Anelastic relaxation effects and elastic instabilities in cgg-type compounds

2016

J Therm Anal Calorim

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 Ba 6 GaNb 9 O 30 (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.

Section: Temperature-dependent Elastic Propertiesmentioning

confidence: 99%

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

Rotaru

Schiemer

2016

J Therm Anal Calorim

“…This involved determining peak positions and peak widths at half height for selected peaks by fitting with an asymmetric Lorentzian function. This approach is analogous to that described by Schreuer et al (2003) and Schreuer and Thybaut (2005). The inverse quality factor, Q −1 = f /f , where f is peak frequency and f is the peak of the width at half height, is a measure of acoustic dissipation within the sample.…”

Section: Resonant Ultrasound Spectroscopymentioning

confidence: 99%

Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃: II. Strain–order parameter coupling and dynamic softening mechanisms

J. Phys.: Condens. Matter

Bryson

Catalán

et al. 2011

Elastic and anelastic behaviour of single crystal and ceramic samples of Pb(Mg(1/3)Nb(2/3))O(3) has been investigated at frequencies of ~0.1-1.2 MHz through the temperature interval 10-800 K by resonant ultrasound spectroscopy (RUS). Comparison with data from the literature shows that softening of the shear modulus between the Burns temperature and the freezing interval is independent of frequency. The softening is attributed to coupling between acoustic modes and the relaxation mode(s) responsible for central peaks in Raman and neutron scattering spectra below the Burns temperature, and can be described with Vogel-Fulcher parameters. Shear elastic compliance and dielectric permittivity show similar patterns of temperature dependence through the freezing interval, demonstrating strong coupling between ferroelectric polarization and strain such that the response to applied stress is more or less the same as the response to an applied electric field, with a frequency dependence consistent with Vogel-Fulcher-like freezing in both cases. Differences in detail show, however, that shearing induces flipping between different twin orientations, in comparison with the influence of an electric field, which induces 180° flipping: the activation energy barrier for the former appears to be higher than for the latter. Below the freezing interval, the anelastic loss also has a similar pattern of evolution to the dielectric loss, signifying again that essentially the same mechanism is involved in the freezing process. Overall softening at low temperatures is attributed to the contributions of strain relaxations due to coupling with the local ferroelectric order parameter and of coupling between acoustic modes and continuing relaxational modes of the polar nanostructure. Dissipation is attributed to movement of boundaries between PNRs or between correlated clusters of PNRs. Overall, strain coupling is fundamental to the development of the characteristic strain, dielectric and elastic properties of relaxors.

“…The slightly less than 'good' fits are assumed to be due to anisotropy of grain orientations or to an anisotropic distribution of pore sizes/orientations. Peak positions and widths were also determined for selected peaks in the RUS spectra from the Cambridge instrument by fitting with an asymmetric Lorentzian function, analogous to the approach described by Schreuer et al (2003) and Schreuer and Thybaut (2005). The Los Alamos spectra had been collected at lower resolution in frequency, and peak widths were estimated by printing relevant segments and measuring the peaks manually.…”

Section: Rus Data Analysismentioning

confidence: 99%

Elastic relaxations associated with the Pm\bar {3}m –R\bar {3}c transition in LaAlO₃: III. Superattenuation of acoustic resonances

J. Phys.: Condens. Matter

Buckley

Taylor

et al. 2009

101

Resonant ultrasound spectroscopy has been used to characterize elastic softening and anelastic dissipation processes associated with the Pm3m <--> R3c transition in single crystal and ceramic samples of LaAlO(3). Softening of the cubic structure ahead of the transition point is not accompanied by an increase in dissipation but follows different temperature dependences for the bulk modulus, (1/3)(C(11) + C(12)), and the shear components, (1/2)(C(11) + C(12)) and C(44), as if the tilting instability contains two slightly different critical temperatures. The transition itself is marked by the complete disappearance of resonance peaks (superattenuation), which then reappear below ∼700 K in spectra from single crystals. Comparisons with low frequency, high stress data from the literature indicate that the dissipation is not due to macroscopic displacement of needle twins. An alternative mechanism, local bowing of twin walls under low dynamic stress, is postulated. Pinning of the walls with respect to this displacement process occurs below ∼350 K. Anelasticity maps, analogous to plastic deformation mechanism maps, are proposed to display dispersion relations and temperature/frequency/stress fields for different twin wall related dissipation mechanisms. These allow comparisons to be made of anelastic loss mechanisms under mechanical stress with elastic behaviour observed by means of Brillouin scattering at high frequencies which might also be related to microstructure.