Investigation of the structural environment of Ta in a silicate glass and water system under high P–T conditions Robert A. Mayanovic a, ⁎, Hao Yan a , Alan J. Anderson b , Giulio Solferino b, 1 a Department of Physics, Astronomy and Materials Science, Missouri State University, Springfield, MO 65897, USA b Department of Earth Sciences, St. Francis Xavier University, P.O. Box 5000, Antigonish, Nova Scotia B2G 2W5, Canada abstract article info Article history: Received 6 February 2013 Received in revised form 7 March 2013 Available online xxxx Keywords: X-ray absorption; High pressure; Silicate glass-melt structure; High field strength elements; Water dissolution in silicate melt In situ Ta L 3 -edge XAS measurements have been made from a Ta (~ 1400 ppm)-bearing peraluminous silicate glass + H 2 O system to 960 °C and ~0.6 GPa. A white-line doublet separated by ~4 eV occurs in the Ta L 3 -edge XANES and results from octahedral crystal field splitting of the Ta 5d levels due to the local structure surrounding Ta coordinated by Q n -species (n = number of bridging oxygen atoms shared between SiO 4 and AlO 4 units) in the silicate glass/melt + H 2 O system. The XANES spectra measured from the hydrous silicate glass/melt and from the silicate-rich aqueous fluid have been analyzed using multi-peak fitting techniques. The white-line doublet intensity varies with increasing P–T conditions of the silicate glass/melt + water system, indicating a shift in the electronic density of states in the vicinity of quasi bound Ta 5d states probed by the 2p 3/2 core photoelectron. Ab initio modeling of the XANES indicates that water dissolution causes distortion of local structure surrounding the 6-fold coordinated Ta–Q n clusters in the hydrous silicate glass/ melt and in the silicate-rich aqueous fluid. Calculation of the angular-momentum projected density of states (l-DOS) shows that the upper doublet level quasi-bound d-DOS is steadily reduced with increasing distortion of the local structure surrounding Ta–Q n clusters. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The introduction of even minor amounts of dissolved water can induce substantial changes in the structural and physical properties of silicate glasses and melts which have yet to be fully understood. Dissolved water (consisting of H 2 O and OH - ) decreases the glass transition temperature, reduces the viscosity and density of melts, in- creases the diffusivity of cations and is the most important volatile component responsible for the eruptive behavior of magmas [1]. Dissolved water in silicate glasses also has importance for technolog- ical applications such as disposal of nuclear waste in glassy materials, solar power generation (e.g., solar tower plants), and performance of optical fibers. Investigations at the atomic level of the dissolution mechanism of water and the associated structural modifications are critical for establishing a better understanding of the effect dissolved water has on the thermodynamic and physical properties of silicate melts and glasses. Moreover, the incorporation of high-field strength elements (Mo, W, Ta, Nb, etc.) can induce substantial structural modifi- cations in silicate melts and glasses. High-field strength elements may potentially affect the polymerization of silicate glasses and melts in varying ways depending upon the nature of oxygen coordination and glass composition. This was well illustrated in a structural study of a variety of silicate glasses showing that Zr 4+ is in most cases 6-fold coor- dinated but exhibits 8-fold coordination by oxygen atoms in the most polymerized glass [2]. In a separate study, the local structural environ- ment of Nb 5+ was found to be sensitive to melt depolymerization and water content, among other factors, in peraluminous and peralkaline glasses [3]. Tantalum silicate glasses show promise for blue laser and luminescent material applications [4], high-transmission optical fibers [5], and for nonlinear-effect photonic structures [6]. Determining the structural role of tantalum and other high-field strength elements in hydrous silicate glasses and melts is fundamentally important to under- standing the evolution of magmas in subduction zones. Using XANES and EXAFS spectroscopy, Pillonen et al. [3] determined that the local structure of Nb 5+ is distorted due to water dissolution in peraluminous (ASI = 1.2) glasses; peralkaline glasses show no such distortion (ASI = 0.6) glasses (ASI: alumina saturation index = Al/ (Na + K) ratio). This is attributed by the authors to a greater degree of depolymerization of the network structure caused by water dissolu- tion in the peraluminous glass than in the peralkaline glass. Our goal is to determine how water dissolution affects polymerization and the local structural environment of isostructural ions Nb 5+ and Ta 5+ in sil- icate melts and how these structural properties compare to those of Nb- and Ta-bearing silicate-rich aqueous fluids. A Ta-bearing peraluminous glass, having a SiO 2 + Al 2 O 3 + Na 2 O+K 2 O + Ta anhydrous com- position, was selected for this study because of its potential for depo- lymerization and distortion of the local structure of Ta in the hydrous melt. Additional advantages of using alkali aluminosilicate Journal of Non-Crystalline Solids 368 (2013) 71–78 ⁎ Corresponding author. Tel.: +1 417 836 5606; fax: +1 417 836 6226. E-mail address: robertmayanovic@missouristate.edu (R.A. Mayanovic). 1 Present address: Department of Earth Science, Mount Royal University, Calgary, Alberta T3E 6K6, Canada. 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.03.011 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol