I. INTRODUCTION
X-ray diffraction technique is widely used to obtain structure of crystalline materials under various conditions of temperature, pressure, electric and magnetic field, etc. [1]. As a phase-I beamline at Shanghai Synchrotron Radiation Facility (SSRF), BL14B1 is dedicated to X-ray diffraction studies on materials science, condensed matter physics and other relevant fields. Polycrystalline powder, surface and interface of film/ultra-thin films, and microstructure of nano-materials have been the main research objectives.
Since its commissioning to users in 2009, BL14B1 has allocated user beamtime of over 20000 h. Over 500 user proposals have been executed. The beamline and the experimental end-station were upgraded for several times to facilitate the user experimentss and to become more convenient to operate. In this paper, the beamline optics, the experimental end-station, the experimental methods, and its commissioning results are reported.
II. SOURCE AND BEAMLINE OPTICS
SSRF is composed of a 3.5 GeV electron storage ring, 3.5 GeV booster and a 150 MeV linac. The beam current in the storage ring is 240 mA. Parameters of the bending magnet, as source of beamline BL14B1, are given in Table 1, and layout of the beamline is shown in Fig. 1.
| Beam energy (GeV) | Natural emittance (nm rad) | (Coupling) | Beam current (mA) | Bending field (T) | Beam size and emittance | |||
|---|---|---|---|---|---|---|---|---|
| σx(μm) | σy(μm) | |||||||
| 3.5 | 3.9 | 1.0% | 240 | 1.27 | 70 | 22 | 114 | 1.97 |
The BL14B1 beamline has three critical components: a collimating mirror, a sagittally focused double crystal monochromator, and a focusing mirror, which are 21, 24 and 30 m away from the source, respectively. The collimating mirror is used to collimate the beam in vertical direction into parallel light, improving the energy resolution. The monochromator consists of a flat water-cooled first crystal and a dynamically bent sagittal crystal to focus the beam in horizontal direction to the sample point. The focusing mirror further focalize the beam in vertical direction to the sample point.
Angular acceptance of the beamline, determined mainly by the water-cooling quad-blade slit (Slit1) in front of the collimating mirror, is 2.5 mrad and 0.12 mrad in horizontal and vertical directions, respectively. Energy range of the beamline is mainly determined by the grazing incidence angle of the collimating mirror. The collimating mirror and focusing mirror are half Rh-coated Si-based cylindrically bend mirror. Grazing incidence angle of 2.8 mrad is used, hence a cutting-off energy of about 22 keV. Reflection of the collimating and focusing mirrors can further suppress higher order harmonics significantly.
Three beryllium windows are placed after the collimating mirror, monochromator and focusing mirror, respectively, to maintain different vacuum levels. A beam block is installed behind the monochromator to block the white light and bremsstrahlung radiation reflected by collimating mirror. Two monochromatic slits are positioned at the back of the focusing mirror (Slit2) and the end of beamline (Slit3) to stop the stray light and improve angular resolution. The first crystal of monochromator absorbs almost all the radiation after collimating mirror. Power density of the crystal reaches 0.4 W/mm2, hence the need of water cooling.
Performance of the beamline is evaluated by measuring absorption and diffraction patterns from standard inorganic powder samples. The energy range is examined at 100 mA and 200 mA using the K-edge absorption spectrum of CaO (4.0385 keV) and RuCl3 (22.117 keV). The energy resolution at 8.9 keV was estimated at 2.47×10-4 by width of the K-edge absorption spectrum of Cu at 100 mA and 200 mA. This is a popular and easy method. However, the precision and range are limited by the natural width. So, 10 keV XRD pattern of NIST SRM660a LaB6 standard powder was used to examine the energy resolution. The instrumental resolution function can be figured out from the diffraction peaks. The measurement result is 1.91×10-4 at 200 mA. Spot size and angular divergence at focus point is measured with a quad-blade slit and an ionization chamber at 100 mA and 200 mA. Spot size is defined as FWHM (Full width at half maximum) of intensity distribution profile, which can be obtained by scanning the slit horizontally and vertically. Using the measurement results of spot size at two positions of the beamline, the angular divergence can be calculated. The spot size is 0.21 mm (H)×0.13 mm (V), while the angular divergence is 1.284×0.090 mrad2 at 200 mA for the focus mode. Photon flux was measured using an ionization chamber, according to the formula I=Eeη N/ε, where I is the intensity of current, E is the X-ray energy, N is the number of photons, e is the electron charge, and η is the absorption efficiency. Photon flux of 4.43×1011 phs/s at 10 keV was obtained. Table 2 lists the main specifications of the beamline.
