The growth conditions of Ce-doped Gd2Si2O7 crystals, using the infrared convergent heating floating zone method, were optimized to make the shape of the crystal-melt interface flat or convex. The shape of the crystal-melt interface, meaning the interface between the molten zone and the grown crystal, was evaluated quantitatively using the convexity (h/r) as a parameter. The h/r value was found to be systematically dependent on the crystal diameter, the focus position of the ellipsoidal mirror for infrared convergent heating, and the lamp power used during crystal growth. For an 8.2-mm-diameter crystal grown using conventional conditions, the h/r value was −0.8, which indicates that the crystal-melt interface had a concave shape. The h/r value increased with the position of the ellipsoidal mirror, going from −0.8 for the −8 mm mirror position, to −0.5 for the +6 mm mirror position. Lastly, when the lamp power increased from 5.8 to 7.0 kW, the h/r value increased from −0.6 to −0.3, which was the maximum value observed, indicating that a flat or convex crystal-melt interface was not obtained.
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