state of matter predicted at pressures above 400 GPa, or to perform ultra-high pressure and temperature X-ray emission spectroscopy in combination with X-ray diffraction in the laser heated diamond anvil cells. In addition to this upgrade, the ESRF will build two new dedicated high pressure beamlines for diamond anvil cells and Paris-Edinburgh research. Time-resolved diffraction is also a priority for the ESRF which propose to build a dedicated beamline covering the timescale from 10 picoseconds to seconds in studies of physical, chemical and biological processes. The 4-circle diffractometer of this beamline will include Laue diffraction and classical diffraction, a very intense focused beam, a new 3 kHz chopper which isolates single pulses of X-rays from the timing modes and a fast readout (0.1 s) Frelon CCD based camera. An other upgrade example concerns the ID20 magnetic scattering beamline with an extend of its experimental conditions in order to provide new research opportunities. It is also planned to build a dedicated station for very low temperatures below 1 K, high magnetic fields up to 17 T and high pressure up to 5 GPa at 1.5 K.

The world's leading spallation neutron source, ISIS, has contributed significantly to many of the major breakthroughs in materials science, physics and chemistry since it was commissioned in 1985. To keep the UK at the forefront of neutron research, it was decided in 2003 to build a second spallation neutron source, ISIS Target Station 2, where three key areas of science are planned to be investigated in priority: soft matter, bio-sciences and advanced materials. ISIS TS-2 project was completed in 2009 on time and to budget. There are currently 7 available instruments (and 4 instruments under building) at ISIS TS-2 which has a capacity for a total of 18 instruments in the future, adding to the 20 instruments already available at ISIS Target Station 1. Concerning the new instruments dedicated to extreme condition measurements, the available WISH instrument [START_REF] Chapon | Wish: the new powder and single crystal magnetic diffractometer on the second target station[END_REF]) is a long-wavelength diffractometer primarily designed for powder diffraction at long d-spacing in magnetic and large unit cell systems, with the option of enabling single-crystal and polarised beam experiments. The WISH sample environments include a dedicated low background cryostat (with ultra-low temperature inserts), a dedicated 13.6 T vertical magnet, but also the standard ISIS furnaces, pressure cells and gas rigs. Futhermore, there is the IMAT instrument [START_REF] Kockelmann | IMAT -an imaging and materials science & engineering facility for TS-II[END_REF], in design phase and operating in 2015, which will be a neutron imaging and diffraction instrument for materials science, materials processing and engineering. IMAT will offer a unique combination of imaging and spatially resolved diffraction modes for tomography-driven diffraction: residual stresses inside engineering-sized samples can be more effectively analysed if the diffraction scans are guided by radiographic data. Finally, the future EXEED instrument [START_REF] Mcmillan | EXEED -an extreme sample environment diffractometer[END_REF] will be a neutron time-of-flight diffractometer optimised for extreme environment studies of materials which will complement the capabilities of WISH on Target Station 2 and PEARL on Target Station 1. The experiments will be performed at high pressure, above 50 GPa using diamond anvil cells, under combined very low temperatures (mK), very high temperatures (2000 K, including laser heating in cells) or intense magnetic field, up to 10 T.

Finally, the neutron reactor installations and instruments at the ILL are undergoing a modernisation phase called the Millennium programme 2001-2014, 30 years after the first experiments in 1972. During the first phase (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) of the ILL upgrade, significant advances had been provided. The efficiency of the instruments has been boosted by a factor of 19. Moreover, 6 new instruments were built and 8 others were upgraded. In the second phase (2008)(2009)(2010)(2011)(2012)(2013)(2014), 7 new instruments are planned to be built and 4 actual instruments upgraded. For extreme conditions experiments, it was planned a new neutron diffractometer XtremeD [START_REF] Rodriguez-Velamazan | XtremeD -a new neutron diffractometer for high pressures and magnetic fields at ILL developed by Spain[END_REF]. This instrument will be optimized for high pressure, up to 30 GPa, and high magnetic field, up to 15 T in continuous mode, studies for both single crystals and powders. The diffractometer, mainly dedicated to study molecular chemistry and magnetism under extreme conditions, will include a large 2D position-sensitive detector and a radial oscillating collimator to suppress background.

