High-Pressure Phase Transitions of Cubic Y_2O_3 under High Pressures by In-situ Synchrotron X-Ray Diffraction

High-pressure phase transitions of cubic Y_2O_3 are investigated using in situ synchrotron x-ray diffraction in a diamond anvil cell up to 36.3 GPa. The pressure-induced phase transitions of cubic Y_2O_3, which display apparent inconsistencies in previous studies, are verified to be from a cubic phase to a monoclinic phase and further to a hexagonal phase at 11.7 and 21.6 GPa, respectively. The hexagonal Y_2O_3 displays noticeable anisotropic compressibility due to its layered structure and it is stable up to the highest pressure in the present study. A third-order Birch–Murnaghan fit based on the observed pressure-volume data yields zero pressure bulk moduli of180(3), 196(7) and 177(7) GPa for cubic, monoclinic and hexagonal phases, respectively.     

Structures and phase transitions of ScH3 under high pressure

The structures and the phase transitions of ScH3 under high pressure are investigated using first-principles calculations. The calculated structural parameters at zero pressure agree well with the available experimental data. With increasing pressure, the transition sequence hcp (GdH3 -type)→ C2/m →fcc→hcp (YH3-type)→Cmcm of ScH3 is predicted first; the corresponding transition pressures at 0 K are 23 GPa, 25 GPa, 348 GPa, and 477 GPa, respectively. The C2/m symmetry structure is a possible candidate but not a good one as the intermediate state from hexagonal to cubic in ScH3 . On the other hand, via the analysis of the structures of hexagonal ScH2.9 , cubic ScH3 , and cubic ScH2 , we find that the repulsive interactions of H-H atoms must play an important role in the transition from hexagonal to cubic.     

Phase-Transition and Magnetic Moment of the Gd3+ Ion in the Gd2Fe17 Compound

The structure and magnetic phase transitions of the Gd2Fe17 compound are investigated by using a differential thermal/thermogravimetric analyzer, x-ray diffraction, and magnetization measurements. The result shows that there are two phase structures for the Gd2Fe17 compound： the hexagonal Th2Nilr-type structure at high temperatures （above 1243℃）, and the rhombohedral Th2Zn17-type structure, respectively. A method to measure the magnetic moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound is presented. The moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound from 77 to 500 K are measured in this way with a vibrating sample magnetometer. A detailed discussion is presented.     

Oxygen vacancy formation and migration in Sr- and Mg-doped LaGaO₃ : a density functional theory study

Oxygen vacancy formation and migration in La0.9 Sr0.1 Ga0.8 Mg0.2O3δ (LSGM) with various crystal symmetries (cubic, rhombohedral, orthorhombic, and monoclinic) are studied by employing first-principles calculations based on density functional theory (DFT). It is shown that the cubic LSGM has the smallest band gap, oxygen vacancy formation energy, and migration barrier, while the other three structures give rise to much larger values for these quantities, implying the best oxygen ion conductivity of the cubic LSGM among the four crystal structures. In our calculations, one oxygen vacancy migration pathway is considered in the cubic and rhombohedral structures due to all the oxygen sites being equivalent in them, while two vacancy migration pathways with different migration barriers are found in the orthorhombic and monoclinic symmetries owing to the existence of nonequivalent O1 and O2 oxygen sites. The migration energies along the migration pathway linking the two O2 sites are obviously lower than those along the pathway linking the O1 and O2 sites. Considering the phase transitions at high temperatures, the results obtained in this paper can not only explain the experimentally observed different behaviours of the oxygen ionic conductivity of LSGM with different symmetries, but also predict the rational crystal structures of LSGM for solid oxide fuel cell applications.     

In situ X-ray diffraction investigation of compression behavior in Gd40Y16Al24Co20 bulk metallic glass under high pressure with synchrotron radiation

The compression behavior of the heavy RE-based BMC Gd40Y16Al24Co20 under high pressure has been investigated by in situ high pressure angle dispersive X-ray diffraction measurements using synchrotron radiation in the pressure range of 0-33.42 GPa at room temperature. By fitting the static equation of state at room temperature, we find the value of bulk modulus B is 61.27±4 GPa which is in good agreement with the experimental study by pulse-echo techniques of 58 GPa. The results show that the amorphous structure in the heavy RE-based BMG Gd40Y16Al24Co20 keeps quite stable up to 33.42 GPa although its compressibility is as large as about 33%. The coexistence of normal local structure similar to that of other BMGs and covalent bond structure similar to those of oxide glasses may be the reason for the anomalous property under high pressure of the Gd4oY16Al24Co2o BMG.     

