Low Cation Coordination in Oxide Melts

L. B. Skinner, C. J. Benmore, J. K. R. Weber, J. Du, J. Neuefeind, S. K. Tumber, and J. B. Parise

Phys. Rev. Lett. 112, 157801 – Published 17 April 2014

Abstract

The complete set of partial pair distribution functions for a rare earth oxide liquid are measured by combining aerodynamic levitation, neutron and x-ray diffraction on $Y_{2} O_{3}$ , and ${Ho}_{2} O_{3}$ melts at 2870 K. The average Y-O (or Ho-O) coordination of these isomorphic melts is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline $Y_{2} O_{3}$ (or ${Ho}_{2} O_{3}$ ). Investigation of ${La}_{2} O_{3}$ , $Zr O_{2}$ , and ${Al}_{2} O_{3}$ melts by x-ray diffraction and molecular dynamics simulations also show lower-than-crystal cation-oxygen coordination. These measurements suggest a general trend towards lower coordination compared to their crystalline counterparts. It is found that the coordination drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations, such as $Si O_{2}$ . These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses.

Received 12 September 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.157801

Authors & Affiliations

L. B. Skinner^1,2,3,*, C. J. Benmore², J. K. R. Weber^2,3, J. Du⁴, J. Neuefeind⁵, S. K. Tumber³, and J. B. Parise¹

¹Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794-2100, USA
²X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
³Materials Development Inc., Arlington Heights, Illinois 60004, USA
⁴Materials Science & Engineering, University of North Texas, Denton, Texas 76203, USA
⁵Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

^*Corresponding author. lawrie.skinner@gmail.com

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Vol. 112, Iss. 15 — 18 April 2014

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Images

Figure 1
(a) Measured partial structure factors of molten $M_{2} O_{3}$ ( $M = Y$ or Ho), from raw data (grey dashed line) and back-Fourier transforms (blue lines, correspond to blue lines in b). Black lines are the $S_{a b} (Q)$ patterns for the MD model. (b) Partial pair distribution functions from raw Fourier transform of the measurements (grey dashed line), modified transforms of measurements (blue lines, see Ref. [11] for modification function), error range (light blue shaded area), and the $g_{a b} (r)$ ’s from the MD model (black lines), all transformed using $Q \leq 18.4 Å^{- 1}$ . The symmetric red dotted curve represents a Gaussian atom distribution with a ${\bar{n}}_{M O} = 3.5 (5)$ , fitted to the low- $r$ side of the $M − O$ peak. This leaves a residual peak centered at 2.56(2) Å, with a ${\bar{n}}_{M O} = 2.0 (5)$ (grey dashed line labeled $Δ$ ). Inset is a portion of the MD model, light green polyhedra are ${YO}_{4}$ , ${YO}_{5}$ , and the dark blue polyhedra are ${YO}_{6}$ , ${YO}_{7}$ units. Numerical data provided in Ref. [48].
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Figure 2
(a) Measured x-ray $T^{'} (r) = 4 π ρ r g^{'} (r)$ pair distribution function patterns (blue dashed lines), where $ρ$ is the number density and the $^{'}$ denotes that the patterns are re-weighted to have equal, $r$ -independent $M − O$ weighting ( $r_{1}$ is the first $M − O$ peak distance). Black lines are the $T^{'} (r)$ from molecular dynamics (MD) simulations ( $Q$ -space patterns and MD potentials given in Ref. [48]). The two dotted curves are the weighted $M − O$ and $M M + OO$ contributions for ${La}_{2} O_{3}$ . (b) Isolated $T_{M O}^{'} (r)$ from the MD simulations. The arrows indicate decreasing cation field strength.
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Figure 3
The shaded red circles are the ${\bar{n}}_{M O}$ determinations made from the oxide liquids studied in this work, plotted against ${\bar{n}}_{M O}$ of their ambient crystal structures. Shaded circle areas are proportional to cation field strength. The solid black circles are literature ${\bar{n}}_{M O}$ measurements for some representative cations $M = Al$ , Si, Te, Ge, Ga, Ti, La, Y, Mg, Ca, Sr, Ba, Pb, Na, K, in oxide glasses [9, 16, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47] (black circle areas represent large, intermediate or low field strength). The light blue shaded area is a guide to the eye. Inset: Average coordination drop from crystal to the liquid or glass, divided into three groups of, large field strength (network formers), intermediate, and low field strength (modifiers). The atomic configurations correspond to liquid $Si O_{2}$ (left) and liquid $Y_{2} O_{3}$ (right) from the MD simulations performed in this work.
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