It is clear, with all the above mentioned new instrumental developments at almost worldwide Large scale facilities, that the challenge for crystallography in the next years is to perform non-equilibrium and extreme condition measurements. All the domains of sciences are concerned in order to increase the knowledge and the understanding of materials behaviours and properties under external perturbations. It can be asserted without ambiguity that the present century is the one of crystallography under extreme conditions.

Conclusion

Since the first crystallographic experiment at the beginning of the 20 th century, scientists have always whished to push back frontiers of measurements. Obtaining the structureproperties relationships in static and dynamic modes is one fundamental goal to increase our knowledge of materials and verify existing theories. This was largely made possible by the arrival of modern Large scale facilities allowing material investigations under extreme conditions. State-of-the-art crystallographic measurements discussed in this review and recent upgrades achieved at most facilities augur that limits will be increasingly crossed in the next years, giving rise to unsuspected highlight researches in all domains of sciences.

The considered perspectives will offer new opportunities in crystallography.

Figure 1 .

 1 Figure 1. Publication number related to neutron or X-ray diffraction experiments under extreme conditions of temperature and pressure during the period 2005-2011 (Web of Science, March 2012).

Figure 2 .

 2 Figure 2. The neutron source of the Institut Laue-Langevin (ILL) and the European Synchrotron Radiation Facility (ESRF) in Grenoble, France (credit: V. Legrand).

Figure 3 .

 3 Figure 3. The high pressure cell developed by Guionneau et al. (2004): (a) view of the high pressure cell mounted on a 4-circle diffractometer equipped with low temperature device and CCD area detector; (b) comparison of the spatial occupied volume when a conventional goniometer head is used instead of the pressure cell. (Reprinted by permission from IOP Publishing Ltd).

Figure 4 .

 4 Figure 4. (a) Schematic experimental setup of the polarized neutron diffractometer; in the insert, the sample holder allowing light irradiation (Reprinted figure with permission from [Goujon et al., Phys. Rev. B 67, 220401(R) 2003]. Copyright 2012 by the American Physical Society). (b) The photocrystallographic experiment used by Legrand et al. (2007) showing the 6-circle diffractometer (BM01 beamline, ESRF -France) equipped with a large CCD area detector, an open flow He cryosystem and a He-Ne laser; inserts show the measured single-crystal (b1) without and (b2) with laser irradiation.Real time investigation of temporally varying molecular structures during chemical reactions and phase transitions is a great challenge due to their ultrashort time scales.Collet et al. shown that it is possible to track complex reactions by time-resolved X-ray diffraction to temporal and spatial resolutions of 100 ps and 0.01 Å. Kind measurements give access to molecular movies during the transformation of matter induced by light irradiation. The authors succeed to describe for the first time the different steps of the dynamical process of several molecular materials and give their thought for the future: "these results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography".

Figure 5 .

