Microscopic probing of order-disorder versus displacive behavior inBaTiO3byFe3+EPR

The cubic-crystalline field-splitting parameter a of ${\mathrm{Fe}}^{3+}$ at a Ti site in ${\mathrm{BaTiO}}_3$ has been measured with EPR in the cubic phase as a function of pressure p and temperature T. From these measurements, the relative explicit volume and temperature dependences of a(p,T) have been obtained. The former is three times those found in MgO, ${\mathrm{KTaO}}_3$, or ${\mathrm{SrTiO}}_3$. The relative explicit temperature dependence(∂ lna/∂T${)}_V$ is positive and four and a half times that found in inert MgO. In contrast, this effect is negative in ${\mathrm{KTaO}}_3$ and ${\mathrm{SrTiO}}_3$, where soft underdamped ferroelectric modes dominate. Both giant effects in ${\mathrm{BaTiO}}_3$ point to a strong local Ti anharmonicity and ferroelectric order-disorder behavior in contrast to ${\mathrm{SrTiO}}_3$ and ${\mathrm{KTaO}}_3$.