# Presentation: 330422

## CRI 

**NEUTRON DIFFRACTION FROM LIQUIDS UNDER EXTREME CONDITIONS**
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- Richard Weber and Jean Tangeman, Containerless Research, Inc., Evanston, IL 60201.Christopher Benmore and Joan Siewenie Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439.

- Containerless techniques completely eliminate container-derived contamination and nucleation of liquids enabling experiments on high temperature, undercooled and non-equilibrium liquids.1,2  Neutron diffraction is well suited to the study of oxides and nitrides due to the large scattering lengths of these elements and it allows isotopic scattering differences3 in many elements to be exploited to complement structural data from X-ray diffraction measurements. In this experiment, we combined containerless techniques with neutron diffraction to study the structure of molten oxide materials. Experiments were performed at the Glass Liquids and Amorphous Diffractometer (GLAD) at IPNS. An aerodynamic levitator with a pure vanadium nozzle and a 30 W CO2 heating laser was integrated with the beam line (see Figs. 1, 2 and 3). 3.5 mm diameter samples of CaO:Al2O3 and SrO:Al2O3 were levitated in pure argon and melted with the laser beam.  Progress of the experiments was followed with a video camera and pyrometer focused on the sample. Scattered neutrons were detected with a bank of 3He detectors.

- Close up view of nozzle assembly
- and mounting fixture.

**5 cm**

**Fig. 1**. Schematic of the facility.  A - sample; B – levitator; C – sample exchange port; D - video camera; E - optical pyrometer; F - laser beam steering mirror; G - laser beam path protection; H - 25 Watt CO2 laser; I – laser beam block and alignment pointer; J – zinc selenide lens; K - gas flow controller.

**Fig. 2. ** **Left**, photograph of experimental set up installed at the GLAD beamline downstream sample position.** Fig. 3**. ** Right**, plan of beamline layout showing the neutron beam path and 3He neutron detector banks which cover a horizontal angular range of 2 = 3o to 117o and ± 11o vertical range.

**Fig. 4**. **Left**: close up view of thick- and thin-walled vanadium levitation nozzles.  The thin-walled nozzles increased the signal intensity by a factor of ~2.  **Right**: image of a 3.5 mm diameter molten CaO:Al2O3 levitated sample at ~2000 K viewed through a pyrometer/video camera.

- Liquid drops were stably levitated and held at *ca*. 2000-2100 K for periods of up to 6 hrs. each to obtain a total of 22 hrs. neutron scattering data. The raw data were corrected for scattering, absorption, multiple scattering, and inelastic effects using the ATLAS software package4 and smoothed using the convolution method.5

**Fig. 5.  Left:** Structure factors, S(Q), for CaAl2O4 and Sr67Al66O166  liquids at 2000-2100 K, open circles-raw data, solid line-smoothed data. **Fig. 6**. **Right:**  Differential distribution function, D(r), obtained by Fourier transformation of the smoothed S(Q) in Fig. 5 above.

- Effects of scattering contributions from the thin-walled nozzle and sample geometry on the derived structure were small. Results are consistent with mainly four coordinate Al3+ ions (rAlO=1.8Å) and 8 or higher coordinate Sr2+ and Ca2+ ions. The liquid consists of polyhedra with a relatively large, broad range of O-O distances indicating a breakdown of the network connectivity compared to the solid phases.6 Reverse Monte Carlo simulation of a CaO:Al2O3 liquid shows evidence of an open structure in the melt, with average coordination numbers consistent with the experimental results, see Fig. 7. below

**Fig. 7**. Snapshots of the complex liquid structure results from the Reverse Monte Carlo simulation based solely on neutron data for liquid at 2000 K. AlO4 tetrahedra in red and mainly isolated CaO7 polydedra in yellow. Neutrons are useful in determining the oxygen related correlations.
- Further work is in progress to refine the model by combining X-ray diffraction data which is most sensitive to the heavier Ca and Al interactions.

- Combining levitation and neutron diffraction provides a new tool to investigate the structure of molten materials in extreme and highly non-equilibrium conditions.
- Liquid Ca and Sr aluminate liquids consist predominantly of tetrahedral Al3+, Sr2+ and Ca2+ ions occur as 6 and higher coordinate species. These neutron results are in agreement with high temperature NMR results and ion molecular dynamics simulations. Planned integration of a higher-power heating laser and potential operation with a much higher neutron flux will expand the range of materials and conditions over which measurements can be performed including the study of transient structure changes, metastable liquids and effects associated with the onset of the glass transition or ordering changes in liquids.

**References**
- 1. J.K.R. Weber and P.C. Nordine, Microgravity Sci. and Tech. **VII**, 279 (1995).
- 2. C. Landron, L. Hennet, J.-P. Coutures, T. Jenkins, C. Aletru, N. Greaves, A.K. Soper and G.Derbyshire, Rev.Sci. Instrum. **71**, 1745 (2000).
- 3. C.J. Benmore and P.S. Salmon, Phys. Rev. Lett., **73**, 264 (1994).
- 4. A.K. Soper, W.S. Howells and A.C. Hannon, in *ATLAS: Analysis of Time of Flight Diffraction Data from Liquid and Amorphous Samples* (Report **RAL 89‐046**, Rutherford Appleton Laboratory, Oxford, UK, 1989).
- 5. C.J. Benmore, B. Tomberli, P.A. Egelstaff and  J. Neuefeind, Mol. Phys. **99**, 787 (2001).
- 6.A.C. Hannon and J.M. Parker, J. Non-cryst. Solids **274**, 102 (2000).
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**Acknowledgments**
**Research at CRI was supported by NASA Physical Sciences Division under contract No. NAS8-00125 and by the Department of Energy under grant No. DE-FG02-01ER86121.  Work at Argonne National Laboratory was supported under contract No. W-31-109-ENG-38 from the Department of Energy.  We thank Ken Volin, Rich Vitt and Nicole Murphy of IPNS for help with design and installation of the instrument and Jim Rix and Kirsten Hiera of CRI for help with design and assembly of the instrument and  sample synthesis.**

*CRI*

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