 5 Figure 5. Schematic views of the experimental setup for real-time studies (a) of metal foam formation (Banhardt et al., 2001) and (b) of structural properties of spider silk[START_REF] Riekel | Spider silk fibre extrusion: combined wide-and small-angle X-ray microdiffraction experiments[END_REF].Radiation Facility, France) and SPRING-8 (Super Photon ring-8 GeV, Japan), teams uncovered subtle details of how cyan fluorescent proteins store incoming energy and retransmit it as fluorescent light[START_REF] Goedhart | Structure-guided evolution of cyan fluorescent proteins towards a quantum yield of 93%[END_REF], or investigated structure-properties relationships of biological mesocrystal in the adult sea urchin spin(Seto et al., 2012). Combination of in situ synchrotron X-ray absorption techniques and molecular dynamics simulations were used to determine the density range of primitive lunar melts at pressures equivalent to those in the lunar interior (4.5 GPa and 1800 K)[START_REF] Van Kan Parker | Neutral buoyancy of titanium-rich melts in the deep lunar interior[END_REF]: these extreme conditions were generated with tiny samples heated thanks to high electric current while squashing them in a press (figure6); knowing the attenuation of the synchrotron X-ray beam through both the solid and molten parts of the sample, the density at high pressure and high temperature could be measured. High pressure in situ X-ray diffraction and specific volume measurements on isotactic poly(4-methyl-1-pentene) melt have uncovered abrupt changes in the pressure dependence of microscopic structure as well as that of macroscopic density[START_REF] Chiba | Pressure-induced structural change of intermediaterange order in poly(4-methyl-1-pentene) melt[END_REF]. It was proved another time that pressure has an essential role in the production and control of superconductivity[START_REF] Sun | Re-emerging superconductivity at 48 kelvin in iron chalcogenides[END_REF]: it is reported that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges at a critical temperature Tc reaching 48.0 K and above 11.5 GPa, after the Tc drops from the first maximum of 32 K at 1 GPa. Metallic liquid silicon at 1787 K was investigated using X-ray scattering[START_REF] Okada | Persistence of covalent bonding in liquid silicon probed by inelastic x-ray scattering[END_REF]: the results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled states. To finish, it can be pointed out the crystallographic structure refinement of a protein, PthXo1 (which is a transcription activator-like, TAL, effector), encoded by an important group of harmful plant pathogens[START_REF] Mak | The crystal structure of TAL effector PthXo1 bound to its DNA target[END_REF]: understanding DNA recognition by TAL effectors may facilitate rational design of DNA-binding proteins with biotechnological applications.

Figure 6 .

 6 Figure 6. Drawing of the high-pressure cell assembly for the synchrotron X-ray experiments used to study the density range of primitive lunar melts (Van Kan Parker et al., 2012). The artificial moon rock samples (orange) were placed inside the ring-shaped, natural diamond sample holder (grey) which in turn was surrounded by a large, disk-shaped container (red). (Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience, copyright 2012)

Figure 7 .

 7 Figure 7. (a) Details of the high-intensity 2-axis diffractometer D20 (Hansen et al., 2008) at the Institut Laue-Langevin (France) with a special version of the helium closed cycle refrigerator (Displex) allowing the use of (c) Paris-Edinburgh high pressure cell. Cryostats are now fully self-governed (b) with automated cold valve, pressure transducer which monitors the exhausting helium gas pressure and temperature controller which adjust the liquid helium flow to the set temperature value.

  concerns the evolution of the cumulative number (neutron and X-ray) of publications during the considered period: about 300 supplementary published papers per year appear between 2005 and 2009, but in the years 2009-2011, a distinct decrease of this number is noticed. This effect is directly related to the number of synchrotron radiation facilities over the world. One can count about 69 particle accelerators and accelerator laboratories in 2005 and about 76 in 2012 (including about 40 synchrotrons in 2011), with most of the new facilities operative in2006-2008.

Table 1 .

 1 The proposed examples are a non-exhaustive list of the actual developments in crystallography under extreme conditions. These examples are chosen on the most powerful Large scale facilities around the world in terms of the most important parameters (energy, current...): SPring-8 (Japan), APS (USA) and ESRF (France) are the chosen synchrotron sources (table 1); ISIS (neutron spallation source, UK) and the ILL (neutron reactor, France) are the chosen neutron facilities. The most important parameters of the 10 world-wide most powerful 3 rd generation light sources available in 2012.

