I've made yttrium aluminum garnet (YAG) precursor powder and in my last batch, I placed one half of the powder to be annealed in a non-preheated oven (letting the powder heat up with the oven) and the other half of the powder, I placed it in the oven once the temperature reached my final desired annealing temperature, 1020ºC. Aside from this difference, everything about the annealing process was the same; Annealing for both were performed in air and in the oven for 2 hours.
The curious thing is, the XRD results showed that the sample placed in the preheated oven still showed traces of secondary phases (YAH and YAM), but sample that was placed in non-preheated oven (the one that was heated up as the oven was being heat up) showed phase pure YAG, which to me is counterintuitive. How can this be? Does it have to do with some sort of kinetic steps in the formation of YAG?