Nucleation sites on platinum surfaces in flux growth

Differential thermal analysis (DTA) experiments were carried out on Pb2V2O7 crystallising from the system Pb2V2O7 1b V2O5 in a sealed platinum crucible. A progressive increase in induction time with soak time was observed, both when the soak temperature was above the liquidus and also after a second soak below the liquidus. A return to the initial induction time occurred after uninterrupted fast cooling to below the eutectic. These results are interpreted as indicating that the melt dissolves preferred nucleation sites on the surface of the platinum crucible, and that, on cooling to the eutectic temperature, platinum reprecipitates to form new nucleation sites. These new nucleation sites are then effective in subsequent flux growth, and thus the previous history of a crucible is more important in flux growth than has been realised before. This interpretation of the removal of nucleation sites by dissolution of platinum accounts for the hitherto unexplained, but generally accepted, practice of a long soak period considerably above the liquidus in order to obtain few crystals in flux growth by spontaneous nucleation.