Structure of Molten CaSiO₃: Neutron Diffraction Isotope Substitution with Aerodynamic Levitation and Molecular Dynamics Study

Abstract

We have performed neutron diffraction isotopic substitution experiments on aerodynamically levitated droplets of CaSiO(3), to directly extract intermediate and local structural information on the Ca environment. The results show a substantial broadening of the first Ca-O peak in the pair distribution function of the melt compared to the glass, which comprises primarily of 6- and 7-fold coordinated Ca-polyhedra. The broadening can be explained by a redistribution of Ca-O bond lengths, especially toward longer distances in the liquid. The first order neutron difference function provides a test of recent molecular dynamics simulations and supports the MD model which contains short chains or channels of edge shared Ca-octahedra in the liquid state. It is suggested that the polymerization of Ca-polyhedra is responsible for the fragile viscosity behavior of the melt and the glass forming ability in CaSiO(3).

Structure of (FexCa1−xO)y(SiODrewitt1, Sanloup2, Bytchkov3 et al. 2013Phys. Rev. B

“…This compares well with the smallest bond distance r CaO = 2.348Å found in crystalline hedenbergite 5 and gives rise to Ca-O correlations that overlap the sixfold coordinated Fe-O peak position, resulting in the observed shift to a slightly higher position r 3 = 2.24(2)Å compared to the fayalite and ferrosilite liquids. The peak at r 4 = 2.55(3)Å is attributed to O-O correlations by comparison with the O-O distance reported for liquid and glassy CaSiO 3 , 64 and is close to the smallest bond length r OO = 2.649Å present in the crystal structure. 5 For the hedenbergite glass, both the four-and sixfold coordination peaks are shifted to slightly higher values of r 2 = 1.96(1)Å and r 3 = 2.27(1)Å, respectively, compared to the liquid counterparts.…”

“…As for the fayalite and ferrosilite liquids, the peak observed at r 2 = 1.93(1)Å for liquid hedenbergite can be attributed to FeO 4 polyhedra. In a recent neutron diffraction with isotope substitution and molecular dynamics (MD) computer simulation study of liquid and glassy CaSiO 3 , 64 the Ca-O bond length was measured as 2.35Å. This compares well with the smallest bond distance r CaO = 2.348Å found in crystalline hedenbergite 5 and gives rise to Ca-O correlations that overlap the sixfold coordinated Fe-O peak position, resulting in the observed shift to a slightly higher position r 3 = 2.24(2)Å compared to the fayalite and ferrosilite liquids.…”

“…From previous measurements of calcium rich silicate and aluminate liquids and glasses, calcium tends to occupy a broad range of coordination sites, which leads to a very broad and often asymmetric pair distribution function. 43,49,64 This increases the level of ambiguity in the measured total pair distribution functions such that the individual partial pair correlations cannot be adequately modelled using a Gaussian fitting procedure. This is illustrated in Fig.…”

“…in which the g CaO (r) and g OO (r) partial pair distribution functions calculated by molecular dynamics computer simulations for both liquid and glassy CaSiO 3 from Ref. 64 are shown, after applying the appropriate x-ray weightings for the hedenbergite composition, together with the experimentally measured T (r) functions for liquid and glassy hedenbergite. In order to obtain more detailed information on the fraction of Fe and Ca coordination species in the liquid and glassy hedenbergite structures, it would therefore be necessary to make more detailed measurements using, e.g., the method of neutron diffraction with isotope substitution and/or combine diffraction measurements with results from molecular dynamics computer simulations.…”

Structure of molten titanium dioxide

“…The only published simulations of liquid TiO 2 were made by Hoang [44,45], using the potentials of Matsui and Akaogi (MA) [46], which were those recommended after a review, by Collins et al [47], of the existing potentials at the time (1996). We also assess the more recent potentials of Pedone et al [48] (Pedone hereafter) and the rigid ion potentials of Teter [49], the latter of which are unpublished, but have been used extensively in the literature, particularly for the simulation of silicate glasses [49,50] and melts [51,52] …”