V. Bornand

Use of resonance Raman spectroscopy to study the phase diagram of Pb Zr 0.52 Ti 0.48 O 3

Evidence is presented for the first time that the sharp and continuous spectral changes observed in PbZr0.52Ti0.48O3 (PZT) between 350 and 10 K with the 647.1 nm wavelength are due to a resonance Raman effect. Such a phenomenon can be explained by means of a self-trapped exciton emission oxygen deficient complex (TiTi - VO-) of PZT powder whose energy is close to the radiation line of the laser. This kind of approach should also be very useful to distinguish the phase transition sequence for other related ferro/ piezoelectric systems.

P–T phase diagram of PbZr0.52Ti0.48O3 (PZT)

Abstract The important piezoelectric material, lead zirconate titanate perovskite PbZr0.52Ti0.48O3 (PZT) was investigated as a function of pressure and temperature by Raman spectroscopy. At ambient pressure, the transition between the low-temperature and high-temperature ferroelectric monoclinic phases occurs near 210 K and is followed by transformation to the tetragonal phase at close to 305 K. The critical pressure for the transition to the disordered polar cubic phase increases with decreasing temperature from 5 GPa at 298 K to 9 GPa at 44 K. The ferroelectric monoclinic phases thus have an extended stability range at low temperatures and high pressure. A strong Raman spectrum is obtained for the cubic phase at high pressure over the temperature range between 44 and 298 K indicating the presence of polar nanodomains. A P–T phase diagram of this material is proposed based on the present results.

Probing high-pressure phase transitions in Ti-based perovskite-type ferroelectrics using visible resonance Raman spectroscopy.

We report unprecedented dramatic changes in the 647.1 nm Raman signal of PbZr(0.6)Ti(0.4)O(3) occurring in the same pressure ranges as the critical pressures of the antiferrodistortive and ferroelectric-paraelectric phase transitions. This huge decrease in intensity of both the Raman modes and the background, observed for both pressure transmitting media used (glycerol or 4:1 methanol ethanol mixture), is shown to originate from the two-step loss of a resonance Raman effect and the concomitant fluorescence. Changes in the local titanium environment (first with the onset of octahedral tilting and then with the removal of polar cation displacements) alter the electronic band structure and modify the resonance conditions. Furthermore, the optimal resonance conditions are found to be particularly narrow, as shown by the fluorescence spectrum of PbZr(0.6)Ti(0.4)O(3) at atmospheric pressure characterized by the presence of a very well-defined sharp peak (fwhm = 8 nm) centered around 647.1 nm. These results thus demonstrate that visible resonance Raman spectroscopy can be used as a quick and efficient technique for probing phase transitions in PbZr(1-x)Ti(x)O(3) (PZT) and other technologically important perovskite-type materials such as PMN-xPT, PZN-xPT relaxors, lead free piezoelectrics, and ferroelectric nanopowders. This technique appears also a good alternative to UV Raman spectroscopy for probing the polar order at the nanoscale in ultrathinfilms and superlattices.

Oriented Growth of LiNbO3 Thin Films for SAW Properties

There is a global interest in developing Surface Acoustic Wave devices of high frequency capability, high power durability and zero temperature dependence of frequency. Lithium niobate has attracted much attention due to its unusual combination of ferroelectric, piezoelectric and acousto-optic coefficients. LiNbO3 thin films offer an enticing potential for high-frequency broadband pass SAW device applications when deposited onto silicon and Al2O3 substrates. In order to optimally benefit from the material properties, it is necessary to grow LiNbO3 thin films with a preferred orientation, for this phase with the c-axis normal to the surface of the substrate. For this purpose, an original 2-step growth process has been developed which involves the creation of a high-nucleation density by radio-frequency sputtering in the early stages of the film growth and an enhancement of both the crystallinity and the texture by reactive chemical sputtering (pyrosol). This work summarizes our studies on the growth of LiNbO3/<111>-Si and LiNbO3/<001>-Al2O3 heterostructures. Emphasis is given on interface control to promote <001>oriented crystallization process. Results from textural, microstructural and composition analyses are presented. By combining both the r.f. sputtering and the pyrosol methods, stoichiometric <001>-oriented heterostructures could be achieved. Depositions performed on <111>-Si templates led to fiber textures, characteristic of oriented polycrystalline materials. <001>-Al2O3 substrates allowed the development of hetero-epitaxial layers, built up of two 60°-rotated domains. The volumic ratio of the rotated domain appear to be in a 50 % range, by a direct integration of the pole contributions. Prefered type of presentation: Oral * Corresponding author, daniel.chateigner@ismra.fr, fax: 33 2 31 951600

Neutron and X-Ray Diffraction Studies of Piezoelectric Materials under Non-Ambient Conditions

In depth study of the crystal structures of piezoelectric materials as a function of temperature, pressure and composition allows for the design and optimization of such materials and defines the conditions of their use in technological applications. Results from studies on two classes of piezoelectric materials are described, the α-quartz group and the ferroelectric perovskite group. The structures of α-quartz-type germanium dioxide and iron phosphate were refined at high temperatures by the Rietveld method using time-of-flight neutron powder diffraction data. The α-β phase transition occurs at 980 K in FePO4, whereas for GeO2, no β phase is observed. The intertetrahedral bridging angle θ and the tilt angle δ in GeO2 exhibit thermal stabilities that are significantly greater than α-quartz. The temperature dependence of these angles is found to be a function of the initial structural distortion in α-quartz homeotypes with the notable exception of α-quartz-type FePO4, which appears to be dynamically unstable. The stability of α-quartz and α-quartz-type germanium dioxide was investigated at high pressure by x-ray powder diffraction. New six-fold coordinated forms were found in both materials. The important, perovskite-type, piezoelectric material PbZr0.52Ti0.48O3 was studied up to 18 GPa by angle-dispersive, x-ray diffraction using an imaging plate and by Raman spectroscopy. A novel phase transition was found in this system at close to 5 GPa. Whereas the x-ray diffraction data indicated no deviation from cubic symmetry above this pressure, a strong Raman signal was present in this phase, which is similar to those observed for ferroelectric relaxors.