Single-Crystal Growth of Some Chalcogenide Spinels

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Abstract
A new technique which has been useful in growing single crystals of a number of the chalcogenide spinels is described. Crystals ACr2X4, where A = Zn or Cd, X = S or Se, are grown by reacting compound AX and CrCl3 in a sealed and evacuated silica vial. The compounds, which are separated to suppress an interface reaction, are heated to 800°–1000°C maintaining about a 50°C gradient along the vial. Spinel crystals form at the low‐temperature end of the vial embedded in a liquid phase which consists principally of ACl2. Commonly a number of crystals grow, usually separated by the liquid phase, but sometimes as a cluster of crystals. This technique yields larger crystals than have been obtained by previous methods (for CdCr2Se4 and CdCr2S4) in which the maximum size was reported to be 2–3 mm. The antiferromagnetic ZnCr2Se4 and ZnCr2S4 crystals have, to our knowledge, been grown for the first time. The growth mode is principally octahedral with well‐developed (111) faces, although the larger crystals tend to be flat, truncated along [100] and [110]. Microscopic examination shows the crystals to be sound, containing few internal voids. The complex growth mechanism is discussed in general terms involving an initial vapor transport of the system components and the resultant liquid‐vapor evironment required for crystal formation. The effect of some substitutional (A‐site) doping experiments in CdCr2S4 crystals is reviewed.