A new growth device in vapour phase to obtain large iodide mercuric single crystals is presented here. The temperature field of this furnace is better defined than in the devices derived from the Scholz process. Using a volume diffraction method recently developed at the "Institute Laue Langevin", we compare the crystalline quality of two crystals produced in our device and in a Scholz type furnace. Both crystals exhibit bad surface layers having a thickness in the millimetre range. But the core of the crystal grown in the new device has a more homogeneous mosaicity than that of the crystal produced in the Scholz type furnace.