| Parameters | Design specifications | Measurement results | |
|---|---|---|---|
| Energy range(keV) | 4–22 | 4–22 | |
| Energy resolution (ΔE/E) | ࣘ 2×10-4 | 1.91×10-4 | |
| Focus Mode | Photon Flux (phs/s@10 keV@300 mA) | ≥ 2×1011 | 4.43×1011 |
| Spot Size (H × V, mm2) | ࣘ0.4×0.4 | 0.21×0.13 | |
| Divergence (H × V, mrad2) | ࣘ2.5×0.2 | 1.284×0.090 | |
| Non-focus Mode | Photon Flux (phs/s@10 keV@300 mA) | ≥2×1011 | 4.43×1011 |
| Spot Size (H × V, mm2) | ࣘ0.4×3.0 | 0.21×1.14 | |
| Divergence (H × V, mrad2) | ࣘ2.5×0.02 | 1.276×0.013 |
III. EXPERIMENTAL END-STATION CONFIGURATION
As shown in Fig. 2, a Huber 5021 six-cycle diffractometer is equipped in the experimental end-station to enable multi-dimensional orientations of samples. Ceramic, crystal, metallic, polymeric and other samples can be effectively measured.
For data collection, a Bede point detector and image plate detector are used to allow the measurements in high-resolution mode or fast mode. In 2012, the Mar345 image plate detector was replaced by an Mar225 CCD and related accessories, such as shutter, software, etc. The new detector is of sub-second time resolution and much faster readout time. A Si microstrip detector (Mythen 1K) is also equipped, together with the data control software and acquisition method. The photon flux is monitored using an on-line ionization chamber. In addition, a Si(111) crystal analyzer improves the angle resolution greatly.
A prominent advantage of the synchrotron-based techniques is the very high photon flux, which is necessary for in situ studies, unlike conventional XRD facilities. The in situ experiments can provide real time studies of the sample at non-ambient condition, which otherwise might change. The in situ experiments in MRI TC-radiation or Linkam high temperature chambers (Fig. 3) provide real time studies of samples in non-ambient condition.
The MRI TC-radiation high temperature chamber (Fig. 3(a)) is mainly used in inert gas environment or in vacuum. A sample can be heated to 800 ℃ under inert gas protection by a direct Mo strip heater. Using a radiation heater under vacuum condition, the temperature can reach 1100 ℃,thus being suitable for measuring powder and metallic samples. The samples are loaded into an Al2O3 crucible. The diffraction experiments are carried out in reflection mode. Typical size of a metallic sample is 10 mm×10 mm×1.5 mm. A K-type thermocouple is soldered to the sample surface to measure the temperature. The MRI chamber can be evacuated to about 10-2 Pa. Fig. 3(b) shows the Linkam high-T chamber. Is can achieve up to 1500 ℃ in vacuum and is suitable for XRD experiment in transmission mode, for materials samples of alloy, polymeric film, etc.
In situ electric field device and in situ high/low-temperature chamber for thin films are available to users. Besides, the beamline has a supporting laboratory equipped with fume hood, glove box, vacuum oven, balance, etc., for sample preparation by the users.
IV. EXPERIMENTS METHODS AND EXAMPLES
The experimental techniques available include high-resolution powder diffraction (HRXRD), X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD), reciprocal space mappings (RSMs), atomic pair distribution function analysis (PDF), and total reflection X-ray fluorescence spectroscopy (TXRF).
A. High-resolution powder diffraction
This method can be used for measuring samples of heterogeneous catalysts, compounds for hydrogen storage, electrode materials for lithium battery applications, ferro-electric compounds, thermo-electric compounds, super- conductor materials, etc. Because of the monochromatic high flux and highly collimated beams, the main advantages of synchrotron-based powder XRD against conventional XRD include no K2 diffraction peaks, higher signal/noise ratio and much better angular resolution.
Professor Chen’s group used experimental data collected at BL14B1 to determine the crystal structure by powder XRD pattern indexing, crystal lattice determination, space group analysis and lattice constant calculation, overlapped diffraction peaks de-convolution, determining the intensity of each diffraction peak and calculating related structure amplitude, etc [2]. The significance of their work is synthesis and characterization of a new hydrogen storage material, light metal ammonia borane, expands the research field of B-N related chemicals, and provides theoretical basis and technical support for new hydrogen storage materials.
B. X-ray reflectivity
XRR technique uses low angle (generally 2θ<10°) θ 2θ scan mode, known also as small angle diffraction. The interface reflection occurs due to different X-ray refractive index of the medium layers. XRR signal results from periodic diffraction of multilayer. Multilayer iterative calculation method of XRR was first given by L. G. Parratt [3]. The matrix calculation method of XRR for non-ideal interface was deduced later [4, 5]. XRR is widely applied to samples of film/ultra-thin film and multilayer, and the electron density, thickness, roughness of films, superlattices, etc. can be obtained. Thickness and optical constants of solgel derived polyvinylpyrrolidone/ZrO2 films were determined by Professor Xu Yao’s group using XRR [6].
