Aluminoborate (AB) powder prepared by the Pechini method is a promising rare earth-free phosphor for white light emitting diodes applications. The photoluminescence emission is attributed to organic molecules, polycyclic aromatic hydrocarbons, trapped in the inorganic matrix. We discuss the replacement of the usual reflux heating of the Pechini synthesis by an autoclave microwave-assisted step. Morphological and structural properties seem not changed by the evolution of the heating method while the emission under UV and blue excitation (385–450 nm) are improved in terms of bandwidth and external quantum yield. The main difference between the reflux and the microwave-assisted autoclave heating is that the NOx gases provided by aluminium nitrate precursor are not evacuated from the reactional medium with this latter. From this statement, the role of the nitrate in the precursors has been investigated. The optical, structural and thermal properties of aluminoborate powders with nitrate (15N nitrogen labelling) or without nitrate are described. In particular, thanks to 15N labelling, thermogravimetric analysis coupled to mass spectrometry and the study of the fluorescence decays suggest the presence of nitrogenous compounds in the composition of the polycyclic aromatic hydrocarbons emitting centres.