Pressure-induced phase transition in α- and ß-BiNbO4.

BiNbO4 has attracted a great deal of interest due to its excellent photocatalytic activities. Besides, it possesses rich polymorphism. Here, the structural stability and structural evolution of orthorhombic α- and triclinic ß-BiNbO4 were investigated via in situ X-ray diffraction patterns and Raman spectra up to 46.7 GPa. Upon compression, both BiNbO4 samples become unstable. α-BiNbO4 transformed into the monoclinic C2/c structure at 10.3 GPa, while ß-BiNbO4 possessed one P1̄-to-P1 isostructural phase transition around 12.7 GPa, and for the first time the crystal structure of each high pressure phase was identified. Both high pressure structures remained stable without obvious symmetry changes during compression to 46.7 GPa. In addition, both phase transitions were reversible upon decompression. These results provide insights to understand pressure-induced reversible phase transition in ABO4 compounds with polymorphism.

Pressure-induced structural transformations and new polymorphs in BiVO4.

BiVO4 has attracted much attention in recent years due to its active photocatalytic and microwave dielectric properties. BiVO4 exhibits a rich structural polymorphism, and its properties strongly depend on the crystalline phase. Therefore, it is of great importance to achieve an easy control of its crystalline phase. In the present work, phase stability and vibrational properties of fergusonite- and zircon-type BiVO4 are investigated up to 41.6 GPa by in situ synchrotron X-ray diffraction (XRD), Raman spectroscopy, and first principles calculation. Upon compression, although having different initial structures, both types of BiVO4 consecutively transform to scheelite- and ß-fergusonite structures. For the first time reported for BiVO4, the ß-fergusonite structure is determined using first principles computational techniques and from refinement of the XRD data. Along the way, one new phase of BiVO4 is theoretically predicted at higher pressures. Moreover, both the fergusonite-to-scheelite and scheelite-to-ß-fergusonite transitions are reversible, while the zircon-to-scheelite transition is irreversible. A large volume collapse is observed associated with each phase transition, and the equations of state for different types of BiVO4 have been determined. These results provide new insights into the relationship between different structural types in the AVO4 family.

Two novel aromatic hydrocarbons: facile synthesis, photophysical properties and applications in deep-blue electroluminescence.

Two efficient novel fluorescent naphthalene and fluorene-based aromatic hydrocarbon isomers (1 and 2) are prepared and investigated for organic electroluminescence. These compounds show bright violet to deep-blue emission, narrow full width at half maximum (52 nm), and high photoluminescence efficiency (e.g. 0.61 in CH2Cl2, 0.67 in film). Alternation of substituent position on the naphthalene moiety can give rise to remarkable emission variation. The relatively large torsion angle between naphthalene and fluorene suppresses the π-π interactions by weakening the intermolecular interactions in the solid state, which can result in highly efficient fluorescence. Moreover, the 1931 Commission Internationale de L'Eclairage coordinates and maximum emission peak for deep-blue electroluminescence based on 1 are (0.16, 0.08) and 410 nm, respectively.

[Study on the Temperature Dependent Phase Transformation of Raman Spectra for Cyclobutanol].

Cyclobutanol (C4H8O) is one of the four-membered ring type molecules, which usually adopts a non-planar equilibrium conformation, and the substituent group OH can adopt two positions relative to the puckered ring, the axial or the equatorial, giving rise to an additional degree of freedom and various molecular conformations. Additionally, temperature is one important thermodynamic parameter that greatly influents the structure and induces the possibility of conformational change or crystal change. As a consequence, there may be a number of phase transitions and molecular conformations for cyclobutanol under different temperature. In this paper, Raman and infrared spectroscopic technique were applied to investigate the vibration modes of cyclobutanol. The results indicate that the main component of the liquid cyclobutanol is equatorial-trans (Eq-t) conformer with a few Eq-g conformers at ambient condition. Then differential scanning calorimetry (DSC) and low temperature Raman spectroscopic were applied to study the phase transition of cyclobutanol during the cooling and heating process. It is observed that the Raman spectra and the intensities of these bands are not significantly changed during the cooling process. The results indicate that there is sill no presence of solidification especially cooling to 140K, which indicates that the cyclobutanol still remains the liquid state and supercooled state is observed during the cooling process. And this supercooled liquid is one metastable state, not in thermodynamic equilibrium. Further cooling to 138 K, the super-cooling liquid cyclobutanol will transform into the glassy state, accompanied with a small change of entropy. During the heating process, as the temperature is raised to 180 K, the Raman peaks became sharper and some new characteristic peaks appeared abruptly and a discontinuous change was observed in bandwidths versus temperature. And these new signatures can be maintained upon to 220 K, and then will disappear as the temperature increasing continuously. This result indicates the one crystal phase transition and a melting transition present at around 180 and 220 K. In addition, it can be observed that the component of Eq-g conformer increases, accompanied with the crystallization during heating at around 180 K. These results were helpful to understand the kinetics of the crystallization process of other small organic molecules.

