Highlights on the reversible nonpolar-to-polar P3121-P31 phase transition at low temperature in NaLa(SO4)2·H2O: mechanism and piezoelectric properties.

We report the existence in NaLa(SO4)2·H2O of a displacive phase transition under 200 K from the nonpolar P3121 to the polar P31 space group. This phase transition was predicted by density functional theory based calculations and experimentally confirmed from infrared spectroscopy and X-ray diffraction. The A2 polar irreducible representation is the primary order parameter. The structural water and hydrogen bonding are the mechanism driving the phase transition. The piezoelectric properties of this new P31 phase have been investigated by first principles based calculations. The highest piezoelectric-strain constants in the zero Kelvin limit are predicted for the d12 and d41 elements with values about 3.4 pC N-1. This compound could be interesting as piezoelectric actuator for cryogenic applications.

Freezing of water confined at the nanoscale.

Freezing of water in hydrophilic nanopores (D=1.2 nm) is probed at the microscopic scale using x-ray diffraction, Raman spectroscopy, and molecular simulation. A freezing scenario, which has not been observed previously, is reported; while the pore surface induces orientational order of water in contact with it, water does not crystallize at temperatures as low as 173 K. Crystallization at the surface is suppressed as the number of hydrogen bonds formed is insufficient (even when including hydrogen bonds with the surface), while crystallization in the pore center is hindered as the curvature prevents the formation of a network of tetrahedrally coordinated molecules. This sheds light on the concept of an ubiquitous unfreezable water layer by showing that the latter has a rigid (i.e., glassy) liquidlike structure, but can exhibit orientational order.

In situ X-ray absorption spectroscopy study of Si(1-x)Ge(x)O2 dissolution and germanium aqueous speciation under hydrothermal conditions.

The dissolution of Si(1-x)Ge(x)O(2) solid solutions under hydrothermal conditions was studied by in situ X-ray absorption spectroscopy. Experiments were performed at the Ge K-edge using a high-pressure cell mounted on the FAME beamline of the European Synchrotron Radiation Facility. Spectra in both transmission and fluorescence mode were collected in isobaric conditions (100 and 150 MPa) up to 475 °C. The local atomic structure around the Ge atom was investigated as a function of the temperature and in pure water and sodium hydroxide solutions. In pure water, the solubility of the cristobalite-type Si(0.8)Ge(0.2)O(2) increases with the temperature and the Ge atom is in 4-fold coordination. In a sodium hydroxide aqueous solution, a complex between Ge and Na atoms forms and gives rise to precipitation of sodium germanates. Under these conditions, the Ge content in the solution decreases with increasing temperature. These results show that a sodium hydroxide aqueous solution, usually used for quartz crystal growth, is not suitable for Ge-containing crystals. The dissolution kinetics and phase transformation of the solid solution were studied as a function of the atomic fraction of Ge. Ge-rich solid solutions dissolve and transform to stable phases faster than Ge-poorer composition, giving rise to important variations of the Ge content in solution.

Vibrational origin of the thermal stability in the high-performance piezoelectric material GaAsO4.

Theoretical calculations and experiments show the absence of libration modes of the tetrahedra in GaAsO(4), the most α-quartz-type distorted material. In consequence, the degree of dynamic disorder at high temperature is very low, making GaAsO(4) 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.

Hydrothermal growth and structural studies of Si(1-x)Ge(x)O2 single crystals.

The substitution of germanium in the α-quartz structure is a method investigated to improve the piezoelectric properties and the thermal stability of α-quartz. Growth of α-quartz type Si(1-x)Ge(x)O(2) single crystals was performed using a temperature gradient hydrothermal method under different experimental conditions (pressure, temperature, nature of the solvent, and the nutrient). To avoid the difference of dissolution kinetics between pure SiO(2) and pure GeO(2), single phases Si(1-x)Ge(x)O(2) solid solutions were prepared and used as nutrients. The influence of the nature (cristobalite-type, glass) and the composition of this nutrient were also studied. Single crystals were grown in aqueous NaOH (0.2-1 M) solutions and in pure water. A wide range of pressures (95-280 MPa) and temperatures (315-505 °C) was investigated. Structures of single crystals with x = 0.07, 0.1, and 0.13 were refined, and it was shown that the structural distortion (i.e., θ and δ) increases with the atomic fraction of Ge in an almost linear way. Thus, the piezoelectric properties of Si(1-x)Ge(x)O(2) solid solution should increase with x, and this material could be a good candidate for technological applications requiring a high piezoelectric coupling factor or high thermal stability.

A high-temperature Raman scattering study of the phase transitions in GaPO(4) and in the AlPO(4)-GaPO(4) system.

Al(1-x)Ga(x)PO(4) solid solutions (x = 0.2, 0.3, 0.38, 0.7) and the pure AlPO(4) (x = 0) and GaPO(4) (x = 1) end members with the α-quartz-type structure were studied by Raman scattering. An investigation as a function of composition enabled the various modes to be assigned, in particular coupled and decoupled vibrations. The tetrahedral tilting modes, which have been linked to high-temperature phase transitions to ß-quartz-type forms, were found to be decoupled. In addition, it is shown that Raman spectroscopy is a powerful technique for determining the gallium content of these solid solutions. Single crystals with x = 0.2, 0.38, and 1.0 (GaPO(4)) were investigated at high temperature. The composition Al(0.8)Ga(0.2)PO(4) was found to exhibit sequential transitions upon heating to the ß-quartz and ß-cristobalite forms at close to 993 K and 1073 K, respectively. Direct α-quartz-ß-cristobalite transitions were observed for the two other compositions at close to 1083 K and 1253 K, respectively, upon heating. The spectra of the ß-quartz and ß-cristobalite forms indicate the presence of significant disorder. Back transformation to the α-quartz-type form occurred readily with a hysteresis of less than 100 K for the composition x = 0.38 and for pure GaPO(4). Rapid cooling was necessary to obtain the metastable α-cristobalite form. In contrast, for Al(0.80)Ga(0.20)PO(4), the α-cristobalite form was obtained even upon slow cooling.

In situ high pressure and high temperature Raman studies of (1-x)SiO(2)xGeO(2) glasses.

The structure of glasses in the binary system SiO(2)-GeO(2) has been studied by Raman spectroscopy. Our results are consistent with mixing of SiO(2) and GeO(2) tetrahedra. The changes induced by temperature and by pressure on the structure are monitored by in situ measurements on the same mixed glasses. Anomalous temperature dependences are observed not only for SiO(2) glass and GeO(2) glass but also for mixed glasses. Particular attention is focused on the pressure densification mechanism in mixed glasses. Via the pressure dependence of the width of the main Raman band, we show that the compression mechanism in mixed glasses is intermediate between that of the end members.