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.

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By framework structures are meant materials consisting of relatively stiff units such as octahedra or tetrahedra, joined by shared oxygen (or other) atoms at the corners. Examples are ZrW 2 O 8 and many aluminosilicates. Rigid rotation of the units often gives a reduction of the volume or of some lattice constant as a purely geometrical effect. The theory of this effect is developed and shown to give a negative contribution to the thermal expansion coefficient. This is in addition to the usual positive contribution from anharmonicity of the interatomic forces. The negative effect varies through the phonon spectrum, being strongest for low frequencies, but the sign of the temperature coefficient may be reversed above a soft mode phase transition.

International audienceWe placed isoflurane, a general anaesthetic, inside palmitoyloleoylphosphatidylcholine (POPC) bilay-ers at clinical concentration, and performed molecular dynamics simulations at atmospheric and raised pressures, using two different thermodynamic ensembles. We also performed a simulation of this system with isoflurane at ten times the clinical concentration. We found that isoflurane did not aggregate inside POPC membranes at 20 MPa, nor at 40 MPa. The implications of these findings for pressure reversal is discussed, in light of the high-pressure neurological syndrome