Based on X-ray diffraction and infrared spectroscopy measurements, BN thin films grown by PECVD on silicon substrates have been studied with the aim of identifying the thin film phase. In a set of samples, while the infrared spectra showed characteristic bands of the hexagonal phase, X - ray diffraction patterns only displayed reflections belonging to the cubic BN phase. Therefore, structural models have been developed to explain the apparent inconsistency between the two sets of experimental data. In particular, static disorder effects - which have been introduced in the model starting from the sp2 hybridization of the ordered hexagonal phase, as suggested by the infra-red spectroscopy results - allowed a consistent interpretation of the X-ray diffraction patterns. For another set of samples, which also showed a characteristic hexagonal signal in the IR data, the XRD pattern could not be indexed with any of the BN phases. In this case, the presence of molecular and ionic phases, associated with impurities, was considered in structural modeling studies.