Local structure around phosphorus and silicon in the CaO–SiO 2 –PO 2.5 system Motohiro Sakamoto a, ⁎, Yutaka Yanaba b , Kazuki Morita b a Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113–8656, Japan b Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153–8505, Japan abstract article info Article history: Received 6 September 2011 Received in revised form 8 November 2011 Available online 3 December 2011 Keywords: Local structure; MAS-NMR; CaO–SiO 2 –PO 2.5 ; Phosphorus; Optical basicity The local structure of phosphorus and silicon in the molten CaO–SiO 2 –PO 2.5 slag system was investigated by magic angle spinning nuclear magnetic resonance (MAS-NMR). The 31 P MAS-NMR spectra revealed that phosphorus was present primarily as the monophosphate complex ion PO 4 3− , with a small amount of diphos- phate ion also present. Their relative ratio to total phosphorus was independent of the phosphate concentra- tion of the sample. In the case of the 29 Si MAS-NMR, the mean number of the non-bridging oxygen atoms associated with tetrahedrally coordinated silicon decreased as the phosphate concentration increased at a fixed CaO/SiO 2 ratio. This indicates that the nonbridging oxygen atoms around the silicon were replaced by bridging oxygen atoms around the phosphorus as the phosphate concentration in the samples increased. To elucidate the basicity dependence of the structure of slag, the relationship between the structure and op- tical basicity was also investigated. The relative ratio of Q n (Q n means the silicon atoms tetrahedrally bonded with “n” number of bridging oxygen atoms) strongly depends on the optical basicity. These optical basicity dependencies of the structures of phosphorus and silicon can be explained clearly by the basicity equalization concept (Duffy and Ingram, 1976) [12]. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The structure of molten slag (oxide) and glass are known to change with their composition. As a result, chemical and physical properties such as basicity, viscosity, and electrical conductance are influenced by structural changes [1]. To elucidate the structural de- pendencies of these properties, the compositional dependence of the structures of slag/glass such as silicate, borate, and phosphate has been widely studied [2–4]. In iron- and steel-making processes, molten slag plays an important role in removing impurities such as phosphorus (P) and sulfur (S) from the molten iron. The species of these impurities are related to the ther- modynamic properties of the slag such as the activity coefficient and ca- pacity of each impurity [5]. These thermodynamic properties are so significant to evaluate ability to remove the impurities qualitatively that the species of such impurities have been investigated in terms of their thermodynamic behavior. Tagaya et al. [6] measured the distribu- tion ratio of phosphorus between molten CaO–CaF 2 –SiO 2 slag and carbon-saturated iron. They confirmed that the distribution ratio was constant up to 2 mass% of phosphorus in the slag and that it increased linearly beyond 2 mass%. To explain this phosphate-concentration de- pendence of the distribution ratio, they presumed that Reaction (1) took place and that the predominant species of phosphorus in the slag was the monophosphate complex ion PO 4 3− , up to 2 mass% of the phos- phorus in the slag. At higher phosphorus concentrations, Reaction (2) occurred and the diphosphate complex ion P 2 O 7 4− was the dominant species in the slag phase. However, up to now, these predominant spe- cies have been simply assumptions made on the basis of thermodynam- ic behavior. P ½ þ 3=2O 2−   þ 5=4O 2 g ðÞ¼ PO 4 3−   ; ð1Þ 2P ½ þ 2O 2−   þ 5=2O 2 g ðÞ¼ P 2 O 7 4−   ; ð2Þ where the symbols [ ], ( ), and (g) represent the metal, slag, and gas phases, respectively. On the other hand, Teixeira et al. [7] recently determine the state of boron in the CaO–SiO 2 slag, which contains a very low boron con- centration (b 0.25 mass%), by magic angle spinning nuclear magnetic resonance (MAS-NMR) and confirmed that MAS-NMR was an effec- tive spectroscopy method for determining the state of impurities in the slags. There are various researches on the local structure around phospho- rus and silicon in a phosphosilicate glass by using MAS-NMR. In the case of alkali phosphosilicate glass, Dupree et al. determined the phosphate- concentration dependence of the structure of silicon and phosphorus in the Na 2 O∙2SiO 2 ∙ pP 2 O 5 glass system (0 ≤ p ≤ 2) [8]. They reported that phosphorus was present as a monophosphate ion together with a di- phosphate ion and that the proportion of diphosphate ion increased Journal of Non-Crystalline Solids 358 (2012) 615–619 ⁎ Corresponding author. Tel./fax: + 81 3 5452 6328. E-mail address: sakagen@iis.u-tokyo.ac.jp (M. Sakamoto). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.11.014 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol