Session L4: NM.3 Novel Properties I

Phase stability and Transitions' mechanisms in MPO$_4$ compounds under high pressures

MPO$_4$ (M $=$ Al, Ga, In, B) compounds exist in several tetrahedrally connected structures at ambient conditions. Our experimental and theoretical investigations show that pressure brings about systematic changes in the nature of polyhedral coordination. Unlike the structural analog SiO$_{2}$ most of MPO$_{4}$ compounds evolve through an intermediate Cmcm phase before transforming to still denser phases in which even phosphorus is also octahedrally coordinated. Our investigations also reveal that over-pressure helps bypass the region of co-existence of parent and daughter phases implying a barrierless path for emergence of higher coordinated states. Results of several investigations have helped obtain a unified understanding of these compounds and will be presented.   

Molecular Routes Syntheses of Nano-structured C and C-N Compounds in High Pressure and Temperature using LH-DAC

Molecular routes syntheses in high pressure and temperature are powerful to obtain novel materials, especially carbon-based inorganic compounds. We have tried to synthesize new nano-structured C and C-N inorganic compounds from heterocyclic compounds in high pressure and temperature using a Laser-Heated Diamond-Anvil Cell (LH-DAC) system and characterized by XRD, SEM-EDX, TEM-EELS, Raman, RBS and ERDA. Some heterocyclic organic compounds were reacted to various kinds of nano-structured carbons such as nano-diamonds, carbon nano-tubes and carbon nano-cones. We have also synthesized graphite-like nano-structured C-N compounds using this technique. They showed petal-like morphology similar to ``carbon nano-wall.'' The thickness of petals was several tens of nm. Petals were found to have a graphite-like layer structure on the basis of TEM experiments. It was found that the c-axis lattice constant changes almost linearly as a function of the analyzed nitrogen content. We will also report synthesis results of nano-structured C/C-N compounds using GPa-range supercritical fluid in DAC.   

A new hexagonal carbon nitride synthesized at high pressure and high temperature

A new hexagonal carbon nitride has been synthesized by subjecting the C$_{3}$N$_{4}$H$_{x\, }$precursor to high pressure and high temperature. The XRD pattern of the new hexagonal carbon nitride is indexed as the hexagonal unit cell with the lattice parameters; $a = b =$ 2.83 {\AA}, $c = $ 9.82 {\AA} (V $=$ 68.10 {\AA}$^{3})$. The unit cell of this new hexagonal carbon nitride differs from the several hexagonal carbon nitrides reported so far by the theoretical and experimental studies. Hart et al. proposed the hexagonal CN structure with 1:1 stoichiometry is based on the known GaSe layer with the unit cell parameters of the hexagonal unit $a = b =$ 2.37 {\AA}, $c =$ 11.38 {\AA} (V $=$ 55.36 {\AA}$^{3})$, respectively, and the space group is $P$6$_{3}$/\textit{mmc}. Bojdys et al. synthesized the graphitic-C$_{3}$N$_{4}$ (g-C$_{3}$N$_{4})$ with the hexagonal unit cell with the lattice parameters;$ a = b =$ 8.43 {\AA}, $c = $ 6.72 {\AA} (V $=$ 414.09 {\AA}$^{3})$, and the space group is $P$6$_{3}$\textit{cm}. These reported hexagonal lattice constants disagree with the new hexagonal carbon nitrides synthesized in this report. We will report the full analysis of the crystal structure of the new hexagonal carbon nitride at the conference.   

High-Pressure Induced New Phases and Properties in Typical Molecular Systems

High pressure introduces new phases by the rearrangement of atoms and reconfigurations of electronic states in materials, often with new physical and chemical phenomena. Study of the new phases in typical molecular systems under high pressure is an interesting subject, such as energy storage materials of solid hydrogen and polymeric nitrogen, hydrogen-rich compound with high-Tc superconductivity under high pressure, high pressure induced metallization of hydrogen, etc. High-pressure structures and pressure-induced phase transitions in the typical molecular solids, such as solid iodine, CHBr3, N2/CN, HBr/HCl, hydrogen-rich compounds (H2S, ZrH2, AsH3, BaReH9, etc.), and group IVA hydrides (Si2H6, Ge2H6, Sn2H6, etc.) are investigated extensively by means of first-principles density functional theory and extensive prediction strategies (molecular dynamics simulation, simulated annealing, soft mode phase transition, random structure-searching method and evolutionary methodology etc.). The new structures and new properties derived from pressure-induced phase transitions in these typical molecular systems have been observed. It is showed that high pressure provides a path for producing new materials with new properties.   

Pressure-induced transformations of confined diatomic molecules inside the one-dimensional channels

Studies of the control and manipulation of atoms/molecules and their assemblies generate remarkable new insights into how physical and chemical systems function. Confining iodine into single crystal zeolite AlPO4-5(AFI), which consists of, well packed, one-dimensional (1D) channels with homogeneous inner diameter of 0.73 nm, has been recently identified to be an effective way to create 1D (I2)n chains. Here, iodine and bromine doped AFI were obtained by a high temperature vapor method. The confined iodine and bromine inside the 1D channels are found to exist as molecular chains, as well as small amount of standing and lying neutral molecules. Using polarized Raman scattering measurement, synchrotron X-ray diffraction and theoretical calculations, we have discovered a unique transition dynamics of the confined species inside the 1D channel of AFI under pressure. The pronounced pressure-induced prolongation of molecular chains, pressure-induced rotation of the confined neutral molecules, and the abrupt transition in the vibrational frequency of the confined iodine due to the change of the interaction between the confined species and host wall have been observed.