$FeTiTaO_6$: A Lead-Free Relaxor Ferroelectric Based on the Rutile Structure

Abstract

Several materials properties of rutile $TiO_2$ (together with its other polymorphs) have been widely investigated in view of both fundamental and practical interest. It is a wide band gap $(E_g=3.05 eV)$ semiconductor that finds application, among others, as a photocatalyst for splitting water[1] into $H_2$ and $O_2$ and remediation of organic pollutants.[2] Among the lesswell-known properties of $TiO_2$ is its abnormally large static dielectric permittivity that shows strong frequency dependence as well as the associated soft $A_{2u}$ mode that decreases with decreasing temperature.[3,4] The latter, however, never becomes completely soft, even at 0 K. Accordingly, rutile is classified as an incipient ferroelectric.[4] The soft $A_{2u}$ mode involves displacement of the positively charged $Ti^{4+}$ against negatively charged $O^{2-}$ that manifests in a large static dielectric permittivity (165–250) along the tetragonal c axis. The special dielectric behavior of $TiO_2$ is different from that of the other rutile oxides such as $GeO_2$ and $SnO_2$, clearly indicating the role of $d^0$ electronic configuration of $Ti^{4+}$ on the dielectric properties. It is known that transition metal oxides with $d^0$ electronic configuration undergo a second-order Jahn–Teller (SOJT) distortion of the $MO_6$ (M = metal) octahedra arising from a mixing of empty $d_0$ states of the transition metal with the O 2p states.[5] The distortion seems to be at the heart of several of the interesting dielectric/ferroelectric properties of $d^0$ transition-metal-containing perovskite oxides[6] such as $BaTiO_3$, $PbTiO_3$, $PbZr_{1-x}Ti_xO_3$, $Pb_3MgNb_2O_9$, and so on