Microscopic probing of order-disorder versus displacive behavior in BaTiO3 by Fe3+ EPR.

Abstract

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(\ensuremath{\partial} lna/\ensuremath{\partial}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 ${\mathrm{KTaO}}_{3}$.