C. Grazing incidence X-ray diffraction
GIXRD is widely used to study super-conductivity, ferro-electric thin film, thermo-electric thin film, semiconducting thin film, wet-epitaxially-grown organic film, polymer thin film transistors, etc. Thin-films as thin as one atomic layer can be grown on single crystal substrates, and analyzed by GIXRD. With a very small incidence angle, one can expect to have much higher cross-section for detection, thus to increase the sample signal and greatly suppress the signal from substrate.
While using the point detector, one can collect data either in-plane or out-plane. Fixing the incidence angle (eg, θ =0.2°), one can scan the 2θ arm horizontally and collect the in plane diffraction signal from the sample. This signal is the essential diffraction signal from the sample, which displays long range ordering perpendicular to the sample surface. One can collect the out-of-plane signal by scanning the 2θ arm vertically. The sample signal, which displays long-range ordering parallel to the normal of the sample surface, can be studied. Using the 2D image plate detector, the in-plane and out-plane modes can be further studied, after the incidence angle of the film is determined using the point detector.
Professor Pei Jian’s group studied the structure of two thin organic films of IIDDT and IIDT [7]. They found four strong diffraction peaks of IIDDT, using GIXRD technique, but just a weak diffraction peak for IIDT. These indicate that IIDDT has a regular long range ordering along the (001) direction, whereas IIDT is more likely with an amorphous phase. Further, the mobility and on/off ratio of the IIDDT, which are critical factors for field-effect transistors, were superior to those of IIDT, thus revealing a clear correlation between the structure and property. The structures of the IIDDT and IIDT films were confirmed using AFM technique.
D. Reciprocal space mappings
Reciprocal space mappings is used to study the domain structure, strain relax, orientation difference and lattice mismatching of crystal or film sample. Professor Yang Ping from Singapore Light Source successfully measured BiFeO3 thin films grown on different substrates at BL14B1, using reciprocal space mapping techniques [8]. For sample of BiFeO3/LaAlO3, the diffraction peak of (103) was a triplet, indicating a monoclinic Mc phase instead of the MA or MB phase reported in previous studies.
E. Atomic pair distribution function analysis
Atomic pair distribution function (PDF) method attracts much interest recent years [9, 10]. By illustrating the atomic pair correlation information, PDF is useful in condensed matter physics and catalyst chemistry, especially for nanoscale systems. PDF gives the atomic correlation information up to several nanometers, hence a complement to EXAFS (Extended X-ray absorption fine structure) where the correlation length usually is limited to the (next) nearest neighbors (a few angstroms).
The PDF measurement in BL14B1 involves usage of diffractometer for large 2θ angle and higher X-ray energy. We have managed to use 18 keV X-ray and a wide 2θ scanning angle (>150°, corresponds to a Q range of ∼17 Å-1) for PDF studies. The setup can be used for amorphous (glass or liquid) and nano-material studies.
F. Total reflection X-ray fluorescence spectroscopy
Total reflection X-ray fluorescence (TXRF) spectroscopy is developed in collaboration with the users. As a complementary method, TXRF analysis, a non-destructive elemental analysis technique, is widely used to determine the depth profiles of trace elements in semiconductor, thin layers on substrate and adsorbed molecules [11-14]. Combined TXRF with XRR, one is able to conduct non-destructive depth profiling of layered samples. Professor Li Wenbin’s group analyzed the structure of Si(substrate)/W/Ni/Ti/N multi-layer using both reflectivity and grazing incidence fluorescence measurements. Combination of the two data sets (refined) gave more precise structural parameters [15].
V. SUMMARY AND OUTLOOK
BL14B1 is a high-resolution X-ray diffraction beamline. The beamline optics and end-station configuration are well suitable for polycrystalline powder and surface/interface of films. Diffraction techniques of GIXRD and RSM, in situ experimental devices especially, have been realized on the beamline. These greatly promote research programs of the beamline users, which has been growing rapidly. However, according to publication analysis across the world, beamlines dedicated to powder diffraction studies did much better in productivity, such as the powder diffraction beamline in Australian Synchrotron, Diamond I11, and Spring-8 BL02B2. As a general purpose diffraction beamline, BL14B1 at SSRF needs to further strengthen its research on powder diffraction. It surely requires efforts in the update of the beamline towards this direction. Especially, phase II beamline construction engineering project will include one surface diffraction beamline and one Laue microdiffraction beamline. It is reasonable for BL14B1 to focus on powder diffraction.
LiNH2BH3-NH3BH3: structure and hydrogen storage properties
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