The Phase Transition of Eu2O3 under High Pressures

Pressure-induced phase transition of cubic Eu2 03 is studied by angle-dispersive x-ray diffraction （ADXD） up to 42.3 GPa at room temperature. A structural transformation from a cubic phase to a hexagonal phase is observed, which starts at 5.0 GPa and finishes at about 13.1 GPa. The phase transition leads to a volume collapse of 9.0% at 8.6 GPa. The hexagonal phase of Eu2 03 maintains stable up to the highest experiment pressure. After re/ease of pressure, the high-pressure phase transforms to a monoclinic phase. The pressure-volume data are fitted with the Birch-Murnaghan equation of state. The bulk moduli obtained upon compression from the fitting are 145（2） GPa and 151（6） OPa for the cubic and hexagonal phases, respectively, when their first pressure derivatives are fixed at 4.     

Phase transition and thermal expansion property of Cr_(2-x)Zr_(0.5x)Mg_(0.5x)Mo_3O_(12) solid solution

Compounds with the formula Cr2-xZr0.5xMg0.5xMo3O12（x = 0.0, 0.3, 0.5, 0.9, 1.3, 1.5, 1.7, 1.9） are synthesized, and the effects of Zr4＋ and Mg2＋ co-incorporation on the phase transition, thermal expansion, and Raman mode are investigated. It is found that Cr2-xZr0.5xMg0.5xMo3O12 crystallize into monoclinic structures for x 〈 1.3 and orthorhombic structures for x _〉 1.5 at room temperature. The phase transition temperature from a monoclinic to an orthorhombic structure of Cr2Mo3O12 can be reduced by the partial substitution of （ZrMg）6＋ for Cr3＋. The overall linear thermal expansion coefficient decreases with the increase of the （ZrMg）6＋ content in an orthorhombic structure sample. The co-incorporation of Zr4＋ and Mg2＋ in the lattice results in the occurrence of new Raman modes and the hardening of the symmetric vibrational modes, which are attributed to the MoO4 tetrahedra sharing comers with ZrO6/MgO6 octahedra and to the strengthening of Mo-O bonds due to less electronegativities of Zr4＋ and Mg2＋ than Cr3＋, respectively.     

High-Pressure and High-Temperature Behaviour of Gallium Oxide

High-temperature and high-pressure behaviours of β-Ga2O3 powder are studied by energy-dispersive x-ray diffrac- tion in a diamond anvil cell （DAC）. It is found that the phase transition from the monoclinic β-Ga2O3 to the trigonal α-Ga2O3 occurs at around 19.2 GPa under cold compression. By heating the powder to 2000 K at 30 GPa, we confirm that α-Ga2O3 is the most stable structure at the high pressure. Furthermore, the structural transition from β-Ga2O3 to α-Ga2O3 is irreversible. After laser heating, the recrystallized Ga2O3 has a preferable （012） orientation. This interesting behaviour is also discussed.     

Oxygen vacancy formation and migration in Sr- and Mg-doped LaGaO3: a density functional theory study

Oxygen vacancy formation and migration in La0.9Sr0.1Ga0.8Mg0.2O3-5 （LSGM） with various crystal symmetries （cubic, rhombohedral, orthorhombic, and monoclinic） are studied by employing first-principles calculations based on density functional theory （DFT）. It is shown that the cubic LSGM has the smallest band gap, oxygen vacancy formation energy, and migration barrier, while the other three structures give rise to much larger values for these quantities, implying the best oxygen ion conductivity of the cubic LSGM among the four crystal structures. In out calculations, one oxygen vacancy migration pathway is considered in the cubic and rhombohedral structures due to all the oxygen sites being equivalent in them, while two vacancy migration pathways with different migration barriers are found in the orthorhombic and monoclinic symmetries owing to the existence of nonequivalent O1 and 02 oxygen sites. The migration energies along the migration pathway linking the two 02 sites are obviously lower than those along the pathway linking the O1 and 02 sites. Considering the phase transitions at high temperatures, the results obtained in this paper can not only explain the experimentally observed different behaviours of the oxygen ionic conductivity of LSGM with different symmetries, but also predict the rational crystal structures of LSGM for solid oxide fuel cell applications.     