	Name	Location	Energy (GeV)	Perimeter (m)	Current (mA)	Emittance (mrad)	Number of straights
	SPRING-8	Japan	8	1436	100	3	48
	APS	USA	7	1060	100	3	40
	ESRF	France	6	844	200	3.8	32
	SSRF	China	3.5	396	300	4.8	20
	ALBA	Spain	3	268.8	250	3.7	24
	Australian Synchrotron	Australia	3	216	200	8.6	14
	DIAMOND	UK	3	560	300	2.7	24
	SPEARS3	USA	3	240	500	18	18
	CLS	Canada	2.9	171	500	18	15
	SOLEIL	France	2.85	354	500	3.1	24

Table 2 .

 2 Estimated time and total cost of the upgrade or construction of some synchrotron and neutron facilities around the world.

	Name	Type of facilities		Period	Total cost
	SPRING-8	synchrotron	upgrade	2012-2019	375 M€
	APS	synchrotron	upgrade	2010-2018	275 M€
	ESRF	synchrotron	upgrade	2008-2017	290 M€
	DIAMOND	synchrotron	construction	2002-2007	460 M€
	SOLEIL	synchrotron	construction	2000-2006	403 M€
	ILL	neutron reactor	upgrade	2001-2014	85 M€
	ISIS-TS2	neutron spallation source	construction	2003-2009	200 M€

Author details

Legrand Vincent LUNAM Université -Université de Nantes -Ecole Centrale Nantes, Institut de Recherche en Génie Civil et Mécanique (GeM), France vincent.legrand@univ-nantes.fr


Motaz Saad

David Langlois

Smaïli Kamel

Sm

Building and Modelling Multilingual Subjective Corpora

Keywords: subjectivity analysis, cross-lingual annotation, language modelling

Building multilingual opinionated models requires multilingual corpora annotated with opinion labels. Unfortunately, such kind of corpora are rare. We consider opinions in this work as subjective or objective. In this paper, we introduce an annotation method that can be reliably transferred across topic domains and across languages. The method starts by building a classifier that annotates sentences into subjective/objective label using a training data from "movie reviews" domain which is in English language. The annotation can be transferred to another language by classifying English sentences in parallel corpora and transferring the same annotation to the same sentences of the other language. We also shed the light on the link between opinion mining and statistical language modelling, and how such corpora are useful for domain specific language modelling. We show the distinction between subjective and objective sentences which tends to be stable across domains and languages. Our experiments show that language models trained on objective (respectively subjective) corpus lead to better perplexities on objective (respectively subjective) test.

Introduction

Opinion mining or sentiment analysis determines the position of a writer with respect to a topic [START_REF] Pang | Opinion mining and sentiment analysis[END_REF]. One of opinion mining tasks is subjectivity identification, which is to classify a given text into subjective or objective [START_REF] Pang | Opinion mining and sentiment analysis[END_REF]. A subjective or opinionated text conveys the opinions of the writer, while an objective text represents facts. For example, the following statement: "I think it is cold" is subjective because it conveys the opinion of a specific person. This opinion can be different from a person to another (a person lives in Mediterranean climate vs. a person lives in tundra climate). On the other hand, the following statement: "the temperature is (-15)" is objective because it represents a fact. However, for controversial statements, humans may disagree between themselves about the subjectivity of these statements. This is why automatic subjective analysis is a challenging task. According to the opinion definition above, subjective and objective texts are very different in terms of writing style. Consequently, language models built from subjective texts may differ from ones built from objective texts. Therefore, opinion language models built from opinion corpora can be useful for certain domains and applications because they are better and more precise than non-domain specific language models. The distinction between multilingual subjective/objective text is investigated in the experiments of this work. One of the wide applications of language modelling is text/speech machine translation, which require multilingual corpora. Since subjective/objective texts are distinct as mentioned earlier, then building multilingual subjective/objective corpora is quite useful for building machine translation systems in this domain. Building opinionated language models requires corpus of both modalities (subjective and objective). So, Section 2. describes an automatic method (Saad et al., 2013a) to annotate multilingual parallel corpus with opinion labels. The parallel corpus represents different domain topics (United Nations resolutions, newspapers, and talks). This work focus on English/Arabic parallel corpus. Annotating this languages pair can be useful because such corpus is not available. Moreover, cross-lingual annotation can be used in information extraction in low-resourced languages. Then, Section 3. shows how statistical language models trained on opinion corpora obtain better performance on test corpora. In addition, the link between subjectivity annotation and language modelling is studied in this section.

