Equation of State of Gallium Oxide to 70 Gpa: Comparison of Quasihydrostatic and Nonhydrostatic Compression

Synchrotron x-ray diffraction and diamond-anvil cell techniques were used to characterize pressure induced structural modifications in gallium oxide. Gallium oxide was studied on compression up to 70 GPa and on the following decompression. The effect of the pressure-transmitting medium on the structural transformations was investigated in two sets of compression and decompression runs, one with nitrogen as a quasihydrostatic pressure-transmitting medium and the other in nonhydrostatic pressure conditions. The x-ray diffraction data showed gradual phase transition from a low-density, monoclinic β-Ga2O3 to a high-density, rhombohedral α-Ga2O3. With the use of nitrogen as a pressure transmitting medium, the β- to α-Ga2O3 transition begins at about 6.5–7 GPa and extends up to ∼40 GPa, confirming recent theoretical calculations. This pressure-driven transition is irreversible and the material decompressed from 70 GPa to ambient conditions was composed, in both sets of experimental runs, of α-Ga2O3 only. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero pressure bulk modulus K0=199(6) GPa, and its pressure derivative K0′=3.1(4) for theβ-Ga2O3 phase, and K0=220(9) GPa and K0′=5.9(6) for the α-Ga2O3 phase for the experiments performed in quasihydrostatic compression conditions. When for the same experiment K0′ is held at 4, then the bulk modulus values are 184(3) and 252(14) GPa for β-Ga2O3 and the α-Ga2O3, respectively. We compare the results of this work with our previous studies on the high-pressure behavior of nanocrystalline gallium oxid

Stability and Equation of State of a Nanocrystalline Ga-Ge Mullite in a Vitroceramic Composite: A Synchrotron X-ray Iffraction Study

Synchrotron x-ray diffraction and diamond anvil cell techniques were used to characterize the phase transformations and to evaluate the structural stability at elevated pressures of a developed nanocrystalline composite. The optically transparent material was built of a germanium oxide-based amorphous host matrix with homogeneously dispersed 13±3 nm Ga-Ge mullite-type nanocrystals, which had a structure similar to the conventional Al2O3-SiO2 mullite. The equation of state of the nanocrystals and the overall structural integrity of the nanocomposite were investigated in quasihydrostatic conditions on compression to 36 GPa and on the following decompression to ambient conditions. The overall pressure-induced changes of x-ray diffraction patterns evidenced that the structural integrity of the material is well preserved up to about 14–16 GPa. The nanocomposite decompressed from 36 GPa to ambient pressure showed a very limited reversibility of the pressure-driven changes. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus, K0, for the nanocrystalline phase of 229(15) GPa which makes this material potentially interesting for structural applications at elevated pressures

Formation of two-dimensional weak localization in conducting Langmuir-Blodgett films

We report the magnetotransport properties up to 7 T in the organic highly conducting Langmuir-Blodgett(LB) films formed by a molecular association of the electroactive donor molecule bis(ethylendioxy)tetrathiafulvalene (BEDO-TTF) and stearic acid CH$_3$(CH$_2$)$_{16}$COOH. We show the logarithmic decrease of dc conductivity and the negative transverse magnetoresistance at low temperature. They are interpreted in the weak localization of two-dimensional (2D) electronic system based on the homogeneous conducting layer with the molecular size thickness of BEDO-TTF. The electronic length with phase memory is given at the mesoscopic scale, which provides for the first time evidence of the 2D coherent charge transport in the conducting LB films.Comment: 5 pages, 1 Table and 5 figure

Low frequency Raman scattering from surface vibrational modes and size of microheterogenities in an amorphous matrix

Polarized room-temperature Raman scattering spectra measured on heat-treated GeO2-based glasses show features resembling particle peaks. They are related to the discrete particle-like structure of thermally treated samples. We have also found that our light scattering spectra have characteristic polarization properties

High-Pressure X-Ray Diffraction Studies Of The Nanostructured Transparent Vitroceramic Medium K2O-Sio2-Ga2O3

Synchrotron-radiation-based, energy-dispersive x-ray-diffraction studies have been performed on a composite containing nanometer-size aggregates embedded in an amorphous matrix, in the pressure range from ambient up to 15 GPa. The optically transparent material containing β-Ga2O3 nanocrystals was developed by the controlled crystallization of a silicon oxide-based amorphous precursor. Transmission electron microscopy and conventional x-ray-diffraction techniques allowed estimating the mean size of a single-crystalline phase to be 14.8±1.9 nm, distributed homogeneously in an amorphous medium. The pressure-driven evolution of x-ray-diffraction patterns indicated a progressive densification of the nanocrystalline phase. A structural modification corresponding to a pressure-induced coordination change of the gallium atoms was evidenced by the appearance of new diffraction peaks. The overall changes of x-ray-diffraction patterns indicated a β-Ga2O3 to α-Ga2O3 phase transformation. The low- to high-density phase transition was initiated at around 6 GPa and not completed in the pressure range investigated. A Birch-Murnaghan fit of the unit-cell volume change as a function of pressure yielded a zero-pressure bulk modulus, K0, for the nanocrystalline phase of 191±4.9 GPa and its pressure derivative, K0′=8.3±0.9. © 2003 The American Physical Society

Stability and equation of state of a nanocrystalline Ga-Ge mullite in a vitroceramic composite: A synchrotron x-ray diffraction study

Synchrotron x-ray diffraction and diamond anvil cell techniques were used to characterize the phase transformations and to evaluate the structural stability at elevated pressures of a developed nanocrystalline composite. The optically transparent material was built of a germanium oxide-based amorphous host matrix with homogeneously dispersed 13 +/- 3 nm Ga-Ge mullite-type nanocrystals, which had a structure similar to the conventional Al2O3-SiO2 mullite. The equation of state of the nanocrystals and the overall structural integrity of the nanocomposite were investigated in quasihydrostatic conditions on compression to 36 GPa and on the following decompression to ambient conditions. The overall pressure-induced changes of x-ray diffraction patterns evidenced that the structural integrity of the material is well preserved up to about 14-16 GPa. The nanocomposite decompressed from 36 GPa to ambient pressure showed a very limited reversibility of the pressure-driven changes. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus, K-0, for the nanocrystalline phase of 229(15) GPa which makes this material potentially interesting for structural applications at elevated pressures