this manner was then brought to a temperature of 1130 u00b0 c which within the scope of the measurement accuracy was constant over the entire volume and wherein the quantity of iodine introduced could be expected according to the ideal gas law to produce an iodine pressure of approximately 3 bar in the system under these conditions transport reactions began to proceed between the individual particles of the granular material which gradually led to a consolidation and to an intergrowth of the entire material after 5 days the reaction was terminated the quartz ampoule was sawed open and the rigid silicon cylinder was removed following the procedure described in example 1 a quartz ampoule constructed in accordance with the figure was filled with 450 g of granular silicon material undoped tellurium content below the level of detection mean particle size 100 to 200 u03bcm as a transport agent tellurium was used in a quantity which ensured a pressure of 0 05 bar at a temperature of 1100 u00b0 c in the reaction space a bulge was provided in the feed pipe to the vacuum pump for receiving the tellurium it was not necessary to cool the tellurium while the granular silicon material was baked out after the system.