Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films.

Alexander Tkach,Olena Okhay,Dmitry Nuzhnyy,Paula M Vilarinho,Jan Petzelt

doi:10.3390/ma14216414

Alexander Tkach, Olena Okhay + Show 3 more

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https://doi.org/10.3390/ma14216414

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Abstract

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol–gel-derived Sr1−1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared spectroscopy in a transmittance mode. Bi doping, known to induce a low-frequency dielectric relaxation in SrTiO3 (ST) ceramics and films, due to off-centre dopant ion displacements generating electric dipoles, is shown to affect the polar phonon behaviour of thin films. We show that in weakly Bi-doped films, the low-frequency polar TO1 mode softens on cooling but less than in undoped ST. In heavily Bi-doped ST films, this mode displays no significant frequency variation with temperature from 300 to 10 K. The polar phonon behaviour of polycrystalline Bi-doped ST thin films is comparable with that of Bi-doped ST ceramics, which exhibit dielectric relaxations and harden soft-mode behaviour instead of the ferroelectric phase transition.

Highlights

Perovskite-structured SrTiO3 (ST) stands out as an incipient ferroelectric with dielectric permittivity increasing continuously on cooling due to the polar-mode softening but without a ferroelectric phase transition [1,2,3,4,5]
This indicates that the mechanism for the reduction of the dielectric permittivity ε under applied field is due to the hardening of the soft mode [2], which arises from the anharmonic restoring forces on Ti ions when displaced from their equilibrium positions [13]
The only difference between the weakly and heavily Bi-doped ST films is a slight shift of the X-ray diffraction (XRD) peaks toward lower 2θ values with Bi content, implying the lattice parameter increase

Summary

Introduction

Perovskite-structured SrTiO3 (ST) stands out as an incipient ferroelectric with dielectric permittivity increasing continuously on cooling due to the polar-mode softening but without a ferroelectric phase transition [1,2,3,4,5]. The zone-centre soft-mode phonon was reported to increase its frequency with applied electric field [2]. This indicates that the mechanism for the reduction of the dielectric permittivity ε under applied field is due to the hardening of the soft mode [2], which arises from the anharmonic restoring forces on Ti ions when displaced from their equilibrium positions [13]

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Conclusion