Elastic and anelastic relaxations accompanying relaxor ferroelectric behaviour of Ba6GaNb9O30 tetragonal tungsten bronze from resonant ultrasound spectroscopy

Andrei Rotaru,Michael A Carpenter,Jason A Schiemer

doi:10.1007/s10973-015-5202-z

Abstract

Abstract Tetragonal tungsten bronze (TTB) structures offer some promise as lead-free ferroelectrics and have an advantage of great flexibility in terms of accessible composition ranges due to the number of crystallographic sites available for chemical substitution. The ferroic properties of interest are coupled with strain, which will be important in the context of stability, switching dynamics and thin film properties. Coupling of strain with the ferroelectric order parameter gives rise to changes in elastic properties, and these have been investigated for a ceramic sample of Ba6GaNb9O30 (BGNO) by resonant ultrasound spectroscopy. Room temperature values of the shear and bulk moduli for BGNO are rather higher than for TTBs with related composition which are orthorhombic at room temperature, consistent with suppression of the ferroelectric transition. Instead, a broad, rounded minimum in the shear modulus measured at ~1 MHz is accompanied by a broad rounded maximum in acoustic loss near 115 K and signifies relaxor freezing behaviour. Elastic softening with falling temperature from room temperature, ahead of the freezing interval, is attributed to the development of dynamical polar nanoregions (PNRs), whilst the nonlinear stiffening below ~115 K is consistent with a spectrum of relaxation times for freezing of the PNR microstructure.

Highlights

The high permittivity of polar dielectrics such as piezoelectrics or ferroelectrics has made these materials useful in electronic devices, with broad application in devices such as capacitors and resonators [1]
It is clear that the advantages of using a mechanical spectroscopy technique like Resonant ultrasound spectroscopy (RUS) lie in the interesting attribute strain coupling phenomena to all structural changes that may be otherwise found from dielectric spectroscopy, magnetic studies, calorimetry, X-ray and neutron diffraction, etc
The elastic and anelastic properties of Ba6GaNb9O30 (BGNO) reveal a material which is significantly stiffer than related ferroelectric structures with nearby compositions, probably due to suppression of the ferroelectric transition

Summary

Introduction

The high permittivity of polar dielectrics such as piezoelectrics or ferroelectrics has made these materials useful in electronic devices, with broad application in devices such as capacitors and resonators [1]. The TTB structure consists of a network of corner sharing BO6 octahedra formed around the perovskitic A1 site that creates further two types of channels: pentagonal A2 channels (which can be occupied by alkali, alkaline earth and rare earth cations) and smaller triangular C channels (mostly vacant, they can be filled/ just partially filled by small low-charged cations like Li?—e.g., K6Li4Nb10O30) These materials, known to exhibit diverse properties as a result of compositional flexibility and by a higher probability for cation ordering, may offer better ways of attaining room-temperature ferroelectricity and (anti)ferromagnetism, multiferroic behaviour and eventually magnetoelectric coupling [2, 8]. Most mechanical resonances of a small object are determined predominantly by shearing motions so that the temperature dependence of fp provides a good representation of the temperature dependence of the shear modulus

Results and discussion

Á A1 h ÀA2 þ

Conclusions