Search

Dicalcium silicate, which is found in steelmaking slag for dephosphorization, exists as the hexagonal α phase at high temperatures. The α‐dicalcium silicate forms a solid solution with tricalcium phosphate in the entire composition range, although the reason for high solubility of phosphorus remains unclear in view of the crystal structure. It has previously been reported that the crystal structure of α‐dicalcium silicate consists of a symmetric arrangement of Ca2+ ions and SiO44− tetrahedra, although other polymorphs exhibit asymmetric arrangements. However, because the occupation probability of each atomic site in the α polymorph is not limited to unity, it has not been qualified how these ions are exactly arranged. In this study, the ionic distribution in the α polymorph of dicalcium silicate was evaluated by first‐principles calculation based on density functional theory (DFT), as well as by molecular dynamics (MD) simulation with a polarizable ion model optimized by DFT calculation. The results indicated that the completely symmetric ionic arrangement, as reported for the α phase, is the most unstable. Electronic‐state calculation and MD simulation indicated that a highly disordered ionic arrangement spontaneously forms in the α‐phase crystal for structure relaxation when held at high temperatures, or when phosphorus is incorporated. [ABSTRACT FROM AUTHOR]

Abstract: Solubility and local structure of transmission elements in calcium silicate compounds has not been well understood. We investigate the local structure of chromium ions dissolved in merwinite (3CaO·MgO·2SiO2) of a monoclinic crystal structure. The acceptance of doping elements into merwinite has not been reported before. We found that chromium ions are soluble in merwinite in air and that chemical valence of the dissolved Cr ions varies with annealing temperature. The absorption edge in the x‐ray absorption near edge structure (XANES) of Cr‐doped merwinite indicated that octahedrally coordinated Cr3+ ions were mainly formed when annealed at 1673 K in air. A pre‐edge peak was also detected, indicating the existence of tetrahedrally coordinated high‐valence Cr ions. Conversely, through annealing of merwinite at 1123 K in air, tetrahedrally coordinated Cr6+ ions were found to be the main form of chromium. XANES spectra simulated by first‐principle calculations were used to explain the structural features in the observed spectra. We propose the coexistence of Cr3+ ions in octahedral Mg2+ sites and high‐valence Cr ions in tetrahedral Si4+ sites. In addition, a change in the chromium ion oxidation state in tetrahedral coordination sites was suggested by XANES spectroscopy of Cr‐doped merwinite synthesized at 1673 K and reannealed at 1123 K. [ABSTRACT FROM AUTHOR]

Abstract: Solubility and local structure of transmission elements in calcium silicate compounds has not been well understood. We investigate the local structure of chromium ions dissolved in merwinite (3CaO·MgO·2SiO2) of a monoclinic crystal structure. The acceptance of doping elements into merwinite has not been reported before. We found that chromium ions are soluble in merwinite in air and that chemical valence of the dissolved Cr ions varies with annealing temperature. The absorption edge in the x‐ray absorption near edge structure (XANES) of Cr‐doped merwinite indicated that octahedrally coordinated Cr3+ ions were mainly formed when annealed at 1673 K in air. A pre‐edge peak was also detected, indicating the existence of tetrahedrally coordinated high‐valence Cr ions. Conversely, through annealing of merwinite at 1123 K in air, tetrahedrally coordinated Cr6+ ions were found to be the main form of chromium. XANES spectra simulated by first‐principle calculations were used to explain the structural features in the observed spectra. We propose the coexistence of Cr3+ ions in octahedral Mg2+ sites and high‐valence Cr ions in tetrahedral Si4+ sites. In addition, a change in the chromium ion oxidation state in tetrahedral coordination sites was suggested by XANES spectroscopy of Cr‐doped merwinite synthesized at 1673 K and reannealed at 1123 K. [ABSTRACT FROM AUTHOR]