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Flux growth utilizing the reaction between flux and crucible

Flux growth involves dissolving the components of the target compound in an appropriate flux at high temperatures and then crystallizing under supersaturation controlled by cooling or evaporating the flux. A refractory crucible is generally used to contain the high temperature melt. The reaction between the melt and crucible materials can modify the composition of the melt, which typically results in growth failure, or contaminates the crystals. Thus one principle in designing a flux growth is to select suitable flux and crucible materials thus to avoid any reaction between them. In this paper, we review two cases of flux growth in which the reaction between flux and Al$_2$O$_3$ crucible tunes the oxygen content in the melt and helps the crystallization of desired compositions. For the case of La$_5$Pb$_3$O, Al$_2$O$_3$ crucible oxidizes La to form a passivating La$_2$O$_3$ layer which not only prevents further oxidization of La in the melt but also provides [O] to the melt. For the case of La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$, it is believed that the Al$_2$O$_3$ crucible reacts with NaAsO$_2$ and the reaction consumes oxygen in the melt thus maintaining an oxygen-free environment.