Philippe Papet

Growth of Piezoelectric Water-Free GeO2 and SiO2-Substituted GeO2 Single-Crystals

Using the slow-cooling method in selected fluxes, we have grown spontaneously nucleated single-crystals of pure GeO(2) and SiO(2)-substituted GeO(2) materials with the α-quartz structure. These piezoelectric materials were obtained in millimeter size as well-faceted, visually colorless, and transparent crystals. Cubic-like or hexagonal prism-like morphology was identified depending on the chemical composition of the single-crystals and on the nature of the flux. Both the silicon substitution rate and the homogeneity of its distribution were estimated by Energy Dispersive X-ray spectroscopy. The cell parameters of the flux-grown GeO(2) and Ge(1-x)Si(x)O(2) (0.038 ≤ x ≤ 0.089) solid-solution were deduced from their X-ray powder diffraction pattern. As expected, the cell volumes decrease as the silicon content substitution increases. A room temperature Infrared spectroscopy study confirms the absence of hydroxyl groups in the as-grown crystals. Unlike what was observed for hydrothermally grown GeO(2) crystals, these flux-grown oxide materials did not present any phase transition before melting as pointed out by a Differential Scanning Calorimetry study. Neither a α-quartz/β-quartz transition as encountered in SiO(2) near 573 °C nor a α-quartz to rutile transformation were detected for these GeO(2) and Ge(1-x)Si(x)O(2) single-crystals.

Vibrational Origin of the Thermal Stability in the Highly Distorted α-Quartz-Type Material GeO2: An Experimental and Theoretical Study

We report an experimental and theoretical vibrational study of the high-performance piezoelectric GeO2 material. Polarized and variable-temperature Raman spectroscopic measurements on high-quality, water-free, flux-grown α-quartz GeO2 single crystals combined with state-of-the-art first-principles calculations allow the controversies on the mode symmetry assignment to be solved, the nature of the vibrations to be described in detail, and the origin of the high thermal stability of this material to be explained. The low-degree of dynamic disorder at high-temperature, which makes α-GeO2 one of the most promising piezoelectric materials for extreme temperature applications, is found to originate from the absence of a libration mode of the GeO4 tetrahedra.

Temperature Dependence of Single-Crystal Elastic Constants of Flux-Grown α-GaPO4

The lattice parameter change with respect to temperature (T) has been measured using high-temperature powder X-ray diffraction techniques for high-temperature flux-grown GaPO(4) single crystals with the alpha-quartz structure. The lattice and the volume linear thermal expansion coefficients in the temperature range 303-1173 K were computed from the X-ray data. The percentage linear thermal expansions along the a and c axes at 1173 K are 1.5 and 0.51, respectively. The temperature dependence of the mass density rho of flux-grown GaPO(4) single crystals was evaluated using the volume thermal expansion coefficient alpha(V)(T) = 3.291 x 10(-5) - 2.786 x 10(-8) [T] + 4.598 x 10(-11)[T](2). Single-crystal high-resolution Brillouin spectroscopy measurements have been carried out at ambient pressure from 303 to 1123 K to determine the elastic constants C(IJ) of high-temperature flux-grown GaPO(4) material. The single-crystal elastic moduli were calculated using the sound velocities via the measured Brillouin frequency shifts Deltanu(B). These are, to our knowledge, the highest temperatures at which single-crystal elastic constants of alpha-GaPO(4) have been measured. Most of the room-temperature elastic constant values measured on flux-grown GaPO(4) material are higher than the ones found for hydrothermally grown GaPO(4) single crystals. The fourth-order temperature coefficients of both the Brillouin frequency shifts T(nuB)((n)) and the single-crystal elastic moduli T(C(IJ))((n)) were obtained. The first-order temperature coefficients of the C(IJ) are in excellent agreement with previous reports on low-temperature hydrothermally grown alpha-GaPO(4) single crystals, while small discrepancies in the higher-order temperature coefficients are observed. This is explained in terms of the OH content in the GaPO(4) network, which is an important parameter in the crystal thermal behavior.

