Anharmonicity of BaTiO3 single crystals

By analyzing the dielectric nonlinearity with the Landau thermodynamic expansion, we find a simple and direct way to assess the importance of the eighth-order term. Following this approach, it is demonstrated that the eighth-order term is essential for the adequate description of the paraelectric-ferroelectric phase transition of BaTiO3. The temperature dependence of the quartic coefficient beta is accordingly reconsidered and is strongly evidenced by the change of its sign above 165 degrees C. All these findings attest to the anomalously strong polarization anharmonicity of this material, which is unexpected for classical displacive ferroelectrics.