Subjective Corpora Annotation

This section describes the method of automatic subjective annotation of parallel corpora with opinion labels. The method requires a pre-annotated monolingual corpus to automatically annotate a parallel corpus. Before describing the method, these corpora are described here.

Corpora Description

The pre-annotated monolingual corpus is a collection of movie reviews. It is in English language and composed of 5,000 subjective and 5,000 objective sentences. The corpus was collected and annotated by [START_REF] Pang | A sentimental education: Sentiment analysis using subjectivity summarization based on minimum cuts[END_REF]. Subjective reviews were obtained from Rotten Tomatoes website www.rottentomatoes.com, while Objective reviews were obtained from IMDb plot summaries www. imdb.com. A Sample of subjective/objective sentences are presented in Tables 1 and2. Regarding parallel corpus, it is described in pretty much sucks, but has a funny moment or two. smart and alert, thirteen conversations about one thing is a small gem works both as an engaging drama and an incisive look at the difficulties facing native Americans. even a hardened voyeur would require the patience of job to get through this interminable, shapeless documentary about the swinging subculture. when perry fists a bull at the Moore farm, it's only a matter of time before he gets the upper hand in matters of the heart.

Table 1: A sample of subjective reviews of movie corpus spurning her mother's insistence that she get on with her life, Mary is thrown out of the house, rejected by Joe, and expelled from school as she grows larger with child. Amitabh can't believe the board of directors and his mind is filled with revenge and what better revenge than robbing the bank himself, ironic as it may sound. the movie begins in the past where a young boy named Sam attempts to save Celebi from a Table 2: A sample of objective reviews of movie corpus AFP 1 , ANN 2 , and ASB 3 news corpora were provided by [START_REF] Ma | Arabic newswire english translation collection[END_REF]. Medar news corpus was provided by www.medar.info. NIST 4 news corpus was provided by (NIST, 2010). UN corpus is resolutions of United Nation and was provided by [START_REF] Rafalovitch | United nations general assembly resolutions: A six-language parallel corpus[END_REF]. TED 5 talks corpus was provided by [START_REF] Cettolo | Wit 3 : Web inventory of transcribed and translated talks[END_REF]. Tatoeba 6 parallel sentences was provided by [START_REF] Tiedemann | Parallel data, tools and interfaces in opus[END_REF].

Corpus |S| |W | |V | English Arabic English Arabic AFP 4K 140K 114K 17K 25K ANN 10K 387K 288K 39K 63K ASB 4K 187K 139K 21K 34K TED 88K 1.9M 1.6M 88K 182K UN 61K 2.8M 2.4M 42K 77K Medar 13K 398K 382K 43K 71K NIST 2K 85K 64K 15K 22K Tatoeba 1K 17K 13K 4K 6K Total 183K 5.9M 5M 269K 480K
Table 3: Parallel Corpora

Automatic Annotation

The method (Saad et al., 2013a) automatically annotates English/Arabic parallel corpus with the help of English corpus which is composed of movie reviews and pre-annotated First, a Naive Bayes classifier is built using the movie corpus, then the classifier is used to automatically annotate English sentences of parallel corpus. Then, the same opinion label is assigned to corresponding Arabic sentences of parallel corpus as illustrated in Figure 1. Corpora which are denoted in Figure 1 are described in Table 4. The Naive Bayes classifier is trained (step 2 in Figure 1) on 3-grams features of each sentence of movie-p1 corpus. So Naive Bayes classifier is used for automatic annotation task as described in our method steps in Figure 1. In sake of robustness, sentence annotation (steps 3 and 4 in Figure 1) is made only if the probability of the highest class label is above 0.8.

