The structural studies of CaMgSi₂O₆, CaCoSi₂O₆ and CaNiSi₂O₆ glasses

Abstract

Local structures around Si, Ca, Mg, Co, and Ni atoms in CaMgSi₂O₆, CaCoSi₂O₆, and CaNiSi₂O₆ glasses were investigated by X-ray diffraction, XAFS, and optical absorption spectra measurements. Si–O interatomic distances of these three glasses are 1.62 Å and coordination numbers of Si atoms are 4. Mg–O, Co–O, and Ni–O interatomic distances are in the range of 2.00 to 2.06 Å and coordination numbers of Co and Ni atoms are estimated to be 4 in the corresponding glass. Ca–O distances are in the range of 2.44 to 2.51 Å and coordination numbers of Ca atoms are estimated to be 6 in the three glasses. Similarities of radial distribution functions obtained from X-ray scattering intensities and XAFS spectra indicate that these glasses have a similar structure and similar local configurations of each cation. The coordination numbers of Ca, Co, and Ni atoms and the interatomic distances of Mg–O, Co–O, and Ni–O in the glasses are smaller than those in the corresponding crystals. These results show that interstitial space of SiO₄ networks of these glasses is too large for these modifier atoms, and these atoms are located on the place apart from the center of space. The result of optical absorption spectra measurements reveals that the difference of the ligand field stabilization energy of Ni atom between CaNiSi₂O₆ glass and crystal is larger than that of Co atom between CaCoSi₂O₆ glass and crystal. This result is related to the difference of the heat of vitrification from these crystals and the different partition of modifier atom into these crystals.