What happens to the crystal structure of barium titanate below 120 °C?

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Figure 1: Ferroelectric properties of barium titanate (BaTiO3). (Left) Above 120° C the structure of the BaTiO3 crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal. — capacitor dielectric and piezoelectric ceramics Figure 1: Ferroelectric properties of barium titanate (BaTiO₃). (Left) Above 120° C the structure of the BaTiO₃ crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal.

Below 120 °C, barium titanate undergoes a structural transition that changes its symmetry from cubic to tetragonal.1

In this process, the following shifts occur within the crystal structure:
* The barium (Ba²⁺) and oxygen (O²⁻) ions shift from their original cubic positions.1
* The titanium (Ti⁴⁺) ion moves away from the center of the cube.12

This displacement results in a loss of the center of symmetry and creates a permanent electric dipole, where positive and negative charges concentrate toward opposite poles of the vertical axis. This spontaneous polarization, known as ferroelectricity, occurs below the Curie point.12