Helical dislocations in gadolinium gallium garnet (Gd3Ga5O12)

Single-crystals of gadolinium gallium garnet (GGG) were grown by the Czochralski technique and cut into wafers. The dislocations present in the wafers were studied by optical microscopy using either the birefringence they induced, or the etch pits formed where they met the sample surface. Helical dislocations, many of which were large enough to be visible to the unaided eye, were present in a number of specimens. The Burgers vectors of the helices were parallel to the <111> and <100> directions. No helices with Burgers vectors parallel to <110> were observed althoughit is known that dislocations with Burgers vectors along <110> are sometimes present in GGG. The sign of the stress parallel to the axis of heliceswas determined with the aid of a sensitive tint plate, and found to be compressive. This means that the helices were of interstitial type.