Annotation Results

The annotation method is applied on the parallel corpus, and results are presented in Table 6. The table presents percentage of annotated sentences with respect to the corpus, the table also shows the class distribution of annotated sentences for each corpus (steps 3 and 4 of Figure 1). As can be seen in the mains, a secondary evaluation is done on a sample of 330 sentences selected randomly and annotated by the first author of this paper (step 7 in Figure 1). News sentences are selected from AFP, ANN, ASB, Medar, and NIST corpora, while non-news sentences are selected from TED, UN, Tatoeba corpora. This manually annotated material is described in Table 8. Then, the classifier built from movie reviews is tested on these sentences (step 8 in Figure 1). The results of the classifier on these sentences are described in

Statistical Language Modelling

In this section, three experiments are conducted in order to show the distinction of subjective/objective text in terms of statistical language modelling. The first one inspects the perplexity of opinionated language models on subjective and objective texts. In the second experiment, we investigate if opinionated language models built from parallel corpora fit the movie domain. In this experience, we compare opinion language models built from movie review domain with language models built from parallel-opinion which is composed of UN resolutions, newspapers, and talks. The third experiment inspects the perplexity of opinionated language models on comparable corpora to explore their subjectivity.

Opinionated Language Models

Regarding to the first experiment, we show that dividing our corpus into two parts (subjective and objective) can be useful for language modelling in terms of perplexity. For that, we measure the perplexity of language models trained on subjective corpus and objective corpus of "parallelopinion".

Words Vocabulary English 4.4M 138K Arabic 3.7M 264K

Table 10: parallel-opinion information

The "parallel-opinion" corpus (the output of steps 3 and 4 of Figure 1) is divided into subjective and objective parts according to the automatic labelling. Each one of them is split into 90% for training and 10% for testing. It can be noted that the perplexity for Arabic is larger than English. According to a study about statistical language modelling for many languages conducted by [START_REF] Meftouh | Modeling Arabic Language using statistical methods[END_REF], Arabic is one of the languages that has high perplexity, because Arabic is agglutinative language and has rich morphology. Second, it can be noted that SLM1/OLM1 models fit better to subjective/objective texts respectively. Indeed, subjective (respectively objective) language models models applied to objective (respectively subjective) test lead to bad performance. It can be concluded from the last result that subjective and objective corpora are very different.

Testing Language Models

In the second experiment, we inspect the opinion language models built from different topic domain. In this experiment, we test movie review corpus with an opinion language models built from opinion parallel corpora (parallelopinion) which come from different domains (UN resolutions, newspapers, talks). Symmetrically, we test the parallel opinion corpus with opinion language models built from movie corpus. We also aim in this experiment to validate our annotation by testing language models on manually annotated corpus (movie corpus). If subjective/objective models fit better to subjective/objective texts respectively, then our annotation of parallel-opinion is good and reliable.

We also aim to inspect if the difference between objective and subjective texts is repeated across various topics (movie reviews vs. news).

In this experiment, we have four language models: SLM1/OLM1 of previous section which are trained on subjective/objective parts of parallel-opinion, and SLM2/OLM2 which are trained on subjective/objective parts of movie corpus. All models are built using 10K most frequent words vocabulary. Table 11 gives the perplexity of testing SLM1/OLM1 on movie and parallel-opinion test corpus, while Table 12 gives the perplexity of testing SLM2/OLM2 on movie and parallel-opinion test corpus. First, we note from the results presented in Tables 11 and12 Actually, this comes from the UN corpus, mostly objective, in which numerous sentences contain common parts (for example "taking note of the outcome of the. . . "); by chance, these common parts are distributed between training and test. We also note that SLM1/OLM1 fit better to subjective/objective parts of movie corpus respectively, and SLM2/OLM2 fit better to subjective/objective parts of parallel-opinion respectively. Second, it can be noted that subjective test corpus (respectively objective) does not fit to objective language model (respectively subjective). It can be concluded that language models built from opinion corpora can fit for movie review, namely, the distinction of subjective/objective text is stable across different topic. Additionally, it can be concluded that the annotation of parallel-opinion is good and reliable, because subjective (respectively objective) models built from parallel-opinion have better perplexity on subjective (respectively objective) corpus manually annotated (the movie corpus). Finally, this experiment confirms the previous results in Section 3.1. which concludes that subjective and objective text are distinct in terms of writing style.

