Relaxor ferroelectric perovskites are highly polarizable and can exhibit giant coupling between elastic strain and an applied electric field. Here, we report an in situ extended X-ray absorption fine structure (EXAFS) study of a PbZn1/3Nb2/3O3 (PZN) single crystal as a function of the electric field. We show that the strong dipoles in the NbO6 octahedra bonds are aligned along the four < 011 > directions close to the orientation of the electric field, while a small reversible polar shift occurs for Zn in the direction of the electric field, i.e., positive or negative. This reversible Zn-O polar shift is proposed to play an important role in both the "easy" switching of the ferroelectric polarization and the giant piezoelectric effect in PZN.

GaAsO4 crystals were grown by a hydrothermal epitaxial process. Micro crystalline GaAsO4 powder was first synthesized by hydrothermal methods by using GaAs as a starting material in a sulfuric acid solvent under oxidizing conditions. Experimental conditions were optimized and the yield of the nutrient synthesis reached 88%. GaAsO4 powder was then used as the nutrient for growing GaAsO4 single crystals by epitaxy on (2(1) over bar 0) oriented GaPO4, AlPO4 and GaAsO4 plates. Crystal growth was performed in a horizontal PTFE-lined autoclave divided into two parts using sulfuric acid as the solvent under a slow heating gradient. Hydrothermal conditions were optimized in order to improve the crystal quality. The influence of the nature and orientation of the seeds on the growth rate was studied. From the solubility curve, the solute supply Delta S as a function of temperature during the growth process was calculated. Transparent crystals were obtained with Delta S between 0.002 and 0.009 mol/L. Large crystals (3.9 Chi 1.8 Chi 1.4 cm(3)) of gallium arsenate were obtained. These crystals were cut into (2(1) over bar 0), (010) and (001) plates. Mapping using Raman spectroscopy was performed in order to study the growth recovery on the seed. GaAsO4 crystals were characterized by infrared spectroscopy in order to quantify the amount of OH groups in the crystal structure. A low value of the absorption coefficient a at 3300 cm(-1) was measured 0.067 cm(-1) indicating a good crystal quality. Optical measurements were performed on GaAsO4 crystals by UV-vis-NIR spectrophotometry. Gallium arsenate exhibits the highest value of birefringence in the alpha-quartz group with Delta n =0.033. The transmission in the spectral region above 250 nm yields a bandgap of 5.85 eV. Piezoelectric properties were measured on resonators. An electromechanical coupling coefficient of 20% was measured which is the highest in the alpha-quartz group. A quality factor QF of 3.2 Chi 10(10) was obtained on a Y-cut plate. The C-66 elastic constant was found to be 20 GPa.

Microporous AlPO4-54·xH2O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H2O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H2O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO4-54·xH2O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IVAl, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.

We have investigated the phonon modes of Ag-3[Co(CN)(6)] using first-principles based methods and high-pressure infrared spectroscopy to understand the origin of its colossal thermal expansion. Griineisen parameters of its zone-center vibrational modes were obtained from the calculations and compared to the experimental ones. We found that optical phonon modes below 100 cm(-1) mainly contribute to its negative thermal expansion, and they also have a significant mixing with acoustic branches. These modes have been assigned to translational motion of the Co-CN-Ag-NC-Co linkages. We also identify a pressure-induced softening of zone-center phonon modes that could account for the experimentally observed phase transition in terms of a first-order displacive instability from the trigona1P-3-1m space group to the acentric Cm space group.

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc(II) cyanide (Zn(CN)(2)) and cadmium(II) imidazolate (Cd(im)(2)), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn(CN)(2) undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn(CN)(2). We find that the fundamental mechanical response exhibited by Zn(CN)(2) is mirrored in the temperature-dependent behavior of Cd(im)(2). Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology.

SAPO-37 with a faujasite-type structure was studied by X-ray diffraction at high temperature and high pressure. A similar degree of negative thermal expansion (NTE) was observed for SAPO-37 with respect to siliceous faujasite with alpha(v(303-773 K)) = -10.7 x 10(-6) K-1. This similarity can be linked to the important role of oxygen displacement in proposed NTE mechanisms. SAPO-37 is more compressible than other faujasites, with a bulk modulus of 18(1) GPa. SAPO-37 also undergoes pressure-induced amorphisation at lower pressures. These results indicate that changes in chemical composition can strongly modify the stability and compressibility of the faujasite network.