Unravelling the Urea-Route to Boron Nitride: Synthesis and Characterization of the Crucial Reaction Intermediate Ammonium Bis(biureto)borate

Ammonium bis(biureto)borate, NH4[B(biu)2] (biu = NH–(C═O)–NH–(C═O)–NH), was synthesized as a phase pure powder from urea and triethylborate in a sophisticated solvothermal process. Its crystal structure was determined via single-crystal XRD and it crystallizes in its own structure type in space group P2/c (a = 9.2855(3) Å, b = 4.8019(2) Å, c = 10.9008(4) Å, β = 110.757(2)°, Z = 2). NH4[B(biu)2] was characterized with FT-IR and UV/vis spectroscopy as well as thermogravimetry. It was identified as a crucial reaction intermediate during the boron nitride synthesis from urea and boric acid. Mixtures of boric acid and urea were studied via TGA and DSC, and furthermore, three series of solid state reactions varying calcination temperature, time, and ratio of starting materials were conducted in order to study the formation and thermal decomposition of NH4[B(biu)2]. Based on these experiments under careful comparison to our studies on the properties of phase pure, solvothermally synthesized NH4[B(biu)2] we suggest a reasonable reaction sequence for the formation of boron nitride starting from urea and boric acid.