We investigated different conditions for the growth of twin-free crystals of AgGaS2 and then examined difference frequency generation (DFG) using the crystals to apply the new technology to coherent light sources tunable from 5 to 12µm. The number of lamellar twins in a crystal decreased with increasing growth rate and vanished at growth rates greater than or equal to about 1 mm/hour. Growth under these conditions allowed us to obtain the twin-free AgGaS2 single crystals. Using these crystals, a DFG output of 5-12µm was obtained by angle tuning on phase matching angle ranges of 32-37° for type-I and 38-43° for type-II respectively. These phase-matching angles coincided with angles of the growth directions from the c-axis. It was found that longer optical crystals capable of producing difference frequency waves tunable from 5 to 12µm could be prepared from the AgGaS2 single crystal boules grown by our VGF method, without suffering cutting losses.