Abstract: The bottom of the crucible has much greater thermal transfer properties, parallel to an axis substantially perpendicular to the bottom, than those of the side walls. The bottom and side walls are formed by materials having the same main chemical constituents. The bottom can be transparent to infrared radiation and the side walls opaque to infrared radiation. The bottom can be made of amorphous silica and the side walls of opaque quartz ceramic. The crucible can also be made of graphite. The device can comprise a graphite felt, arranged between the bottom of the crucible and cooling means, and compression means of the graphite felt. It is thus possible to define a temperature gradient comprised between 8° C./cm and 30° C./cm in the liquid phase.