Charge states of Ca atoms in β-dicalcium silicate

Abstract

In order to study the crystal structure of β ‐ Ca 2 SiO 4 , time-of-flight neutron powder diffraction experiments were carried out at temperatures between room temperature (RT) and 600 °C. Rietveld refinement at RT has shown that β ‐ Ca 2 SiO 4 is monoclinic based on P2 1/ n symmetry and two different types of Ca sites, Ca(1) and Ca(2). All interatomic distances within 3 Å were calculated, with the valences of Ca(1) with seven Ca O bonds and Ca(2) with eight were estimated to be 1.87+ and 2+ by the Zachariasen–Brown–Altermatt formula (bond valence sum). Applying charge neutrality the two charge states of Ca in β ‐ Ca 2 SiO 4 are [Ca(1)SiO 4] 2− and Ca(2) 2+, respectively. Furthermore, the [Ca(1)SiO 4] 2− unit has the shortest Ca–O distance, and its length kept constant at 2.23 Å at all temperatures. In the short-range structure analysis at RT, the shortest Ca–O bond was also observed in a radial distribution function. These results imply that the [Ca(1)SiO 4] 2− unit has covalency on the shortest Ca–O in addition to Si–O.