SYNTHESIS AND GROWTH OF HGCA2S4 CRYSTALS

Mercury thiogallate, HgGa2S4, is an exotic II-III-VI2 defect-chalcopyr ite semiconductor which offers a combination of attractive properties for nonlinear optical frequency conversion in the infrared. The synthe sis of this compound, however, is complicated by the high vapor pressu res associated with mercury and sulfur, and previous crystal growth an d phase equilibria studies give conflicting results. Here we report th e use of a novel double-walled ampoule technique for pre-synthesizing 90 g quantities of starting material without the need for a high-press ure furnace. After synthesis, crystal growth by the horizontal-gradien t freeze technique was performed in a transparent furnace in order to monitor and control the crystallization process. The incongruent melti ng behavior of stoichiometric HgGa2S4 was confirmed. Crystallization f rom HgS-rich compositions containing 35, 40, and 46 mol% Ga2S3 failed to produce single-phase material, but small high-optical-quality HgGa2 S4 crystals measuring 1 x 2.5 x 5 mm could be extracted from the direc tionally solidified poly-phase ingots.