Evidence of an Elastic Instability between the Amorphous and the Microcrystalline State of Ca-modified Lead Titanate as revealed by Brillouin Spectroscopy

Abstract

Brillouin spectroscopy was used to probe the hypersonic properties of Ca-modified lead titanate between the pure amorphous and the coarse grained state. The samples were thin films rf sputtered onto (100)-silicon wafer at ambient temperature, followed by an annealing process in the range from 673 to 873 K. An elastic instability of the longitudinally polarized sound mode suggests the existence of a discontinuous structural transformation from the amorphous to the nanostructured state. [S0031-9007(97)02610-0] PACS numbers: 81.05. Ys, 68.35.Rh, 68.60.Bs, 78.35. + c Against the background of an increasing interest in nanostructured materials (nsM) the physical properties of the intermediate metastable structures between the pure amorphous and the microcrystalline states require special attention. One possible way to realize such intermediate states is to produce sputtered amorphous (dielectric) films and to anneal these films successively in an appropriate way. It is likely that the annealing-induced transformation from the initial amorphous state to the final coarse grained state may pass through one or more so-called metastable nanocrystallized states (e.g. [1][2][3][4][5]). It is a matter of current physical interest (i) whether these intermediate nanostructured states (nsS) can be fixed, (ii) how the transformation from the amorphous to the microcrystalline states takes place, and (iii) what are the properties of these intermediate states. Although the amorphous state as well as the nsS are metastable, the possibility of a structural phase transformation between the crystalline and the amorphous states has been proposed in the literature [1,5]. Based on free-energy simulations, Wang et al. [6], only recently, have predicted a phase transition from the nanocrystalline to the glassy state. A hint for the existence of such a transition may be drawn from the work of Rehn et al. [1]. These authors have reported an elastic shear instability for ion damaged Zr 3 Al.It is well established now that the evolution of the nsS is accompanied by an anomalous behavior in the elastic properties when compared to the amorphous and the microcrystalline reference states [2,4,7,8]. Experimental and theoretical results indicate a significant influence in the size of nanoparticles on the elastic properties of the consolidated nsM (e.g., [2,4,[9][10][11][12][13]). In turn, the elastic properties are expected to reflect the predicted transition from the amorphous to the nsS. The aim of the current paper is to show evidences for such a transformation in rf-sputtered films of Ca-modified lead-titanate ͑Pb 0.76 Ca 0.24 ͒TiO 3 (PTC) at a critical annealing temperature ͑T ca ഠ 823 K͒. As a sensitive probe we used the lon-gitudinally polarized acoustic phonon mode as detected by high resolution Brillouin spectroscopy (BS).PTC is -in the single or coarse grained polycrystalline state-a ferroelectric perovskite with a phase transition from the paraelectric to the ferroelectric state at T c ഠ 530 K [14]. At room temperature, the material is t...

Crystallisation of Pb1−xCaxTiO3 ferroelectric thin films as a function of the Ca2+ content

“…This behaviour has previously been reported for PCT24 films, where the elastic instability was observed at similar temperatures. 19,20 Therefore, BS results indicate that complete crystallisation of the PCT50 films occurs at temperatures of ∼500 • C, like in films with low calcium. Reflections detected by X-ray diffraction in films treated at lower temperatures correspond to perovskite (Fig.…”

Elastic properties of single-crystalline and consolidated nano-structured yttrium oxide at room temperature

“…This leads to the conclusion that the additional consolidation process of the nc-Y 2 O 3 sample is responsible for the above mentioned inner surfaces which strongly dominate the macroscopic elastic behaviour and therefore lead to the significant softening of c 11 of the consolidated sample. This hypothesis is supported by the above mentioned unexpected appearance of strong Mie scattering of the nc-Y 2 O 3 sample due to two-dimensional voids which have no equivalence in the perfectly consolidated samples [10,14]. This result shows that if one wants to compare the physical properties of nc-and sc-samples, the influence of structures caused by the preparation process of nc-samples can lead to wrong conclusions about their physical properties.…”

“…In [14] Krüger et al showed that for Ca-modified lead titanate (PCT) c 11 of nc-samples of PCT can be higher as well as lower than c 11 of mc-samples depending on the grain size of the nc-material, but both values maximally differ by a factor of 0.7. For nc-and mc-diamond, the difference of the elastic stiffness coefficients is even smaller [10].…”

“…For nc-and mc-diamond, the difference of the elastic stiffness coefficients is even smaller [10]. The nc-and mc-samples of PCT investigated by Krüger et al [14] were produced by successive annealing of pure amorphous material whereas the diamond samples [10] were produced by chemical vapour deposition.…”

Acoustoelastic anomaly in stressed heterostructures

“…1 More recently ultrasonic waves have also been employed to measure the elastic properties of polycrystalline and epitaxial thin films. [2][3][4][5] Though still rare, such measurements are particularly important for heterostructures as the elastic constants play a key role for the understanding of the interdependence of stress and strain-with particular impact for electronic and optoelectronic devices. So far it is not clear whether the elastic constants of thin films deviate from the respective bulk values, which usually are used to calculate stress from strain and vice versa.…”