Influence of Ytterbia Content on Residual Stress and Microstructure of Y2O3--ZrO2 Thin Films Prepared by EB-PVD

Y2O3 stabilized ZrO2 （YSZ） thin films with different Y2O3 molar contents （0, 3, 7, and 12 mol%） are deposited on BK7 substrates by electron-beam evaporation technique. The effects of different Y2O3 contents on residual stresses and structures of YSZ thin films are studied. Residual stresses are investigated by means of two different techniques： the curvature measurement and x-ray diffraction method. It is found that the evolution of residual stresses of YSZ thin films by the two different methods is consistent. Residual stresses of films transform from compressive stress into tensile stress and the tensile stress increases monotonically with the increase of Y2O3 content. At the same time, the structures of these films change from the mixture of amorphous and monoclinic phases into high temperature cubic phase. The variations of residual stress correspond to the evolution of structures induced by adding of Y2O3 content.     

Hydrothermal synthesis, characterization and Raman studies of Eu activated Gd2O3 nanorods

Eu³⁺ (8 mol%) activated gadolinium oxide nanorods have been prepared by hydrothermal method without and with surfactant, cityl trimethyl ammonium bromide (CTAB). Powder X-ray diffraction (PXRD) studies reveal that the as-formed product is in hexagonal Gd(OH)₃:Eu phase and subsequent heat treatment at 350 and 600 °C transforms the sample to monoclinic GdOOH:Eu and cubic Gd₂O₃:Eu phases, respectively. The structural data and refinement parameters for cubic Gd₂O₃:Eu nanorods were calculated by the Rietveld refinement. SEM and TEM micrographs show that as-obtained Gd(OH)₃:Eu consists of uniform nanorods in high yield with uniform diameters of about 15 nm and lengths of about 50-150 nm. The temperature dependent morphological evolution of Gd₂O₃:Eu without and with CTAB surfactant was studied. FTIR studies reveal that CTAB surfactant plays an important role in converting cubic Gd₂O₃:Eu to hexagonal Gd(OH)₃:Eu. The strong and intense Raman peak at 489 cm⁻¹ has been assigned to A_g mode, which is attributed to the hexagonal phase of Gd₂O₃. The peak at ∼360 cm⁻¹ has been assigned to the combination of F_g and E_g modes, which is mainly attributed to the cubic Gd₂O₃ phase. The shift in frequency and broadening of the Raman modes have been attributed to the decrease in crystallite dimension to the nanometer scale as a result of phonon confinement.     

Electrical Properties of La-Doped Al2O3 Films on Si （100） Substrates as a High-Dielectric-Constant Gate Material

Amorphous La-doped Al2O3 （La： Al2O3） thin films are deposited on n-type （100） Si substrates by rf magnetron co-sputterlng. The composition of the deposited films is measured by energy dispersive x-ray spectroscopy： Capacitance-voltage measurement shows that the dielectric constant k of La-doped Al2O3 films ranges from 8.5 to 11.6 with the increasing La content, and the highest k value of 11.6 is obtained for the 20.14% La content film. In the structure of the Al/La：Al2O3/Si metal oxide semiconductor, the dominant conduction stems from the space- charge-limited current at different temperatures. In addition, the wavelength dependence of the transmittance is studied by ultraviolet spectroscopy and the band gap of all the deposited films is above 5.5eV. The results demonstrate that La-doped Al2O3 can meet the requirement of next-generation gate materials.     

Optical, structural and fluorescence properties of nanocrystalline cubic or monoclinic Eu:Lu2O3 films prepared by pulsed laser deposition

Eu³⁺-doped lutetium oxide (Eu:Lu₂O₃) nanocrystalline films were grown on fused-silica substrates by pulsed laser deposition. Depending on deposition conditions (oxygen pressure, temperature and laser energy), the structure of the films changed from amorphous to crystalline and the cubic or monoclinic phases were obtained with varying preferential orientation and crystallite size. The monoclinic phase could be prepared for the first time at temperatures as low as 240 °C and in a narrow range of parameters. Although this phase has been previously reported for powder samples, it occurs only for high pressures and high temperatures preparation conditions. The refractive indices were measured by m-lines spectroscopy for both crystalline phases and their dispersion curve fitted by the Sellmeier expression. The specific Eu³⁺ fluorescence properties of the different phases, monoclinic and cubic, were registered and show modifications due to the disorder induced by the nanometric size of the crystallites, emphasised in particular by quasi-selective excitation in the charge transfer band.     

