Being a best known thermoelectric material and a topological insulator at ambient condition, magic bismuth telluride (Bi2Te3) under pressure transforms into several superconducting phases, whose structures remain unsolved for decades. Here, we have solved the two long-puzzling low high-pressure phases as seven- and eightfold monoclinic structures, respectively, through particle-swarm optimization technique on crystal structure prediction. Above 14.4 GPa, we experimentally discovered that Bi2Te3 unexpectedly develops into a Bi-Te substitutional alloy by adopting a body-centered cubic disordered structure stable at least up to 52.1 GPa. The continuously monoclinic distortion leads to the ultimate formation of the Bi-Te alloy, which is attributed to the Bi→Te charge transfer under pressure. Our research provides a route to find alloys made of nonmetallic elements for a variety of applications.
Bulk quantities of nitrogen-rich graphitic carbon nitride are synthesized via a facile reactive pyrolysis process with a mixture of melamine and cyanuric chloride as the molecular precursors. Scanning electron microscopy and transmission electron microscopy studies show that micrometer-sized hollow vessels with high aspect ratios have been successfully elaborated without the designed addition of any catalyst or template. The composition of the prepared carbon nitride determined by combustion method is C(3)N(4.91)H(1.00)O(0.22), with the N/C ratio to be 1.64, indicating a high nitrogen content. X-ray diffraction pattern reveals the regular stacking of graphene CN(x) monolayers along the (002) direction with the presence of turbostratic ordering of C and N atoms in the a-b basal planes. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy investigations provide further evidence for graphite-like sp(2)-bonded building blocks based on both triazine and heptazine ring units bridged by 3-fold coordinated nitrogen atoms. The optical properties of the sample are investigated by UV-vis absorption and photoluminescence spectroscopy, which show features characteristic of pi-pi* and n-pi* electronic transitions involving lone pairs of nitrogen atoms. Thermogravimetric analysis curves of the synthesized graphitic carbon nitride hollow vessels show typical weight loss steps related to the volatilization of triazine and heptazine structural units.
Using the angle-dispersive synchrotron x-ray powder diffraction technique in a diamond anvil cell, the high-pressure behaviors of antimony telluride (Sb(2)Te(3)) are explored up to 52.7 GPa at room temperature. Three high-pressure phases have been observed, at about 8.0 GPa, 13.2 GPa and above 21.6 GPa, respectively. Furthermore, the crystalline structures of these high-pressure phases are determined as monoclinic sevenfold C2/m phase, eightfold C2/c phase and disordered body-centered cubic structure (space group Im - 3m) respectively. The phase-transition sequences and pressures observed are well explained by first-principles calculations. The pressure dependence of the volume of all high-pressure phases is described by a third-order Birch-Murnaghan equation of state. All the high-pressure phases are metallic and the metallic character for β-, γ- and δ-Sb(2)Te(3) increases in turn based on the results of the electronic density of states calculated for each high-pressure phase.
The structural transformations occurring to water from low-density (LDW) to high-density (HDW) regimes have been studied by Brillouin scattering for the first time at temperatures up to 453 K and at pressures up to the solidification point. At ambient temperature (293 K) a discontinuity in pressure response of the sound velocity is observed. Furthermore, there are evident breaks in the linear behavior of log10 C11 versus log10(rho/rho0) when pressure increases up to 0.29, 0.21, and 0.19 GPa at the temperature of 293, 316, and 353 K, respectively. It is supposed to indicate the structural transition from LDW to HDW, and the possible transition boundary between LDW and HDW is in good agreement with the molecular-dynamics simulation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.