Na0.5Bi0.5TiO3 (NBT)-based ferroelectric systems have received considerable attention over the years as potential Pb-free piezoceramics. Despite years of extensive investigations, the complexity of the structure-property correlations in Na0.5Bi0.5TiO3 (NBT)-based piezoelectric continues to puzzle the scientific community. The structural complexities in NBT-based systems owes its origin to the presence of in-phase octahedral tilt on different length scales which interferes with the long-range ferroelectric order. In unpoled NBT, the in-phase tilt is short range order. For the two important morphotropic phase boundary (MPB) systems (1-x)Na0.5Bi0.5TiO3 – (x)BaTiO3 and (1-y) Na0.5Bi0.5TiO3 – (y)K0.5Bi0.5TiO3, the propensity of the in-phase octahedral tilt increases and a long range order corresponding to the P4bm phase sets in below the MPB compositions of the two series. Structural analysis revealed that while the P4bm model can accurately explain the superlattice peaks for the sub MPB compositions, they fail to explain them for the post MPB compositions.  Accordingly, we identify two different types of P4bm phases in both the solid solution series: (i) ordered-P4bm phase and (ii) disordered-P4bm phase. Using a set of complimentary techniques like X-ray and neutron diffraction, and Transmission electron microscopy (TEM), we found that the onset of the disordered-P4bm phase is linked with the long range R3c ferroelectric order becoming short ranged. We have also found abrupt changes in the ferroelectric, piezoelectric, depolarization temperature when the disordered -P4bm phase disappear above a certain critical composition.