GaAsO4 crystals were grown by a hydrothermal epitaxial process. Microcrystalline 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 (21¯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 ΔS as a function of temperature during the growth process was calculated. Transparent crystals were obtained with ΔS between 0.002 and 0.009mol/L. Large crystals (3.9×1.8×1.4cm3) of gallium arsenate were obtained. These crystals were cut into (21¯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 α at 3300cm−1 was measured 0.067cm−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 α-quartz group with Δn=0.033. The transmission in the spectral region above 250nm yields a bandgap of 5.85eV. Piezoelectric properties were measured on resonators. An electromechanical coupling coefficient of 20% was measured which is the highest in the α-quartz group. A quality factor QF of 3.2×1010 was obtained on a Y-cut plate. The C66 elastic constant was found to be 20GPa.

Quartz-type iron phosphate (FePO4) was studied by total neutron scattering and Raman spectroscopy up to 1150 K in order to investigate disorder and the mechanism of the alpha-beta transition. The increasingly large underestimation of P-O and Fe-O distances in Rietveld refinements of the average long-range structure as compared to the bond lengths obtained from the pair distribution function as a function of temperature is a clear indication of the presence of significant dynamic disorder, particularly from 850 K up to and above the alpha-beta transition near 980 K. A significant broadening of the Fe-O distance distribution is also observed. Reverse Monte Carlo modeling confirms the presence of such disorder with broadened angular distributions in this temperature range, in particular for the Fe-O-P and Fe-P-Fe distributions. The Raman spectrum, calculated using density functional theory, is in very good agreement with experiment. These calculations indicate that there is an inversion of two low-energy A(1) vibrational modes with respect to AlPO4. The principle mode, which exhibits strong damping in the Raman spectrum above 850 K, is thus not a tetrahedral libration mode, but a mode that principally involves large amplitude translations of the Fe atoms along with a degree of oxygen displacement. The transition mechanism from a dynamic point of view is thus different from the transitions in SiO2 and AlPO4. The strong damping of this mode is also further evidence of a high degree of dynamic disorder, which is different from the disorder observed in SiO2 and AlPO4. This mode does not exhibit any significant softening with temperature near the phase transition, which is further evidence that the alpha-beta transition is not of the simple displacive type. The difference in behavior between FePO4 and the other quartz homeotypes arises from the weaker bonding between the 3d(5) transition metal cation and oxygen.

Quartz-type iron phosphate (FePO${}_{4}$) was studied by total neutron scattering and Raman spectroscopy up to 1150 K in order to investigate disorder and the mechanism of the \ensuremath{\alpha}-\ensuremath{\beta} transition. The increasingly large underestimation of P-O and Fe-O distances in Rietveld refinements of the average long-range structure as compared to the bond lengths obtained from the pair distribution function as a function of temperature is a clear indication of the presence of significant dynamic disorder, particularly from 850 K up to and above the \ensuremath{\alpha}-\ensuremath{\beta} transition near 980 K. A significant broadening of the Fe-O distance distribution is also observed. Reverse Monte Carlo modeling confirms the presence of such disorder with broadened angular distributions in this temperature range, in particular for the Fe-O-P and Fe-P-Fe distributions. The Raman spectrum, calculated using density functional theory, is in very good agreement with experiment. These calculations indicate that there is an inversion of two low-energy A${}_{1}$ vibrational modes with respect to AlPO${}_{4}$. The principle mode, which exhibits strong damping in the Raman spectrum above 850 K, is thus not a tetrahedral libration mode, but a mode that principally involves large amplitude translations of the Fe atoms along with a degree of oxygen displacement. The transition mechanism from a dynamic point of view is thus different from the transitions in SiO${}_{2}$ and AlPO${}_{4}$. The strong damping of this mode is also further evidence of a high degree of dynamic disorder, which is different from the disorder observed in SiO${}_{2}$ and AlPO${}_{4}$. This mode does not exhibit any significant softening with temperature near the phase transition, which is further evidence that the \ensuremath{\alpha}-\ensuremath{\beta} transition is not of the simple displacive type. The difference in behavior between FePO${}_{4}$ and the other quartz homeotypes arises from the weaker bonding between the 3${d}^{5}$ transition metal cation and oxygen.

Theoretical calculations and experiments show the absence of libration modes of the tetrahedra in GaAsO4, the most α-quartz-type distorted material. In consequence, the degree of dynamic disorder at high temperature is very low, making GaAsO4 of high interest for high-temperature applications. This paper shows the importance of the theoretical calculations of vibration in oxide materials. In this way, it could be possible to extend this result to other materials and predict the thermal stability of the materials and their potential applications at high temperature.

We explore two basic issues behind the 1-bond → 2-mode percolation scheme that has recently lead to a unification of the classification of the Raman and infrared spectra of usual zincblende semiconductor alloys. In doing so we focus on the model ZnBeSe alloy, for which the percolation scheme was originally developed. First, we show by using inelastic neutron scattering that the well-resolved 1-bond → 2-mode percolation doublet of the short Be–Se bond detected close to the zone center by Raman and infrared spectra remains observable throughout the whole Brillouin zone up to the zone edge. This testifies for an origin at the bond scale. Second, high-pressure is used to disable a Fano interference which screens the Zn–Se Raman signal, revealing a distinct Zn–Se percolation doublet. This provides experimental evidence that the 1-bond → 2-mode percolation scheme is generic, and may, in principle, apply to all bonds in an alloy.

AbstractStructure and dynamic disorder of the title compound are studied by total neutron scattering coupled with reverse Monte Carlo modeling.

Sessions C702More detailed analysis reveals, however, how compositional disorder sets up a local strain field and couples to soft modes preventing their condensation into a low symmetry phase.We study, by means of DS experiments and atomistic modelling different ferroelectric compounds that exhibit this type of behaviour [1].In the case of KNb 1-x Ta x O 3 substituting ferroelectrically active Nb atoms with Ta (up to high values of x) does not alter significantly the tendency for the system to host polar BO 6 chains.It does however constrain the transverse correlation of chains (the fact observed experimentally for similar, BaTiO 3 -related compounds [2]).For PbMg 1/3 Nb 2/3 O 3 molecular dynamics simulations for different realizations of B-site partial disorder enables extraction of the conditions necessary for the existence of polar order.At the same time it is shown that the DS observed for this compound is not directly related to so-called polar nano-regions.PbZn 1/3 Nb 2/3 O 3 and PbZr 1-x Ti x O 3 are among the other materials being analyzed.