ABSTRACT
This paper explores the connection between the H3BO3 flux concentration and the co-existence of Eu2+ and Eu3+ dopants within CaMgSi2O6 crystals (diopside). The samples were synthesised using a solid-state synthesis method under varying atmospheric conditions, including oxidative (air), neutral (N2), and reductive (H2/N2 mixture) environments. Additionally, some materials underwent chemical modification by partially substituting Si4+ with Al3+ ions acting as charge compensation defects stabilizing Eu3+ luminescence. Depending on the specific synthesis conditions, the materials predominantly displayed either the orange-red luminescence of Eu3+ (under oxidising conditions) or the blue luminescence of Eu2+; however, the comprehensive results confirmed the co-existence of Eu3+/Eu2+ luminescence in both cases. This work shows that varying flux concentrations added during synthesis significantly affect the relative strength of Eu2+ and Eu3+ emissions in a manner dependent on the synthesis atmosphere. The emission of Eu2+ increases with a higher flux concentration in materials synthesised under oxidative and neutral atmospheres independent of the chemical modification. In contrast, for materials obtained under a reductive atmosphere, the changes in the Eu3+ emission intensity depended on the presence or absence of Al3+ ions namely the increase of flux increased the Eu3+ intensity in the case of unmodified materials and decreased in the Al-modified ones. All observed effects were qualitatively explained considering the double role of the flux in the studied system, which besides facilitating the diffusion of chemical species during synthesis acts as a charge compensating agent by creating B'Si centres stabilizing Eu3+ emission.
ABSTRACT
A series of strontium orthotitanate (Sr2TiO4) samples doped with 2% of a mole of europium, praseodymium, and erbium were obtained using the solid-state synthesis method. The X-ray diffraction (XRD) technique confirms the phase purity of all samples and the lack of the influence of dopants at a given concentration on the structure of materials. The optical properties indicate, in the case of Sr2TiO4:Eu3+, two independent emission (PL) and excitation (PLE) spectra attributed to the Eu3+ ions at sites with different symmetries: low - excited at 360 nm and high - excited at 325 nm, while, for Sr2TiO4:Er3+ and Sr2TiO4:Pr3+, the emission spectra do not depend on the excitation wavelength. The measurements of X-ray photoemission spectroscopy (XPS) indicate the presence of only one type of charge compensation mechanism, which is based on the creation of strontium vacancies in all cases. This suggests that the different charge compensation mechanisms cannot easily explain the presence of Eu3+ at two non-equivalent crystal sites. The photocurrent excitation (PCE) spectroscopy investigations, that have not been reported in the literature so far, show that among all the studied dopants, only Pr3+ can promote the electrons to the conduction band and give rise to electron conductivity. The results collected from the PLE and PCE spectra allowed us to find the location of the ground states of lanthanides(II)/(III) in the studied matrix.
