Structural and electrical properties of layered perovskite type Pr₂Ti₂O₇: experimental and theoretical investigations

Abstract

In this communication we report the details of the structural and thermal properties of monoclinic layered perovskite type Pr₂Ti₂O₇ (PTO) using ambient to higher temperature XRD and Raman spectroscopic studies. The monoclinic (P2₁) structure is found to be the stable structure of PTO compared to the orthorhombic Pna2₁, Cmc2₁ or Cmcm and monoclinic P2₁/m structures. The crystal structure is further supported by the ab initio total energy calculations using density functional theory (DFT) formalism. The total energy calculation and structural relationship favour the ferroelectric (P2₁) to paraelectric (P2₁/m) displacive transition. The calculated electric polarization as observed from the displacement of ions is ∼8.3 μC cm⁻². The calculated electron density of states indicated a band gap of about 2.7 eV, which closely agrees with that measured by UV-Vis diffuse reflectance spectroscopy. Variable temperature XRD and differential thermal analysis studies revealed no structural transition to Cmc2₁ in the temperature range from ambient to 1473 K as reported for analogous rare-earth titanates, like La₂Ti₂O₇ and Nd₂Ti₂O₇. A partial decomposition of PTO to cubic perovskite type structure is observed at around 1673 K. The measurement of field dependent electric polarization indicates the ferroelectric nature of PTO. The electrical properties of PTO have also been investigated by ac impedance spectroscopic studies from 173 to 1073 K. The low temperature dielectric data indicate two different types of relaxations, one at a lower frequency region and strongly temperature dependent while the other at a higher frequency region (>1 kHz) and nearly temperature independent. The low and high frequency relaxations have been attributed to the thermally activated polarization process arising from the grain boundaries and dipolar orientations, respectively. The activation energy for a thermally activated low frequency relaxation process is 0.38 eV, which is similar to the interfacial polarizations due to ionic movements. An appreciable contribution of ionic conductivity in PTO is observed at still higher temperature (∼700 K). The activation energy for ionic conductivity is about 0.60 eV.