A unified electrical SPICE model for piezoelectric transducers

The widespread use of piezoelectric materials in an increasing number of applications requires the development of advanced realistic electrical models. Until now, models were limited taking into account only one ceramics operation mode, i.e., thickness or planar. On the other hand, it establishes that the robustness of the piezo-electronic systems is likely to weaken if the models making it possible to conceive do not improve, in particular by taking into account further real phenomena. This article proposes to merge, in a new electrical model, the two operation modes. It is demonstrated that the electrical behavior of the proposed model is in very good agreement with the real ceramic behavior.

A 2-D KLM Model for Disk-Shape Piezoelectric Transducers

Piezoelectric materials are widely used for many applications such as sensors, actuators. Today, their integration in microelectronics processes like CMOS requires the development of advanced realistic behavioral models. Until now, these models were limited to only one ceramics operation mode, i.e., thickness or planar. Moreover, the robustness of piezo-electronic systems cannot be adequately addressed as long as models are not improved in particular by taking into account further real phenomena. This article proposes to merge, in a new KLM model, the two operation modes. It is demonstrated that the behavior of the proposed model is in very good agreement with the real ceramic behavior.

A 2-D VHDL-AMS Model for Disk-Shape Piezoelectric Transducers

Piezoelectric materials are widely used for many applications such as sensors, actuators. Today, their integration in microelectronics processes like CMOS requires the development of advanced realistic behavioral models. Until now, these models were limited to only one ceramics operation mode, i.e., thickness or planar. Moreover, the robustness of piezo-electronic system cannot be adequately addressed as long as models are not improved, in particular by taking into account further real phenomena. This article proposes to merge, in a new behavioral model, the two operation modes. It is demonstrated that the electrical behavior of the proposed model is in very good agreement with the real ceramic behavior.

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.

Cubic Sr2ScGaO5 Perovskite: Structural Stability, Oxygen Defect Structure, and Ion Conductivity Explored on Single Crystals

Oxygen-deficient Sr2ScGaO5 single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO6 octahedra and GaO4 tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature-dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr2ScGaO5 to be thermodynamically stable only above 1400 °C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr2ScGaO5 remains surprisingly kinetically stable upon heating from ambient temperature to 1300 °C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10-4 S/cm at 600 °C, while oxygen 18O/16O isotope exchange indicates a free oxygen mobility to set in at around 500 °C.

Growth of α-GaPO4 and α-GeO2 single crystals by the flux method

Colorless and transparent single crystals of the piezoelectric materials α-GaPO 4 and α-GeO 2 have been grown by the spontaneous nucleation method from Li 2 Mo 3 O 10 flux at high temperature. Lattice parameters of the hexagonal-phase crystals have been measured by a single crystal X-ray diffraction method. Results of the infrared measurement have indicated the possibility of growing almost hydroxyl-free α-GaPO 4 crystals by the flux method. Thermal analyses have pointed out the phase transformation α-quartz GaPO 4 ↔ β-cristobalite GaPO 4 .

Phase Transitions at high-pressure and structural description of the macroscopic ferroelectric properties of the Pb(Zr 1−x Ti x )O 3 solid solution

PbZr1−x TixO3 (PZT) is an important class of ferroelectrics materials with the ABO3 perovskite-type structure due to their exceptional dielectric and piezoelectric properties. It is well known that PZT ceramics and thin films are sensitive to the level of stress induced by both external elastic and electric fields which fundamentally modifies their physical properties. In order to improve the understanding of structure-properties relationship, Pb(Zr1−x Tix)O3 materials were investigated at high-pressure by X-ray and neutron diffraction. Upon increasing pressure a series of low-symmetry structures are identified for both Ti and Zr rich compositions and the high pressure behavior is consistent with a reduction and a rotation of the spontaneous polarization and the onset of octahedral tilting leading to unit cell doubling. To ameliorate the knowledge of the origin of exceptional piezoelectric properties of PZT materials with compositions at the morphotropic phase boundary (MPB), in situ structural picture of the ferroelectric properties as a function of the applied electric field is reported (data were obtained by synchrotron Xray diffraction). Finally, the ferroelectric fatigue of PZT ceramics with composition close to the MPB was also investigated and the structural contribution to the fatigue was pointed out as a reduced degree of tetragonal-to-monoclinic transformation which accounts for the reduction of the piezoelectric/ferroelectric efficiency.