The furnace was operated with an argon flow rate of approximately 10 ml/min. The temperature was gradually increased from room temperature to 660 °C for one hour, then maintained at that temperature for three hours. Subsequently, the temperature was raised to 900 °C for another hour and held for six hours. This process was repeated, with the temperature increasing to 1100 °C for one hour and held for three hours. Finally, the furnace was allowed to cool down to room temperature at a controlled rate.