Piezoelectricity is inherent only in noncentrosymmetric materials, but a piezoelectric response can also be obtained in centrosymmetric crystals if subjected to inhomogeneous deformation. This phenomenon, known as flexoelectricity, can significantly affect the functional properties of insulators, particularly thin films of high permittivity materials. We have measured strain-gradient-induced polarization in single crystals of paraelectric SrTiO3 as a function of temperature and orientation down to and below the 105 K phase transition. Estimates were obtained for all the components of the flexoelectric tensor, and calculations based on these indicate that local polarization around defects in SrTiO3 may exceed the largest ferroelectric polarizations. A sign reversal of the flexoelectric response detected below the phase transition suggests that the ferroelastic domain walls of SrTiO3 may be polar.

No abstract

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.

No abstract

The effects of grain size on the elastic properties of quartz through the α–β phase transition have been investigated by resonant ultrasound spectroscopy. It is found that there are three regimes, dependent on grain size, within which elastic properties show different evolutions with temperature. In the large grain size regime, as represented by a quartzite sample with ∼100–300 µm grains, microcracking is believed to occur in the vicinity of the transition point, allowing grains to pull apart. In the intermediate grain size regime, as represented by novaculite (1–5 µm grain size) and Ethiebeaton agate (∼120 nm grain size), bulk and shear moduli through the transition follow closely the values expected from averages of single crystal data. The novaculite sample, however, has a transition temperature ∼7 °C higher than that of single crystal quartz. This is assumed to be due to the development of internal pressure arising from anisotropic thermal expansion. In the small grain size region, agates from Mexico (∼65 nm) and Brazil (∼50 nm) show significant reductions in the amount of softening of the bulk modulus as the transition point is approached from below. This is consistent with a tendency for the transition to become more second order in character. The apparent changes towards second order character do not match quantitative predictions for samples with homogeneous strain across elastically clamped nanocrystals, however. Some of the elastic variations are also due to the presence of moganite in these samples. True ‘nanobehaviour’ for quartz in ceramic samples thus appears to be restricted to grain sizes of less than ∼50 nm.