VOLUME 86, NUMBER 21 PHYSICAL REVIEW LETTERS 21 MAY 2001 Liquid Alumina: Detailed Atomic Coordination Determined from Neutron Diffraction Data Using Empirical Potential Structure Refinement C. Landron, 1,2 L. Hennet, 1,2 T. E. Jenkins, 3 G. N. Greaves, 3 J. P. Coutures, 4 and A. K. Soper 5, * 1 Centre de Recherches sur les Matériaux à Haute Température, 45071, Orléans Cedex 2, France 2 LURE, 91405, Orsay Cedex, France 3 Department of Physics, University of Wales, Aberystwyth, SY23 3BZ, United Kingdom 4 Institut de Sciences des Matériaux et Génie des Procédés, Université de Perpignan, 52 Avenue de Villeneuve, 66860 Perpignan Cedex, France 5 ISIS Department, Rutherford Appleton Laboratory, Oxon OX11 0QX, United Kingdom (Received 24 October 2000) The neutron scattering structure factor S N Q for a 40 mg drop of molten alumina (Al 2 O 3 ) held at 2500 K, using a laser-heated aerodynamic levitation furnace, is measured for the first time. A 1700 atom model of liquid alumina is generated from these data using the technique of empirical potential struc- tural refinement. About 62% of the aluminum sites are 4-fold coordinated, matching the mostly triply coordinated oxygen sites, but some 24% of the aluminum sites are 5-fold coordinated. The octahedral aluminum sites found in crystalline a-Al 2 O 3 occur only at the 2% level in liquid alumina. DOI: 10.1103/PhysRevLett.86.4839 PACS numbers: 61.12.Ex, 07.20.Ka, 61.20.Ne There has recently been much interest in developing laser heated gas levitation techniques to examine the struc- tural properties of high temperature and supercooled liq- uids in general [1], and liquid alumina in particular [2–5]. This is driven by the search for liquid-liquid phase tran- sitions [6], and for local atomic structural changes which occur as a liquid is supercooled [7]. There is often a con- siderable density deficit between the crystalline and amor- phous states. Typically this is 10–15% [8], but it is larger for ionic systems, reaching around 27% in the case of alu- mina at its melting point, 2326 K [9]. Many of the mate- rials of interest have melting points in excess of 2000 K which, combined with their generally high reactivity, pre- cludes the use of a container in a conventional furnace [1,10 – 12]. Levitation therefore presents a real opportunity for synthesis and in situ characterization of pure liquids or solids at highly elevated temperatures. Materials have been melted with good reproducibility by using acoustic levita- tion [12–14], electromagnetic levitation [15,16], electro- static levitation [17,18], as well as aerodynamic levitation [10,11]. The possibility of using aerodynamic levitation in association with CO 2 laser heating [12] to look at the structure of a melt has been demonstrated in recent investi- gations of refractory oxides [19], including alumina [4,20], using x rays as the structural probe. We present here the first structural data on molten alu- mina obtained using neutron diffraction under contactless conditions. The stronger scattering of neutrons by oxy- gen compared to aluminum serves to highlight the essen- tial structural features of molten aluminum oxide and so complements the existing x-ray data, where the scattering from aluminum is stronger. By employing the empirical potential structure refinement (EPSR) computer simula- tion procedure [21,22] we are able to obtain for the first time a structural model for liquid alumina which is en- tirely consistent with the structure factors obtained by both neutrons and x rays, the physical density, and the findings from NMR. The neutron experiments were carried out on the SANDALS diffractometer at the ISIS spallation neutron source (U.K.). The starting material was a high-purity (99.9%) powder of alumina pressed under isostatic pressure to 250 MPa. The spherical specimens, with a nominal diameter 2.7 mm, corresponding to a weight of 40 mg, were processed by melting them in a CO 2 laser beam and then cooling to room temperature. This size of sample presents a major challenge for a neutron diffrac- tion experiment since the amount of material that can be supported by the levitator is around 120th to 1100th of the amount of material used in a conventional experiment on a liquid. The main change, compared to [10], is the use of horizontal incidence, after a double focusing reflection, of the laser beam onto the sample. As the sphere rotates about a vertical axis, the temperature of the sample is stable and uniform, and the material fully molten: any crystallinity in the sample is readily detectable as sharp Bragg peaks in the diffraction pattern. The argon gas flow used to levitate the sample was precisely adjusted versus temperature by a remote control computer in order to optimize the stability of the sample [23]. The infrared radiation emitted by a 125W CO 2 laser was focused onto the sample by means of spherical mirrors. Our heating system was capable of achieving temperatures well above the melting temperature of alumina T m  2327 K under both oxidizing and reducing conditions [24], for continu- ous periods of 20 min. Temperature measurement was performed by means of a calibrated pyrometer. Figure 1 shows the diffraction pattern from molten alu- minum oxide obtained in this experiment, after subtracting the scattering from the boron nitride levitator nozzle. Only neutron detectors above the scattering plane of the nozzle were used to accumulate data, since for detectors below 0031-9007 01  86(21)  4839(4)$15.00 © 2001 The American Physical Society 4839