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In this study, the structural phase transitions are investigated as a function of composition and temperature for polycrystalline x (Ba 0.7 Ca 0.3)TiO 3 -(1 − x)Ba(Zr 0.2 Ti 0.8)O 3 (x = 0.40, 0.45, 0.50, 0.55, and 0.60) through a combination of Raman spectroscopy, synchrotron X-ray diffraction, and dielectric spectroscopy. The aim is to gain insight into the complex phase boundary region responsible for the excellent electromechanical properties. The results demonstrate the correlation between local site substitutions based on the stoichiometric variations to the microstructure and dielectric properties. The dielectric response has been correlated with the BCT/BZT content displaying a maximum depending on the phase content. Additionally, in situ temperature-dependent Raman, permittivity, and pair distribution function (PDF) studies were performed from −60 °C to 130 °C to highlight the structure and phase evolution. In particular, the in situ temperature-dependent Raman measurements reveal sudden discontinuities in the vibrational modes that correspond to the structural changes in the perovskite structure. Therefore, the results of the permittivity response, based on the average of a large volume, are consistent with the local structural changes obtained by other techniques. Indeed, correlations of Raman, X-ray diffraction, and PDFs obtained from synchrotron X-ray total scattering data, along with permittivity measurements allowed the identification of the discrete ferro- to paraelectric phase transitions and a more robust characterization. The rotation and distortion of the octahedral caused by oxygen displacement are driving forces behind symmetry changes and phase transitions, explaining the mechanism of polymorphic phase transition based on A- and B-site substitution. Therefore, this work provides a comprehensive understanding of temperature and composition-dependent phase transitions in BCZT. [Display omitted] • The polymorphic phase transitions in BCZT ceramics are investigated as a function of composition and temperature. • The mechanism of polymorphic phase transition is explained based on A and B site substitution. • Variations in octahedra distortion due to the Ti/Zr substitution explain the symmetry change as a function of composition. • in situ T-dependent Raman data show sudden discontinuities in the vibrational modes that agree with the phase transitions. • Symmetry in BCZT can be estimated using three-phase transition equations based on barycenter of Raman stretching mode. [ABSTRACT FROM AUTHOR]