Testing Comparable Corpora

Regarding the third experiment, opinionated language models built using parallel-opinion are inspected on comparable corpora collected by (Saad et al., 2013b). We use two comparable corpus: AFEWC and eNews. AFEWC is collected from Wikipedia, and eNews is collected from euro-news website http://www.euronews. com. These corpora are composed of English/Arabic comparable articles aligned at article level. We aim in this experiment to explore the subjectivity of AFEWC and eNews using two ways: annotating with the Naive Bayes classifier, and testing with language models. We also want to show whether the results of annotation and language model tests accord to each others or not. We take a random subset of sentences composed of these corpora for our experiments. To confirm this objectivity of the comparable corpora, we test them with opinion language models SLM1 and OLM1 built from opinion corpora (parallel-opinion). Results of these tests are presented in Figures 4 and5. The Figures present the perplexity of SLM1/OLM1 tested on AFEWC and eNews comparable corpora. We first note that the perplexity values for all test are high. This is maybe because language models are built from different corpus and domain than the test one. We also note from the figures that OLM1 fits better to English/Arabic AFEWC and eNews comparable corpora (has lower perplexity) than SLM1. Classification results and language models tests in the Table 13 and the Figures 4 and 5 confirm the distinction of subjective/objective text. Their results accord to each others. This leads us to confirm that comparable corpora eNews and AFEWC are more objective.

Conclusion

We have presented a method for cross-lingual annotation with subjective/objective labels. We tested successfully our classifier with manually labelled data. By training and testing statistical language models, we also showed that subjective/objective texts are statistically different in terms of writing style. We tested several models and corpora of different genres and we always obtained the same conclusion: subjective/objective texts are distinct. Moreover, results of the classifier and of objective/subjective language models highlighted that our comparable corpora (AFEWC and eNews: data extracted from Wikipedia and news website in English and Arabic) are more objective than subjective.

In the future, this work can be extended to polarity in order to go towards a review system which is able to retrieve multilingual articles about a topic, and to classify these articles in terms of opinion.

Figure 1 :

 1 Figure 1: Parallel corpus annotation and evaluation

Table 3 ,

 3 where |S| is the number of sentences, |W | is the number of words, and |V | is the vocabulary size. The corpora are collected from different sources and represent different genres of text.

Table 4 :

 4 Corpora description

Table 5 :

 5 We used 90% of movie-p1 for training and kept 10% for testing. A sample of subjective/objective n-gram features subjective objective . . . entertaining . . . . . . the story of . . . . . . interesting . . . . . . order . . . . . . entertainment . . . . . . decides . . . . . . you are . . . . . . discover . . . . . . but it . . . . . . she is . . . A sample of subjective/objective n-gram features of the classifier of the classifier are presented in Table5. As can be noted from the table, subjective n-gram features are expressions that the reviewer uses to express what he/she thinks about the movie, while objective n-gram features are used by the reviewer to describe the events of the movie. The accuracy of this classifier on the 10% of movie-p1 is 0.926 and objective F-Measure (F 1) is 0.926 and subjective F 1 is 0.927.

	. . . me . . .	. . . with her . . .
	. . . fans . . .	. . . led . . .
	. . . it is not . . .	. . . kill . . .
	. . . is so . . .	. . . his family . . .
	. . . I . . .	. . . to kill . . .
	. . . if it . . .	. . . one day . . .

