upgrade of NPD at MLNSC - Springer Link

Appl. Phys. A 74 [Suppl.], S163–S165 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201929

Applied Physics A Materials Science & Processing

Building a high resolution total scattering powder diffractometer – upgrade of NPD at MLNSC T. Proffen1,∗ , T. Egami2 , S.J.L. Billinge3 , A.K. Cheetham4 , D. Louca5 , J.B. Parise6 1 Los Alamos National Laboratory, LANSCE-12, MS H805, Los Alamos, NM 87545, USA 2 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA 3 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-1116, USA 4 Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA 5 Department of Physics, University of Virginia, Charlottesville, VA 22901, USA 6 Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11790, USA

Received: 17 July 2001/Accepted: 31 August 2002 –  Springer-Verlag 2002

Abstract. The neutron powder diffractometer NPD at the Manuel Lujan Neutron Scattering Center (MLNSC) at Los Alamos National Laboratory is the neutron powder diffractometer with the highest resolution in the United States. We are currently upgrading NPD by adding a large number of position-sensitive detectors in the backscattering position. The upgraded instrument will make it possible to collect data from a powder sample over a wide range in Q with high Q resolution at high data rates. This design will optimize the instrument for total scattering measurements, including Bragg as well as diffuse scattering, allowing us to determine the crystallographic average structure as well as the local structure, often responsible for the properties of complex materials. PACS: 61.12.-q

1 Why total scattering? Modern functional materials often contain defects and are quite disordered. In fact, it is often the defects that give them their interesting properties [1]. Pulsed neutron powder diffraction is widely recognized as a powerful tool in determining the structure of complex materials in a relatively short time with high accuracy. The Rietveld method [2] has become the standard tool to determine the crystal structure from powder diffraction data. It should be noted, however, that even though the Rietveld analysis is a full profile refinement of the powder diffraction pattern, it only uses the Bragg scattering contribution to the diffraction pattern. On the other hand the actual diffraction intensity measured by a powder diffraction experiment consists of two parts: Bragg- and diffuse scattering. Bragg scattering contains information about the long range average structure of the material. The diffuse scattering part contains information about the local atomic arrangements, or two-body correlations, such as chemical short ∗ Corresponding

author. (Fax: +1-505/665-2676, E-mail: [email protected])

range order or correlated motion of atoms (see e.g. [3, 4]). In the Rietveld method the diffuse scattering, as well as instrumental background, are described by an arbitrary background function and discarded from analysis. Thus, only the average structure can be obtained from standard Rietveld refinements, although very recently attempts have been made to include a physical description of the diffuse scattering as part of the Rietveld background function [5]. The idea of the total scattering method is to actually measure both the Bragg and diffuse components of diffraction. The local structure of materials can very efficiently be characterized by a real-space approach in which the local atomic structure is described by the atomic pair-density function (PDF). The PDF is the Fourier transform of the total diffraction intensity, including both the Bragg peaks and the diffuse scattering intensities. Its utility has been demonstrated by a number of recent studies using pulsed neutron sources to determine the local structure of complex solids (e.g. [6–9], for more general information see [10]). It is clear that the understanding of the real structure of complex materials can be achieved only through the combined knowledge of average and local atomic structures. 2 Why high resolution? The capability of pulsed neutron high Q-resolution powder diffraction method in materials research has been amply demonstrated in recent studies of magnetic oxides such as the manganites that exhibit colossal magnetoresistance (CMR), superconducting cuprates, and catalytic zeolites [11], just to name a few. High resolution in reciprocal space is frequently required to resolve small distortions that alter the symmetry in many complex systems such as ferroelectric or superconducting oxide compounds (e.g. [12, 13]). Routine Rietveld analysis is usually only carried out to a maximum momentum transfer (Q = 4π sin θ/λ) of 12 Å−1 or so, since with a normal Q-resolution the Bragg peaks become indistinguishable due to peak overlap at high Q. Figure 1 shows the effect of Q-resolution by comparing the normalized scattering intensity S(Q) of Ni at room temperature obtained on NPD at

S164

NPD (∆d/d~0.15%) GLAD (∆d/d~0.60%)

0.5

-10

1

-5

1.5

G (r) 0

S (Q)

2

5

2.5

10

NPD (∆d/d~0.15%) Glad (∆d/d~0.60%)

40

15

20

25 Q (Å-1)

Fig. 1. Normalized diffraction pattern, S(Q) of Ni at high Q for NPD at MLNSC and GLAD at IPNS

the Manuel Lujan Jr. Neutron Scattering Center (MLNSC) and the diffractometer GLAD at the Intense Pulsed Neutron Source (IPNS). It is obvious that with high resolution obtained from instruments such as NPD, one can expect to greatly increase the amount of information regarding the details of the structure of complex materials. In order to obtain information about the local structure of a material via the PDF analysis, the PDF G(r) is obtained by Fourier transform of the normalize scattering intensity S(Q): 2 G(r) = π


∞ Q [S(Q) − 1] sin(Qr) dQ .

