In-situ x-ray and neutron, diffraction combined with computation modelling have been used to elucidate the details of the structural phase transitions in TBa₂Cu₃O₆₊ₓ for 0 < x < 1. In this study it is found that in the elevated temperature range [200-300°C], with increasing oxgen content, the structure changes from the well known tetragonal (Tet.) to the orthorhombic II (OII) superlattice phase (under thermodynamical equilibrium condition). This transition is found to be first order (a matter of considerable controversy) with the expected linear change in the volume fraction of each phase across the corresponding two phase mixed region (Gibbs phase rule). The possibility of the existence of a stable entirely OII phase region at 300°C around x ≈ 0.7 is perhaps the most notable result of this study, which this result implies a substantial correction on the YBCO's structural phase diagram. From the full analyses of the data, the possibility of the formation of a superstructure arong the 'c' direction under the aforementioned conditions cannot be ruled out. At higher temperature [500-600°C] the measurements relate to the second order Tet. to orthorhombic I(OI) phase transition. The interesting issue here is the nature of the twining microstructure that forms just after the transition. The computation modeling and peak broadening analyses of the data, reveals the interference effect on the diffracted peak shapes from the thin stripe twinned sample at the early stages of transition from the Tet. to OI phase.