The present invention relates to a method for the production of crystal layers or bulk crystals of group III nitride or of mixtures of different group III nitrides by means of precipitation, at a first temperature T1 in a first temperature range, from a group-III containing fused metal on a group-III-nitride crystal seed placed in the fused metal or on a foreign substrate placed in the fused metal, with the admixture of nitrogen in the fused metal at a pressure P.With the method a solvent additive is added to the fused metal which increases the conversion rate of group III metal to group III nitride in the fused metal. The fused metal runs through at least one temperature cycle with a first and a second process phase in which cycle the fused metal cools after the first process phase from the first temperature to a second temperature T2 below the first temperature range and at the end of the second process phase is heated from the second temperature back to a temperature within the first temperature range. The described method permits producing group III nitride crystal layers with a thickness of >10 μm, respectively massive crystals with a diameter of >10 mm at dislocation densities of