42.5

30

(1)

0

Theoretically the integration should be carried out to Q →∞, but in reality scattering data can only be obtained up to a maximum value Q max . This truncation of the integral introduces so called termination ripples into the PDF, however those can be modelled analytically [14]. Obviously the real space resolution, not to be confused with the resolution in reciprocal space discussed above, of the PDF will depend on the value Q max that can be achieved. However, using pulsed neutrons or high energy X-rays, values of Q max > 50 Å−1 can easily be achieved. The influence of the Q resolution on the PDF can easily be seen by inspecting Fig. 2 showing G(r) for Ni obtained from the data shown in Fig. 1. In all cases the data were processed using PDFgetN [15] and the scattering data for both instruments were terminated at Q max = 35 Å−1 . Note that the PDF in Fig. 2 is shown for medium range distances 40 < r < 50 Å. The Q resolution results in an exponential dampening of the PDF peaks as function of r [16]. Using high Q-resolution data for PDF analysis allows one to access medium range distances, 10–100 Å, opening up a new territory of research. Compared with the XAFS technique which is capable of determining only the first and second nearest neighbor distances, the PDF method has an advantage of describing the short- as well as medium-range mesoscopic local structure. This capability will be further enhanced by high Q-resolution, and will connect the PDF and the crystallographic methods seamlessly.

45 r (Å)

47.5

Fig. 2. Atomic pair distribution function, G(r), of Ni obtained from data shown in Fig. 1 for 40 < r < 50 Å. The scattering data were terminated at Q max = 35 Å−1 for both data sets

3 Upgrading NPD The NPD instrument has the highest resolution among neutron powder diffractometers in the United States. However, NPD has only a relatively small number of detectors at high diffraction angles. In this configuration the data collection time for the Ni data shown in Fig. 1 required in excess of 20 hours. It should be kept in mind that Ni is a strong neutron scatterer and in some cases even longer counting time might be required to obtain high quality pair distribution functions. These long counting times make the systematic exploration of the local structure of a material as a function of external parameters such as temperature or pressure virtually impossible. The current upgrade of NPD will add 168 position sensitive detectors (PSD) to the instrument. We use one-dimensional 3 He PSD tubes with a gas pressure of 10 atm. The active length of each tube is 50.8 cm. The data acquisition time is expected to be reduced by a factor of 5. A summary of the cur-

Table 1. Overview of current detector configuration of NPD at MLNSC. A pixel corresponds to one single ended 3 He tube. The tubes are arranged in double packed rows. The low angle banks are currently not equipped with detectors Bank angle

Pixel size w × h (cm)

No. pixels

Coverage cm2

±148◦ ±90◦

1.3 × 30.5 1.3 × 30.5

62 62

2480 2480

Table 2. Overview of upgraded detector configuration of NPD at MLNSC. The new configuration used the old single ended tubes as well as 3 He PSDs. The position resolution is expected to be better than 1.3 cm Bank angle

Pixel size w × h (cm)

No. pixels

Coverage cm2

±148◦ ±119◦ ± 90◦ ± 40◦

1.3 × 2.6 1.3 × 2.6 1.3 × 30.5 1.3 × 30.5

3200 3200 96 32

10800 10800 3840 1280

S165

eg 148 d

119 deg

90 deg

Neutrons

NPD will be a world-level high-resolution diffractometer for materials research and education. The new instrument will have a unique capability of simultaneous high Q Rietveld analysis and PDF analysis, making it possible to determine the average as well as local structures of complex materials with high accuracy allowing researchers to gain a deeper insight into the structure-property relationship in modern materials. Acknowledgements. The authors would like to thank Don Brown for his help with the data collection on NPD. This work was funded by the National Science Foundation through DMR00-76488 at the University of Pennsylvania and DMR00-75149 at Michigan State University. This work has benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory, funded by the U.S. Department of Energy, BES-Materials Science, under Contract No. W-31-109-Eng-38, and from the use of the Los Alamos Neutron Science Center at the Los Alamos National Laboratory, funded by the U.S. Department of Energy under Contract No. W-7405-ENG-36.

References Fig. 3. Schematic detector configuration of upgraded instrument NPD

rent and upgraded detector configuration is given in Table 1 and Table 2. A schematic layout of the detectors is shown in Fig. 3. In addition the upgrade will add a frame overlap chopper to the instrument to prevent contamination of the data due to frame overlap. The total budget of about $1.1 M is sponsored in part by the National Science Foundation and involves a collaboration including the University of Pennsylvania, State University of New York at Stony Brook, University of Virginia, Michigan State University, University of California, Santa Barbara, and Los Alamos National Laboratory. 4 Outlook The upgrade of the diffractometer NPD is currently under way. It is expected that the upgraded instrument will be available to users at the Manuel Lujan Jr. Neutron Scattering Center from the beginning of the 2003 run cycle. The upgraded

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