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Structural Transformations on Vitrification in the Fragile Glass-Forming System CaAl 2 O 4

Abstract : The structure of the fragile glass-forming material CaAl 2 O 4 was measured by applying the method of neutron diffraction with Ca isotope substitution to the laser-heated aerodynamically levitated liquid at 1973(30) K and to the glass at 300(1) K. The results, interpreted with the aid of molecular dynamics simulations, reveal key structural modifications on multiple length scales. Specifically, there is a reorganization on quenching that leads to an almost complete breakdown of the AlO 5 polyhedra and threefold coordinated oxygen atoms present in the liquid, and to their replacement by a predominantly corner-sharing network of AlO 4 tetrahedra in the glass. This process is accompanied by the formation of branched chains of edge and face-sharing Ca-centered polyhedra that give cationic ordering on an intermediate length scale, where the measured coordination number for O around Ca is 6.0(2) for the liquid and 6.4(2) for the glass. Calcium aluminates ðCaOÞ x ðAl 2 O 3 Þ 1Àx (0 x 1) have been extensively studied on account of their geological , technological, and scientific importance [1-20]. For example, they are a significant component of the Earth's mantle so that the liquid structure is of interest for understanding magma-related processes [21], they are an integral component of aluminous cement [22], the glasses have a favorable infrared transmission window that extends up to a wavelength $6 m [23] giving them optical applications [24,25], and the rare-earth-metal-doped materials exhibit persistent luminescence [26]. From a glass physics perspective, calcium aluminates are very fragile glass for-mers [1,4] and, in contrast to strong network glass formers such as SiO 2 , large structural alterations should accompany the rapid change in viscosity and other dynamical properties as the glass transition temperature T g is approached [27]. Experimental information on the extent of structural transformation is therefore essential to understanding the processes occurring around T g and the material properties to which they are linked. An experimental exploration of liquid aluminates is, however, challenging because of the high temperatures involved. The containerless method of aerodynamic levitation offers a way forward, and by minimizing heterogeneous nucleation, it extends the narrow glass-forming region centered at x ¼ 0:65 in the calcium aluminate system to include the equimolar composition CaAl 2 O 4 [16] which has a fragility index of m ¼ 116 [1,28]. At this composition , the O:Al ratio is 2:1 such that it is just feasible to form an ideal network of fully connected corner-sharing AlO 4 tetrahedra where the oxygen atoms are twofold coordinated, as in the crystalline phase which has a tridymite-like structure where the tetrahedra form a fully polymerized network of six-membered rings [29]. This has motivated a range of experimental and computer simulation studies on the liquid and glass structure [2-20]. It has, however, proved difficult to measure unambiguously the Al and Ca coordination environments. For example, in the liquid state 27 Al nuclear magnetic resonance (NMR) experiments observe the fast exchange limit such that individual Al coordination environments cannot be identified [2-5], and in diffraction experiments , the nearest-neighbor Ca-O and other pair correlations are strongly overlapped [16-19]. The powerful method of neutron diffraction with isotope substitution has been used to probe directly the coordination environment of Ca in ðCaOÞ 48 ðSiO 2 Þ 49 ðAl 2 O 3 Þ 3 glass [30,31], but the method is usually limited to large samples [32]. In this Letter we show, however, that the neutron diffraction with isotope substitution method can be used to measure the detailed atomic structure of a single aerodynamically levitated laser-heated drop of liquid CaAl 2 O 4 at 1973(30) K. The structure of the glass at 300(1) K is also investigated. The results, interpreted with the aid of molecular dynamics (MD) simulations, characterize the nature of the structural transformations that occur on vitrification on both the local and intermediate atomic length scales. The total structure factor measured by neutron diffraction is given by FðQÞ ¼ P P c c b b ½S ðQÞ À 1Š,
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James Drewitt, Louis Hennet, Anita Zeidler, Sandro Jahn, Philip Salmon, et al.. Structural Transformations on Vitrification in the Fragile Glass-Forming System CaAl 2 O 4. Physical Review Letters, American Physical Society, 2012, 109 (23), ⟨10.1103/physrevlett.109.235501⟩. ⟨insu-01905955⟩

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