Crystal Growth of Refractory Chalcogenides by Floating Zone Method under Normal Pressure

Abstract

Single crystals of various chalcogenides with high melting point and high volatility were grown under a flowing Ar gas at normal pressure by a floating zone method, utilizing a 5 kW Xe arc lamp and an ellipsoidal mirror as an image furnace. Homogeneous crystals with large size, typically 3 mm Φ× 30 mm, were obtained for CaS, SrS, BaS, CaSe, SrSe, BaSe, EuS, EuSe and for solid solution systems of EuS-SrS and EuS-EuSe. Characterizations by X-ray powder diffraction and optical measurements indicated the superior qualities of these crystals to those by the conventional crystal growth methods. Despite the high growth rate, typically 50to 70 mm/h, the homogeneity of the solute distribution was sufficiently good to qbserve the sharp reflection spectra due to the band edge exciton transitions. In the case of CuxMo₆S₈ (x=1.6∼3.2), the microscopic fibrous precipitation of Mo was observed in the otherwise single-crystalline matrix phase. The precipitation could not be avoided unless the atmosphere was slightly pressurized to about 10 atm. The effect of volatilization was discussed in terms of stoichiometric and non-stoichiometric ones. The former turned out to raise rather minor difficulties to the crystal growth process; on the other hand, an adverse effect such as eutectic solidification could result from the change in the melt composition due to the latter.
As a whole, the present method operated under normal pressure seemed to b e highly potential for the growth of single crystals even with high volatility, provided that their volatilization was nearly stoichiometric.