Ca₂Al₂SiO₇ synthesis, local order and optical properties studied using synchrotron radiation

Full text: Ca₂Al₂SiO₇(CAS) is a very interesting oxide that can be used as host material for rare earth and transition metal dopant ions. When doped, CAS exhibit optical properties such as scintillation and fluorescence allowing several applications. This oxide is generally synthesized by solid state reaction, sol-gel routes or crystal growth. The silicon source used to prepare CAS usually SiO₂, for solid state reaction and crystal growth, or TEOS in sol-gel or other chemical routes. In previous works new methodology which combines solid state reaction and proteic sol-gel method was proposed, where coconut (Cocos nucifera) water is used as the starting solvent and the polymerizer agent mixed with non soluble reactants as SiO₂. This work aimed the full characterization of the CAS samples produced with this hybrid route starting with the in situ XRD measurements of the synthesis process. These kinetic studies were performed in the XRD2 beamline at the LNLS. Previous DTA/TG results showed a crystallization onset around 927°C that may be related to the formation of CAS, and that confirmed by conventional powder XRD. XRD in situ showed that up to 700°C SiO₂ in quartz structure is preserved. Above 800°C, intermediate phases are formed and diffuse by thermal effect and react among themselves producing in the end the CAS phase. The kinetic studies also showed at 1160°C all SiO₂ diffused and react with other phases. Morphological and structural studies were carried out in the synthesized samples by electron microscopy at LME/LNNano and CMNano-UFS. The results indicated that nanoparticles were obtained by this synthesis process. EXAFS studies around the K edges of matrix elements, Ca and Si, showed the local order around them in samples calcined at different temperatures. XANES results around the Mn K edge and Ce and Eu LIII edges were used to check the dopant oxidation states. XEOL spectra showed typical emission of the dopants and different behaviors for XEOL excitation spectra when different absorption edges were measured. These results can be used to build up a model to explain the scintillation mechanism of the doped CAS samples. Decay time constants of the XEOL could be measured using the LNLS single bunch mode at both SXS and XASF2 beamlines. The XEOL decay curves showed characteristic decay constant around few tenths of nanosecond sputting these materials in the group of the fastest known scintillators. (author)