Photoemission Spectroscopy and Electronic Structures of LiMn2O4

We study the electronic structures of LiMn2O4 by x-ray and ultraviolet photoelectron spectroscopy （XPS, UPS） and resonant photoelectron spectroscopy （RPES）. XPS data suggest that the average oxidation state of Mn ions is 3.55, probably due to the small amount of lithium oxides on the surface. UPS and RPES data imply that Mn ions are in a high spin state, and RPES results show strong Mn3d-O2p hybridization in the LiMn2O4 valence band.     

Structural and Optical Properties of Nanocrystal In2O3 Films by Thermal Oxidation of In2S3 Films

In₂S₃ nanocrystalline films are prepared on glass substrates by the spray pyrolysis technique using indium chloride and thiourea as precursors. The deposition is carried out at 350°C on glass substrates. The films are then annealed for two hour at 200, 400, 600, and 800°C in O₂ flow. This process allows the transformation of nanocrystal In₂O₃ films from In₂S₃ films and the reaction completes at 600°C. These results indicate that the In₂O₃ film prepared by this simple thermal oxidation method is a promising candidate for electro-optical and photovoltaic devices.     

Superconducting Transition Temperature and Metal--Semiconductor Transition Temperature in YBa2Cu4O8/La0.67Ca0.33MnO3/YBa2Cu4O8 Trilayer Films

YBa₂Cu₄O₈/La_0.67Ca_0.33MnO₃/YBa₂Cu₄O₈(YBCO/LCMO/YBCO) trilayer films were prepared by magnetron facing-target sputtering. For the first time, the oscillatory behaviour of superconducting transition temperature T_c,ON with the thickness of LCMO (d_L) has been observed. The strongest nonmonotonic information in the T_c,ON--d_L curves appears clearly when d_L is larger than the critical thickness d_L^CR. The metal--semiconductor transition temperature can only be detected at d_L>d_L^CR. The dependence on the ferromagnetic spacer layer in YBCO/LCMO/YBCO systems suggests strongly the interplay of ferromagnetic and superconducting couplings.     

Photoluminescence of cubic and monoclinic Gd2O3:Eu phosphors prepared by flame spray pyrolysis

In this paper, europium-doped gadolinium phosphor, which is a potentially bifunctional material with both fluorescent and magnetic properties, has been prepared in a one-step procedure via flame spray pyrolysis, and its crystal structure, morphology, and PL intensity were investigated. All the prepared phosphors were submicron-sized with spherical shapes and either a pure cubic or pure monoclinic phase. In order to observe the effects of temperature on the crystal phases of the prepared phosphors, we applied a H₂ vs. N₂/O₂ diffusion flame, with the maximum flame temperature ranging from T_max=1375 to 2050 K. The temperature profiles under various flame conditions are also reported herein to further elucidate the rapid synthesis process. The PL intensity in the cubic phase improved linearly with increasing flame temperature until the transition to a monoclinic phase. The peak of the photoluminescence(PL) spectrum from the phosphors prepared at T_max=1733 K in the cubic phase was narrower and twice as strong as the peak of the PL spectrum from the phosphors prepared at T_max=2050 K in the monoclinic phase. This paper provides important data showing the relationship between the synthesis temperature and the phase transition in Gd₂O₃:Eu in the continuous one-step use of flame spray pyrolysis.     

Preparation of Mg55Ni35Si10 Amorphous Powders by Mechanical Alloying and Consolidation by Vacuum Hot Pressing

Amorphous Mg55Ni35Si10 powders are fabricated by using a mechanical alloying technique. The amorphous powders are found to exhibit a relatively high crystallization temperature of 380℃. The as-milled amorphous Mg55Ni35Si10 powders are consolidated successfully into bulk body by vacuum hot pressing technique. Limited nanocrystallization is noticed. The Vickers microhardness range of the Mg55Ni35Si10 bulk sample is 7834 to 8048 MPa. Its bending strength and compressive strength are 529 MPa and 1466 MPa, respectively.     

Dielectric Properties of Fine-Grained BaTiO3 Ceramics under High Pressure

We investigate the temperature dependence of the dielectric constant of BaTiO3 ceramic with coarse to nanograin size under different hydrostatic high pressures up to 5000 bar in the range between room temperature and 200℃. The ferroelectric-to-paraelectric phase transition temperatures Tc are determined from the peak of dielectric constant versus temperature. The values of average grain-size are estimated from the SEM images. It is found that the magnitude of dTc/dp varies considerably from sample to sample depending on grain size. The Curie point Tc of the sample with small grain size decreases more sharply than that of samples with larger one.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.