Anomalous elastic behaviour of Pb0.76Ca0.24TiO3 (PTC) at the transition from the amorphous to the nanocrystalline state compared to Pb0.88La0.08(Zr0.65Ti0.35)O-3 (PLZT)

Thin amorphous films of PTC and PLZT of 1-3 mu m thickness were produced by rf-sputtering at ambient temperature. Different grain sizes between the subnanometer and the micrometer region could be prepared by a subsequent annealing process. The increasing size of the crystallites was characterised by profile analysis of x-ray powder diffraction spectra. It is expected that the small grain size of compact nanocrystalline materials will have drastic influences on their elastic properties. High resolution Brillouin spectroscopy was used to probe the hypersonic properties of the two materials at ambient temperature. For the PLZT samples the elastic modulus cll shows the expected increase at the transition from the amorphous to the nanocrystalline state. On the contrary the PTC samples exhibit a surprising elastic instability in the same region.