Abstracts

The host matrix, Ca₂MgSi₂O₇, belongs to the melilite group, being tetragonal and with space group P-42­₁m. Its standard structure presents three different sites: a tetrahedral Si⁴⁺ site, a tetrahedral Mg²⁺ site, and an octacoordinated Ca²⁺ site. However, there are studies on its incommensurately modulated structure, which is stable at temperatures lower than 85ºC. In these situations, Si⁴⁺ tetrahedra rotate at a non-commensurate periodicity, leading to distortions in the Mg²⁺ tetrahedra and the formation of non-equivalent Ca²⁺ sites with coordination numbers equal to 6 and 7. The dopant, Mn²⁺, has an adequate ionic radius to substitute for the Ca²⁺ and Mg²⁺ sites. The activator presents spin-forbidden d-d transitions, with the ⁴T₁→⁶A₁ transition being dominant when subjected to weak crystalline field strength. In addition, the energy of the ⁴T₁ level decreases as the crystalline field strength increases, leading to emission with longer wavelengths. The compounds were synthesized through a solid-state reaction with the precursors CaCO₃, MgO, MnCO₃, and SiO₂. The mixture was heated at 1350ºC for 8 hours. Mixing and heating twice under the same conditions improved the formation of the phase of interest, reducing the proportion of spurious phases. For materials doped with 2.5% at. Mn²⁺, the emission spectra present two bands originating from the dopant, which are believed to be from the dopant replacing the site of square antiprismatic geometry of Ca²⁺ for the lower energy emission (~ 690 nm) and the tetrahedral site of Mg²⁺ for the higher energy (~590 nm). The broad emission at approximately 480 nm must be due to a defect in the matrix, being observed in the compound synthesized without dopant. Emission spectra obtained every 5ºC at varying temperatures from 30ºC to 100ºC showed that the ratio between the band at 480 nm and that at 590 nm grows linearly from 50ºC to 80ºC, from 80ºC up to 100ºC this ratio grows at an even greater rate. The ratio was measured by the integrated area of these bands. These results show a new way of evaluating the transition from the incommensurable to the commensurable phase.