Abstract
Coherent X-ray diffraction patterns were recorded by using an X-ray free-electron laser to illuminate barium titanate nanocrystals as a function of time delay after laser excitation. Rather than seeing any significant thermal expansion effects, the diffraction peaks were found to move perpendicular to the momentum transfer direction. This suggests a laser driven rotation of the crystal lattice, which is delayed by the aggregated state of the crystals. Internal deformations associated with crystal contacts were also observed.
Highlights
Barium Titanate, BaTiO3 (BTO), is an important ferroelectric material, which was first used to explain the coupling of lattice distortions with phonon mode softening, leading to its ferroelectric structure [1]
By directly viewing the crystal in 3-D, we find that both the Bragg density slices and displacement slices are complex and continuously changing inside a single crystal grain following laser excitation
Our X-ray Free Electron Laser (XFEL) experiment clearly shows that rotations of the BTO nanocrystals are induced by laser excitation on a 100 ps time scale, appropriate for an acoustic response of the material
Summary
Barium Titanate, BaTiO3 (BTO), is an important ferroelectric material, which was first used to explain the coupling of lattice distortions with phonon mode softening, leading to its ferroelectric structure [1]. It has the classical phase diagram for a perovskite oxide material, with a cubic structure at high temperature, becoming tetragonal below TC = 393 K and orthorhombic below T2 = 273 K [2]. BTO has clear structural phase transitions, there are studies showing local rhombohedral distortion even in the cubic phase [4,5].
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