Photoluminescence of monolayer MoS2 on LaAlO3 and SrTiO3 substrates.

In an atomically thin-film/dielectric-substrate heterostructure, the elemental physical properties of the atomically thin-film are influenced by the interaction between the thin-film and the substrate. In this article, utilizing monolayer MoS(2) on LaAlO(3) and SrTiO(3) substrates, as well as SiO2 and Gel-film as reference substrates similar to previously reported work [Nano Res, 2014, 7, 561], we systematically investigate the substrate effect on the photoluminescence of monolayer MoS(2). We observed significantly substrate-dependant photoluminescence of monolayer MoS(2), originating from substrate-to-film charge transfer. We found that SiO2 substrate introduces the most charge doping while SrTiO(3) introduces less charge transfer. Through the selection of desired substrate, we are able to induce different amounts of charge into the monolayer MoS(2), which consequently modifies the neutral exciton and charged exciton (trion) emissions. Finally, we proposed a band-diagram model to elucidate the relation between charge transfer and the substrate Fermi level and work function. Our work demonstrates that the substrate charge transfer exerts a strong influence on the monolayer MoS(2) photoluminescence property, which should be considered during device design and application. The work also provides a possible route to modify the thin-film photoluminescence property via substrate engineering for future device design.

In situ crystallization of ionic liquid [Emim][PF6] from methanol solution under high pressure.

The solubility of 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) in methanol under high pressure is newly measured quantitatively according to the correlation between the ratios of Raman intensity and the concentrations. In situ crystallization and cation conformation of [Emim][PF6] from methanol solution under high pressure have been investigated by using Raman spectroscopy in detail. Remarkably, crystal polymorphism was observed and two crystalline phases (phases I and II) coexisted under high pressure up to ∼ 1.4 GPa. However, only phase II was obtained by recrystallization at ∼ 2 GPa. Our findings may facilitate the development of an effective way for crystallization and purification of ionic liquids under high pressure.

In situ observation of multiple phase transitions in low-melting ionic liquid [BMIM][BF4] under high pressure up to 30 GPa.

In situ characterization of phase transitions and direct microscopic observations of a low-melting ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF(4)]), has been performed in detail by Raman spectroscopy. Compression of [BMIM][BF(4)] was measured under hydrostatic pressure up to ~30.0 GPa at room temperature by using a high-pressure diamond anvil cell. With pressure increasing, the characteristic bands of [BMIM][BF(4)] displayed nonmonotonic pressure-induced frequency shifts, and it is found to undergo four successive phase transitions at around 2.25, 6.10, 14.00, and 21.26 GPa. Especially, above a pressure of 21.26 GPa, luminescence of the sample occurs, which is connected with the most significant phase transition at around this pressure. It was indicated that the structure change under high pressure might be associated with a conformational change in the butyl chain. Upon releasing pressure, the spectrum was not recovered under a pressure up to 1.16 GPa, thereby indicating that this high-pressure phase remains stable over a large pressure range between 30 and 1.16 GPa in low-melting ionic liquid [BMIM][BF(4)]. Although the sample was kept under the normal pressure for 24 h, the spectrum was recovered, and it showed that the phase transition of [BMIM][BF(4)] was reversible. In other words, such a low-melting ionic liquid [BMIM][BF(4)] remains stable even after being treated under so a high pressure of up to 30 GPa.

[Preparation and luminescence of Eu3+ /Yb3+ codoped ZrO2 powders].

Samples of Eu3+ /Yb3+ co-doped ZrO2 powders were prepared by co-precipitation method. The dependence on the sintering temperature and doping concentration of the structure and luminescence was studied. The results confirmed that the sintering temperature has significant influence on the crystalline phases of ZrO2. As the sintering temperature increased the tetragonal phase was transformed into monoclinic phase. After sintered at 1150 degrees C, single monoclinic phase was observed. In contrast, with the increase in the doping concentration of Yb3+, the crystalline phase was also changed, and the monoclinic phase was transformed back to tetragonal phase. With 1% Eu3+ and 10% Yb3+ doping, single tetragonal phase presents. It was observed that the luminescent properties of Eu3+ ions in two structures were different. Experiment results reveal that the luminescence can be affected by both sintering temperature and doping concentration. With single Yb3+ doping, no NIR emission was observed under ultraviolet light excitation (270 nm). However, with Eu3+/Yb3+ codoping, NIR emission around 980 nm from Yb3+ ((2)F(5/2)-->(2)F(7/2)) was observed under the same excitation. Furthermore, it is confirmed that Yb3+ has the same excitation spectrum with Eu3+. This down-conversion result indicates that there is an energy transfer process between Eu3+ and Yb3+. Cooperative energy transfer process and cross-relaxation process were assigned as the possible mechanism for the near-infrared emission of Yb3+.