The
Super-D2B High Resolution Powder Diffractometer
E.
Suard and A.W. Hewat, ILL
The
most cited ILL work ever concerned the establishment of the “charge reservoir”
concept for oxide superconductors, and the understanding of magnetic and charge
ordering in CMR materials, both done on D2B.
Yet these experiments are at the limit of the resolution and intensity
of the machine. We propose an order of
magnitude improvement, using a new type of pseudo-2D detector covering a much
larger solid angle but at the same time permitting very high resolution. The required new technology has already been
prototyped, and the costs and time scale well defined. (An MS Word version is available for printing).
Of the more than
5000 publications produced by ILL since 1981, 50 papers were cited many more
times than others1; as much as 20% of this high profile work was done using
D1A/D2B !
(Zeolite catalysts, Superconductors, Giant Magnetic Resonance materials,
Ice-phases etc).
High temperature
oxide superconductors
By studying small structural
changes as a function of oxygen stoichiometry, we concluded2 with Bell labs. that
oxidation-reduction of a “charge reservoir” layer could control the hole doping
in copper oxide layers responsible for super-conductivity. These ideas lead
immediately to a successful search for new materials with different “charge
reservoir” layers, such as those with oxides of bismuth, thallium and mercury.
These subtle effects
in heavy metal oxides are much better studied with neutrons, which are more
sensitive to scattering from oxygen, especially since new materials are at
first available only as powders. A
common problem is that chemists can often only make very small quantities
of new materials.
For example, the
Raveau group have recently synthesized a new copper oxide at high pressure, but
only 100 mg of sample is available, requiring an impractically long measuring
time on the present D2B with the necessary high resolution.
High temperature superconductors
are only one example of the many interesting materials found in recent years
whose properties depend on order-disorder of the mixed valence states of one or
more of the atoms. A recent example is that of charge and magnetic ordering in
CMR manganites. Ordering is only within
domains, so the average structure of a “single crystal” appears unchanged, and
again it involves subtle movements of the heavy and light atoms, which are best
seen with neutrons. Whether ordering appears as “stripes” or “ladders” has
generated great interest, and measurements on D2B, coupled with synchrotron
powder diffraction, has been essential to answer these questions3. The problem here is that
the highest resolution is needed to resolve the line splitting, and the present
D2B flux/resolution is not really sufficient.
What other new science will be possible with
super-D2B ?
The superconductor and CMR work has made neutron diffraction one of the
best known techniques for the study of metal-insulator transitions, one of the
hottest current topics in solid state science.
A recent Phys.Rev.Letter4 reported D2B work
on charge disproportionation in rare-earth nickelates; a complete understanding
requires a study with different rare earths as a function of temperature.
Unfortunately only mg quantities of these materials are available since they
are produced under high pressure, yet high resolution is needed to see the small
structural distortions. Synchrotron
diffraction has been tried, but failed to measure the small displacements of
the light oxygen atoms with sufficient precision.
Much more exiting
new science would be possible with such a machine. For example, new forms of meta-stable ice have recently been
discovered at ILL on D2B by Kuhs and Finney, but again working at high pressure
means very small samples, and high resolution is needed to sort out the
different ice phases. The chemistry of
hydrocarbons in zeolite catalysts is another hot topic studied by Fuess, Baehtz
and others. These synthetic zeolites
are not available as crystals, and again we are interested in the coordination
of light atoms, which cannot be located with X-rays without good single
crystals.
What can we do to increase both flux and
resolution on super-D2B?
We propose to
build a unique new type of pseudo-2D high resolution position sensitive
detector. In the horizontal plane, high
resolution would be obtained by scanning a large bank of very fine 5 minute
collimators. In the vertical plane
lower resolution would be obtained using a stack of linear wire position
sensitive detectors. We have already
had prototypes of 300 mm high collimators built by Euro-Collimators, and the
detectors too, are commercially available in Europe. The cost is then well
known: 1.35 MFF for the collimators and 1.25 MFF for the detectors to which we
must add 0.4 MFF for shielding, to be made locally.
Such a detector would
collect 6 times as many neutrons for the same resolution, and allow the highest
possible resolution to be used for most problems. Following this proposal, a similar detector has been planned for
the new Munich reactor, but with lower resolution and flux. (Indeed, if we do
not modernize our own equipment, we will lose the advantage of our high flux
reactor).
The
proposed new high-resolution pseudo-2D detector for ILL’s super-D2B.
