Determination of the Na$_3$AlF$_6$ –Y$_2$O$_3$ Phase Diagram and Its Implications for Low-Temperature YAG/Nd:YAG Synthesis
Résumé
A new way of preparing YAG, Y$_3$Al$_5$O$_{12}$, materials at low -temperatures has been discovered using a molten Na$_3$AlF$_6$ –Y$_2$O$_3$ reaction mixture. For the successful synthesis of YAG, a precise examination of the cryolite part of the phase diagram of the Na$_3$AlF$_6$ –Y$_2$O$_3$ system was carried out up to 45 mol% of Y$_2$O$_3$ using thermal analysis with a larger amount of sample (12 g). The phase diagram of the Na$_3$AlF$_6$ –Y$_2$O$_3$ system was discovered to likely be a simple eutectic system with one inflection point on the liquidus curve (coordinates: 22.0 mol% Y$_2$O$_3$, 920 °C) and one eutectic point (coordinates: 43.0 mole% Y$_2$O$_3$, 620 °C). The spontaneously solidified samples of Na$_3$AlF$_6$ –Y$_2$O$_3$ after thermal analysis have been investigated using solid -state NMR ($^{19}$F, $^{23}$Na, and $^{27}$Al) spectroscopy and X -ray powder diffraction over a broad range of compositions. The minimal synthesis temperature used in this work for the preparations of YAG was 630°C and the Y$_2$O$_3$ concentration was 43 mol%. Besides the synthesis of YAG, the molten Na$_3$AlF$_6$ –Y$_2$O$_3$ system with the addition of Nd$_2$O$_3$ has been successfully used also for the preparation of the neodymium -doped YAG powders (Nd:YAG). Rietveld refinement has been used to quantitatively assess the incorporation of neodymium into YAG and β-Na(Y$_{1.5}$Na$_{0.5}$)F$_6$ materials.