Table 7 :

 7 table, 81% of all sentences are annotated, 45% of these are subjective and 55% are objective. Evaluation on movie-p2 (step 6)

		Annotated
	Corpus	sentences Subjective Objective
	AFP	90.6%	9.4%	90.6%
	ANN	89.9%	18.6%	81.4%
	ASB	91.7%	17.8%	82.2%
	TED	88.7%	74.8%	25.2%
	UN	89.6%	15.7%	84.3%
	Medar	89.6%	25.8%	74.2%
	NIST	79.4%	20.6%	79.4%
	Tatoeba	86.4%	59.3%	40.7%
	Total	81%	45%	55%
	Table 6: Parallel corpus annotation
	A preliminary evaluation (step 6 of Figure 1) of the annota-
	tion process consists in: (1) training Naive Bayes classifiers

using parallel-opinion corpus. (2) testing the obtained classifiers on movie-p2. Results are presented in Table

7

. The classification accuracy is 0.79, subjective F 1 is 0.81, and objective F 1 is 0.74. To make sure that the classifier built from movie domain can correctly identify the subjectivity of sentences of parallel corpus which is composed of news and non-news do-

Table 9 .

 9 The classification accuracy of news sentences is 0.718, subjective F 1 is 0.717, and objective F 1 is 0.720, while the classification accuracy of non-news sentences is 0.658, subjective F 1 is 0.667, and objective F 1 is 0.649. As can be seen from the results, the classifier built from movie reviews can classify sentences from other domains: the classifier can detect subjective and objective sentences from news corpus and from non-news corpus. In other words, the classifier does not modelize the genres of the corpus (news/non-news) but the objectivity/subjectivity. Another validation by language models is presented in the next section.

	Corpus	Subjective Objective
	News	112	101
	Non-News	60	57
	Total	330
	Table 8: The sample of parallel sentences annotated by a
	human (step 7)
	Corpus	Accuracy subjective F 1 objective F 1
	News	0.718	0.717	0.720
	Non-News	0.658	0.667	0.649
	Table 9: Evaluation on parallel corpus annotated by a hu-
	man (step 8)

Table 10

 10 The vocabulary for SLM1 and OLM1 is made up of the union of words of STrC and OTrC, and composed at most of of 138K English words and 264K Arabic words as presented in Table10. This work has been done for English and Arabic languages.

	describes the number of words and the vocabulary size
	of the training parts of the corpus. Therefore, we have a
	subjective training corpus (STrC), an objective training cor-
	pus (OTrC), a subjective test corpus (STeC) and an objec-
	tive test corpus (OTeC). A 3-gram language model is built
	on STrC (called SLM1) and another one on OTrC (called
	OLM1). SRILM toolkit (Stolcke and others, 2002) is used
	to build language models, with Kneser-Ney discounting
	method.

tive STeC and objective OTeC test corpora for several vocabulary sizes. Each vocabulary is made up of the most frequent words. For each figure, the language model is tested on Arabic/English subjective/objective test texts, so we have four performance curves. The numbers on the right side of the figures present the value of the last point of each curve.

Table 12 :

 12 Testing models built from movie corpus are higher when models and test corpora are from different domain topics (movie reviews vs. UN resolutions, newspapers, talks). It can be noted also that the perplexity of OLM1 for objective test part of parallel-opinion is low.

	that the perplexities

Table 13 :

 13 Subjective/Objective sentences distribution of the subset of the comparable corpora (30K words)We annotate automatically each sentence in AFEWC and eNews with subjective/objective labels using the Naive Bayes classifier built using parallel-opinion. Subjective/objective distribution of eNews and AFEWC comparable corpora is presented in Table13. The table presents the percentage of subjective and objective sentences in the subset of the comparable corpora. As can be seen from the table, for both comparable corpora, and for both languages, there are more objective sentences than subjective ones. This is coherent because Wikipedia and news mostly tend to be objective: they describe facts.

	The subset size is 30K words.
	Subjective Objective
	AFEWC English	24%	76%
	AFEWC Arabic	18%	82%
	eNews English	23%	77%
	eNews Arabic	11%	89%