References
For more information, see
the ILL’s Diffraction Group Millennium Proposal (http://www.ill.fr/dif/2000/)
1) According to the US
Institute for Scientific Information (see: http://www.ill.fr/dif/citations/)
2) Cava, R. J., Hewat, A.
W., Hewat, E. A., Batlogg, B., Marezio et al. (1990) Physica C. 165, 419.
Structural anomalies oxygen ordering and
superconductivity in oxygen deficient Ba2YCu3Ox.
3)
Hwang,H.Y.,Cheong,S.W.,Radaelli,P.G.,Marezio,M. and Batlogg,B. (1995) Phys.
Rev. Letters. 75, 914.
Lattice effects on the magnetoresistance
in doped LaMnO3.
Caignaert,V.,Suard,E.,Maignan,A.,Simon,C.
and Raveau,B. (1996) J.Mag.& Mag.Mat.153:
L260.
Neutron diffraction evidence for
antiferromag. ordering in the CMR manganites Pr0.7Ca0.3-xSrMnO3.
Radaelli,P.G.,Cox,D.E.,Capogna,L.,Cheong,S.W.,Marezio,M.
(1999) Phys. Rev. B.
Wigner Crystal and stripe models for the
magnetic and crystal. superstructures of La0.333Ca0.667MnO3.
4)
Alonso,J.M., Garcia-Munoz,J.L., Fernández-Díaz,M.T. et al. (1999) Phys.
Rev. Letters 82, 3873.
Charge disproportionation in RNiO3 perovskites: metal-insulator & structural transition in YNiO3
Other people’s opinions of the super-D2B
project.
At the
April 1999 Science Council meeting, the magnetism college chairman (R. Cywinski) in his report gave an
order of priority for the Millennium programme, with first priority going to
super-D2B, followed closely by D3-polarimetry.
He wrote “The subcommittee
expressed its strongest support for the super-D2B high-resolution
diffractometer upgrade... the principal advantage of the proposed order of magnitude
increase in intensity would not be that more experiments could be performed,
but that new science would ensue”.
He referred to charge density ordering, incommensurate and complex
magnetic structures, the interplay of magnetic and structural transitions,
small moment systems, and new possibilities for exploring complex systems in
the parameter space of temperature, pressure, magnetic field and composition.
The
Crystallography college chairman (W.
Kuhs) wrote “Over one hour was
devoted to the discussion…the highest priority was given ex aequo to Thermal
LADI and the D2B upgrade. Both
proposals were considered of outstanding quality, both in its scientific case
and its technical feasibility”. In a letter to D. Dubbers, he wrote “I have seen the minutes of the last
scientific council… that the scientific case should be strengthened… I had the
strong impression that a good scientific case was made in the powder workshop…”. He referred to the work of his group on recently discovered
phases of ice and clathrates at high pressure, and pointed out that increased
intensity was needed for the study of such meta-stable and transient phases.
The
chairman of the Science Council Powder Review (C. de Novion) wrote in his interim report on instrument upgrades,
after organizing a workshop attended by many participants from both pulsed and
synchrotron sources “The super-D2B
upgrade project, presented within the new ILL Millennium Programme, was
considered as a first priority, in particular the new (and higher) detector set. This would allow to use routinely the D2B
instrument in its highest resolution mode”. In his final report in October, he wrote “It is rated as first priority by the Subcommittees 5a and 5b, which
answer partially at least, to the requirement of the S.C. (“scientific case
needs to be strengthened…). The first
part of the project, new detectors and collimators, is well defined and has a
good scientific return on investment ratio”.
Indeed,
the new chairman of the Crystallography college (M. Latroche) wrote in his October report “… the subcommittee still strongly supports the D2B project. Structural determination by powder
diffraction is a basic work that cannot be bypassed…” In a letter to D.
Dubbers, he wrote “Our research group has
shown the complementarity between neutron and synchrotron techniques by solving
complex structures using joint refinement… from D2B and BM16 at ESRF. These structures cannot be solved from
synchrotron data alone, and neutron data were essential…”
The
Magnetism college chairman (R. Cywinski)
further wrote in October “The
subcommittee expressed its strongest and unequivocal support for the super-D2B
high resolution diffractometer upgrade”. “Several Experiments proposed during this round underlined the need for
both high resolution and high count rate”.
He went on to give examples of D2B highlights including a new one
dimensional cuprate that can only be synthesized in very small quantities under
very high oxygen pressure. He concluded, “…
longer term issues should not cloud the case for an urgent, well argued, and
readily achievable upgrade to the present D2B”.