Synthesis of vanadium spinel compound like MnV2O4 in vacuum ampoule by solid state reaction at high temperature
Check the gas pressure at the temperature of the furnace; ideal gas equation is a good first approximation. You might also have evaporation of liquid phase which would additionally increase pressure. Also beware that temperature gradients can cause mechanical stress.
Yes gas (vapour) pressure is the reason. You can be able to resolve it by reducing the sample amount or increasing the ampoule length. Also should use better quality quartz. Hope it may work...
Don't discount the possibility of defects in the ampoule, either present in the glass itself or introduced during the sealing process!
There are several possibilities
1) High vapour pressure of the synthesized material.
2) Reaction of the synthesized material with the quartz tube at annealing temperature.
3) Low quality of the quartz tube can have defected.
you can try using
1) A proper crucible inside the quartz tube which will make sure there will be no reaction between the sample and tube.
2) double quartz sealing.
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