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Abstract
It is well known that BaTiO3 exhibits four phase transitions (1460°C, 135°C, 5°C, -90°C). The most extensively discussed transition is the paraelectric-ferroelectric one at 135°C: displacive with an over-damped soft mode, order-disorder or relaxator ferroelectric? Homovalent and heterovalent substitutions change the phenomenological Landau approach of the transition, the lattice dynamics, the electronic structures and the covalency of oxygen bonds.